Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia
Taxonomy & Systematics
RAFFLES BULLETIN OF ZOOLOGY 70: 134–248

Date of publication: 1 April 2022
DOI: 10.26107/RBZ-2022-0009
http://zoobank.org/urn:lsid:zoobank.org:pub:BE6164AE-1C24-4E01-8B7B-D80764F147B3

The pea crab genus Arcotheres Manning, 1993 (Crustacea: Brachyura:
Pinnotheridae) from Singapore and Peninsular Malaysia, with areappraisal of diagnostic characters and descriptions of two new genera
Peter K. L. Ng1* & Shane T. Ahyong2,3
Abstract. Study of a large collection of pinnotherid crabs collected primarily from Singapore and Peninsular Malaysia
revealed substantially more variation in the carapace shape, third maxilliped, and other taxonomic characters thanpreviously acknowledged. While carapace shape and form, structures of the epistome (especially the posterior
margin), antennal article 1, eyes (especially the cornea), third maxilliped, male and female chelae, ambulatory
pereopods, male pleon, as well as features of the male first and second gonopods are useful taxonomic characters,
the degree of variation must be better accounted for so that accurate species and genus assignments can be made.
The female-to-male ratio is also indicative for several species. Also significant is that the host of the pinnotherid
species is taxonomically correlated and many taxa are quite specific to the mollusc genus, family, or superfamily.
As a result of this reappraisal of the taxonomic characters used, 14 names are synonymised under five taxa.
The following new synonymies are recognised: Pinnotheres hirsutissima Johnson, 1963, a nomen nudum, under
Holotheres semperi (Bürger, 1895); P. rhombifer Bürger, 1895, P. latissimus Bürger, 1895, P. arcophilus Bürger,
1895, P. paralatissimus Dai & Song, 1986, and Arcotheres guinotae Campos, 2001, under Arcotheres palaensis(Bürger, 1895); P. modiolicola Bürger, 1895, under Arcotheres similis (Bürger, 1895); P. winckworthi Gordon,
1936, P. vicajii Chhapgar, 1957, P. casta Antony & Kutyamma, 1971, and P. obscuridentata Dai & Song, 1986,
under Arcotheres exiguus (Bürger, 1895); and P. latus Bürger, 1895, P. borradailei Nobili, 1906, and P. ridgewayi
Southwell, 1911, under P. globosum Hombron & Jacquinot, 1846, which is referred to a new genus, Magnotheres.
In addition, we establish a new genus, Plenotheres, for an unusual member of Arcotheres Manning, 1993, which
lives in mangrove clams of the families Cyrenidae Gray, 1840, and Glauconomidae Gray, 1853. The poorly known
Japanese species, Pinnotheres corbiculae Sakai, 1939, which lives in corbiculid clams and superficially resembles
Plenotheres coarctatus, new combination, is redescribed from a male specimen and here referred to Nepinnotheres
Manning, 1993. Likewise, Pinnotheres tsingtaoensis Shen, 1932, is also referred to Nepinnotheres.
Key words. systematics, Pinnotheroidea, Southeast Asia, new genera, synonymies, character states

INTRODUCTION

Gordon (1936) treated the taxonomy of three species frombivalves: P. edwardsi De Man, 1887, P. similis Bürger,
1895, and a new species, P. spinidactylus. In his book on
the Malayan (including Singapore) seashore fauna, Chuang(1961) listed two species, Pinnotheres palaensis Bürger,
1895, and Xanthasia whitei De Man, 1888, from bivalves,and P. semperi from sea cucumbers.

The earliest record of pinnotherid crabs from Singapore wasby Hombron & Jacquinot (1846), who described Pinnotheres
globosum from a “mussel”. This was also the first brachyuran
crab recorded from the island (see Ng & Corbari, 2019).
Lanchester (1900) subsequently recorded four species: two,P. semperi Bürger, 1895, and P. ortmanni Bürger, 1895,
from sea cucumbers; and two, P. modiolicola Bürger, 1895,
and P. arcophilus Bürger, 1895, from bivalve molluscs.

The two pinnotherids associated with sea cucumbers(Echinodermata: Holothuroidea), Holotheres semperi(Bürger, 1895) and Buergeres ortmanni (Bürger, 1895),
have been treated by Ng & Manning (2003) and Ahyong& Ng (2007b, 2020). The mollusc-associated pinnotherids,
Serenotheres Ahyong & Ng, 2005, and Tridacnatheres
Ahyong & Ng, 2005, with one species each from Singapore
and Malaysia, have been revised by Ahyong & Ng (2005),
so there is no need to elaborate on them here.

Accepted by: Jose Christopher E. Mendoza
Lee Kong Chian Natural History Museum, Faculty of Science, National University ofSingapore, 2 Conservatory Drive, Singapore 117377, Republic of Singapore; Email:peterng@nus.edu.sg (*corresponding author)
1

Australian Museum Research Institute, 1 William Street, Sydney NSW 2010, Australia;
Email: shane.ahyong@austmus.gov.au
2

In his review of commensal and semi-parasitic decapod
crustaceans of Singapore, Johnson (1963) expressed doubt
about many of the old identifications. He commented: “Thespecies which inhabitats [sic] the sea-cucumber Holothuria

School of Biological, Earth & Environmental Sciences, University of New South
Wales, NSW 2052, Australia
3© National University of Singapore
ISSN 2345-7600 (electronic) | ISSN 0217-2445 (print)

134

RAFFLES BULLETIN OF ZOOLOGY 2022
scabra is likewise very common. Usually a pair of individuals,
male and female are present. This species is remarkable bothfor its unusual habitat and its extreme hairiness. Chuang
records it as P. semperi. I have tentatively identified itas P. hirsutissima. Once again these identifications must
be considered to be doubtful.” (Johnson, 1963: 286). The
source of Johnson’s (1963) name “P. hirsutissima” is notknown—there is no pinnotherid species with this nameand perhaps he intended to eventually describe it as a new
species. In any case, “P. hirsutissima” is not an available
name as it has no description or diagnosis, and the namehas never been used since. As noted above, Ng & Manning(2003) confirmed that Chuang’s (1961) record is correctly
identified as the species currently known as Holotheres
semperi (Bürger, 1895). Pinnotheres hirsutissima Johnson,
1963, a nomen nudum, is here formally synonymised under
Holotheres semperi (Bürger, 1895).

epicaridean parasites and the carapace deformed, only thecarapace length is provided.
Specimens examined are deposited in the Australian Museum,
Sydney, Australia (AM); Institute of Zoology at the Chinese
Academy of Science, Beijing, China (CAS); Natural History
Museum and Institute, Chiba, Japan (CBM); Cambridge
University Museum of Zoology, Cambridge, United Kingdom(CUMZ); Kanagawa Prefectural Museum of Natural History,
Japan (KPM); Muséum national d’Histoire naturelle, Paris,
France (MNHN); The Natural History Museum, London,
United Kingdom (NHM); National Museum of Nature andScience, Tokyo, Japan (NSMT); Senckenberg Museum
Forschunginstitut (including the Zoologisch Museum
Göttingen collection), Frankfurt-am-Main, Germany (SMFZMG); National Museum of Natural History, Smithsonian
Institution, Washington, D.C. (USNM); Natural History
Museum of Denmark (formerly the Zoologisk Museum,
Københavns Universitet, Copenhagen) (NHMD); Zoological
Reference Collection of the Lee Kong Chian Natural History
Museum, National University of Singapore (ZRC); andZoological Survey of India, Calcutta, India (ZSI).

The Lee Kong Chian Natural History Museum (previously
the Raffles Museum of Biodiversity Research) at the
National University of Singapore has a large collection of
pinnotherids dating back to the early 1900s, although manyof the old specimens are in poor condition. Over the last
decade, the first author actively collected and examinedpea crabs (Pinnotheridae) in and around Singapore andPeninsular Malaysia, as well as neighbouring waters, toascertain the regional diversity. Much of the recent material
was obtained as a result of various malacological surveys
around the island led by resident malacologist S. K. Tan andhis colleagues, while others came from bivalves purchasedfrom markets, most of which were harvested in nearby
Peninsular Malaysia. Most of the species living in bivalvesfrom Peninsular Malaysia and Singapore belong to the genus
Arcotheres Manning, 1993a, with only a few species assigned
to Nepinnotheres Manning, 1993b, Serenotheres Ahyong& Ng, 2005, and Tridacnatheres Ahyong & Ng, 2005 (seeAhyong & Ng, 2007b; Ng, 2018a; Ng et al., 2019a).

Freshly collected pinnotherids should be carefully preserved.
In the past, as with many parasites, they were often placedin 5–10% formalin (not always buffered), but nowadays,
they are mostly preserved directly in 70–80% ethanol. For
fresh specimens, it is useful to carefully clean them of
mucus, debris, and mud prior to fixation. Otherwise, themucus and mud can harden or become sticky in preservative,
making subsequent cleaning more difficult and increasing
the likelihood of damage to appendages, spines, and setae.
Specimens of species with firmer and better chitinised
exoskeletons can be directly preserved in 80% ethanol,leaving appendages sufficiently flexible for taxonomic
examination, yet suitable for DNA extraction. Softer-bodied
species, however, require more careful treatment. If collected
alive, specimens can be euthanised slowly by refrigeration
or freezing in water (preferably seawater), or rapidly (almost
instantly) by immersion in ice water. Photography whilst
fresh is also recommended to record not only colour in life,
but also the natural form in case of carapace collapse during
fixation. The use of high-concentration ethanol as a first step
is not recommended because it can desiccate the specimens
too rapidly, potentially collapsing the carapace and making
the appendages stiff and brittle. Specimens will best retain
their true shape and form if they are: 1) initially fixed in a
5–10% formalin-seawater solution with subsequent transfer
to 80% ethanol, or; 2) preserved through an increasingly
concentrated ethanol series (60% through 80%) over at
least several hours (starting with as low as 60% diluted
with seawater) to prevent rapid osmotic changes that lead to
deformation or collapse, with final storage in 80% ethanol
diluted with fresh water. Regardless of the method used, thelegs, especially P5, tend to fold back on themselves and canbecome severely twisted as the preservative or fixative acts on
the specimen. After about 5–10 minutes in the preservative,
the legs of the specimen should be repositioned and spreadout, and left for another 15–30 minutes to become “fixed”
in place prior to the next preservation steps. If material

The present paper examines the taxonomy of the available
specimens of Arcotheres, mainly from Singapore andMalaysia (except for A. purpureus (Alcock, 1900), which
was covered in detail by Ahyong & Ng, 2021). The extensive
material on hand for many of the species, including types andmales, allows us to substantially reappraise the charactersused thus far to differentiate species, clarifying the taxonomy
of many species and resulting in many synonymies. We also
show that two of the species formerly assigned to Arcotheres
are referable to new genera.
MATERIAL AND METHODSThe terminology used follows Manning (1993b) and Davieet al. (2015) with the following abbreviations: cl = maximum
carapace length; cw = maximum carapace width; MXP3 =third maxilliped; P2–P5 = pereopods 2–5 (first to fourth
ambulatory legs), respectively; Gl = male first gonopod;
and G2 = male second gonopod. Measurements provided,in millimetres (mm), are of the maximum carapace widthand length, respectively. When the specimen is infected by135

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia
is required for specialised molecular work, then whole
individuals or a tissue sub-sample should be prepared fromthe outset for that purpose.

nor Tesch (1918) considered pereopodal asymmetry intheir descriptions. Bürger’s (1895) figures only show theambulatory legs for the right side, while Tesch’s (1918)
figures do not mention or depict asymmetry at all. The
carapace width-to-length ratio was used as diagnostic, but
De Man (1929) challenged this given that the soft carapace
is easily deformed. Gordon (1936: 163), Griffin & Campbell(1969: 160), and others also criticised the accuracy andusefulness of Tesch’s key to Pinnotheres. Gordon (1936:
164, fig. 2a) even had doubts about the reliability of the
proportions of the MXP3 propodus and dactylus as she notedthat this character varies in some species.

The taxonomy of the bivalves follows Huber (2010).
STATE OF PLAY
The foundations for pinnotherid taxonomy were primarilylaid by Otto Bürger (1895) in his seminal paper on Southeast
Asian pea crabs, which not only described many species
but also introduced several important characters such as
the segment proportions of MXP3. Although Bürger (1895)
used the proportions of the P2–P5 to distinguish species,
he did not realise that the ambulatory legs are naturally
asymmetrical in some taxa, some markedly so. De Man (1921,
1929) discussed the matter and was the first to suggest thatasymmetrical legs may not be an anomalous condition but
a natural occurrence. In another major contribution, Gordon(1936) established the taxonomic reliability of asymmetry as
expressed in species now assigned to Arcotheres, and Sakai(1939) reported on pereopodal asymmetry in A. boninensis(Stimpson, 1858). In the 1990s, Raymond Manning restarted
work on their taxonomy and in two key papers (Manning,
1993a, b), he emphasised that the above characters allowed
for more genera and species to be recognised. Over the last
three decades, there has been a relative explosion of studies
on pinnotherids, with numerous taxa described and systematic
positions clarified (e.g., see Campos, 1989, 2009, 2013, 2016,2017, 2018; Stevens, 1990; Pohle & Marques, 1998; Campos& Manning, 2001; Ng & Manning, 2003; Ahyong & Ng,
2005, 2007a, b, 2009, 2020, 2021; Komatsu & Ohtsuka,
2009; Komatsu & Takeda, 2009; Becker & Türkay, 2010;
Campos & Hernández-Ávila, 2010; Ng & Ngo, 2010; Jiang& Liu, 2011; Ahyong et al., 2012; Ng & Kumar, 2015;
Salgado-Barragán, 2015; Ng & Ho, 2016a, b; Ahyong, 2018,2020a, b; Trivedi et al., 2018a, c, 2019, 2020; Campos &Hernández-Aguilera, 2019; Cuesta et al., 2019; Ng et al.,2019a, b; Felder & Palacios Theil, 2020a, b; Komai et al.,2020; Salgado-Barragán & Barragán-Zepeda, 2021; SalgadoBarragán et al., 2021). Molecular phylogenetic studies havebeen particularly useful in elucidating interrelationships andthe higher classification of pinnotherids (e.g., Palacios-Theilet al., 2009; Ocampo et al., 2013; Palacios Theil et al., 2016;
Tsang et al., 2018; Palacios Theil & Felder, 2019, 2020a, b).

Manning (1993b) provided a synoptic key to the known
pinnotherid genera (with an emphasis on West African taxa),
but did not distinguish between several genera due to a lackof characters. To date, no reliable generic key is available,
and certainly no species key to genera such as Pinnotheres,
Arcotheres, and Nepinnotheres has been attempted. For theselarge genera, many named taxa have yet to be redescribed
and the types re-examined. In many cases, especially in thelarge genera, species are still being reassigned following
redescription of type material and correction of errors (e.g.,
Ng et al., 2017; Trivedi et al., 2020; Ahyong & Ng, 2021;
present paper).
From an ecological perspective, the latest review of symbiosis
in the Brachyura (Castro, 2015: 558) noted that mostpinnotherids can be treated as commensal symbionts, butobserved ambiguity over whether some should be treatedas parasites. Johnson (1963), for example, treats them as“semi-parasites”, whereas some carcinologists regard themas true parasites (e.g., Silas & Alagarswami, 1967; Ng &Manning, 2003). While it is true that the ecology of many
pinnotherid species is poorly studied and their effect on thehost is not comprehensively known, a good body of data
(mainly on European and American species) suggests thatthe relationship is usually detrimental to some degree forthe host, whether they are molluscs or echinoderms (e.g.,
Seed, 1969; Kruczynski, 1972; Anderson, 1975; Bierbaum& Shumway, 1988; Tablado & López Gappa, 1995; Hamel
et al., 1999, 2019; Cruz-Kaled et al., 2004; Narvarte & Saiz,
2004; Saeedi & Ardalan, 2010; Mena et al., 2014). For thetime being, we regard them as symbionts.
AVAILABILITY OF SPECIMENS

Not surprisingly, given the poor state of knowledge ofmany pinnotherids, there is no good comprehensive key
to the Pinnotheridae. Tesch (1918) was the last to attempt
this, preparing a key to the known genera and 58 speciesof Pinnotheres that he regarded as valid at the time. Tesch(1918), however, did not actually examine types or materialof many species, relying only on published accounts in mostcases. During his time, the natural asymmetry in ambulatory
legs in many species had also not yet been recognised.
De Man (1921: 261, 262, 264) observed asymmetry but
suggested that it was an abnormality due to parasitism bybopyrid isopods, an idea he later discarded (De Man, 1929:14) when he had more specimens. Neither Bürger (1895)

Finding pinnotherids in bivalves is very much a hit-and-missaffair, with most bivalves not containing crabs. However,there have been a few cases where the relative incidenceof pinnotherids was higher. For example, the presence ofArcotheres placunicola Ng, 2018a, in the window-pane
oyster Placuna ephippium (Philippson, 1788) can be up to
70% in parts of Singapore, but in other parts of the world,
there are almost no records of these crabs. As such, theinfestation of pinnotherids in bivalves can be quite localised,possibly affected by season, water currents, habitat, hostcondition, etc. This is an important consideration becauseeven A. placunicola did not seem to be so common in the
136

RAFFLES BULLETIN OF ZOOLOGY 2022
1950s and 1960s, when various molluscs were collected formalacological work and symbiosis studies, at least on thebasis of published accounts and ZRC specimens. Yet overthe last decade, A. placunicola has become very common in
Singapore, with an infection rate of about 30–50% for the
Placuna collected (unpublished data). Conversely, species
such as A. exiguus (Bürger, 1895) are rarely seen, evenwhen large numbers of their hosts, the venerid clams Marcia
recens and Paphia undulata, are sampled. In some instances,a kilogram of these molluscs with some 50 clams can yield
up to 10 crabs, but in most cases, there are no crabs at all.
The same has been observed for Plenotheres coarctatus(Bürger, 1895) in the glauconomid clam Glauconome rugosa
Hanley, 1843. This probably has much to do with where theclams were collected.

compared when fresh or very recently preserved, although
this is often not possible with old type material. While many
pinnotherid workers are aware of these problems, in lieu ofdetermination of additional reliable alternatives, the suite of
useful taxonomic characters remains limited.
The present study relies on a large series of specimens ofseveral well-known species, obtained sometimes from many
localities. In many cases, live specimens were studied andcomparisons based on fresh material.
Female-to-male ratios. When there is a good series of
specimens, the female-to-male ratio of many of the species
treated here is noteworthy and has taxonomic significance.
Although sampling bias cannot be excluded, the large series
available to us suggests a real pattern is present. In species
like A. palaensis, males are very rare—over 90 female
specimens were examined, but only two males are known.
The same is true for males of Plenotheres coarctatus (1:21),
although proportionally more males are known. Males are
more common in other species: A. placunicola (1:2), A.
similis (1:5), A. exiguus (1:5), and A. rayi (1:8). Interestingly,
in Magnotheres globosus, the male:female ratio across all
lots is 1:11, but among Australian specimens, only 1:5.
This difference is probably a sampling artefact given that
Australian M. globosus is represented by only 12 specimensfrom four lots. Why the sex ratio in the Pinnotheridae isso skewed is not currently known, but males are usuallyless common than females. For this reason, unlike in mostbrachyurans, pinnotherid taxonomy is based mainly on
female characters.

Almost universally, pinnotherid males are seldom found, but
the incidence of males is dependent on the species. In the caseof A. placunicola, a large number of males were collected
and heterosexual pairs frequently obtained from one host.
For species such as A. similis and A. exiguus, males are also
not rare, and while not collected together, a good numberof males are also collected when the females of the species
are common. Males of A. palaensis, Pl. coarctatus, and M.
globosus, on the other hand, are extremely rare, with only
a few specimens known. We are of the opinion that males
in Arcotheres are permanent entities and not a transitional
ontogenic stage. There have been suggestions that at least
some pinnotherids may be protandrous, starting as males andbecoming female later in life. While this may be the case forsome pinnotherids (see Ahyong & Ng, 2008), in Arcotheres,
we do find small females that resemble males in habitus
but do not have the associated gonopods (see Fig. 15A–C).

Male or female? In many pinnotherid species, and especiallyfor the Indo-West Pacific taxa, subadult females closely
resemble males superficially, with a more rounded carapace,well-developed front, eyes visible dorsally, and a narrowtriangular pleon; so much so that they can be indistinguishable
unless the pleopods are examined. We are of the opinion
that some of the old records of “males” in the literature are
actually juvenile females. At this early stage, the gonopods
are usually undeveloped but the number and setation/formof pleopods are still visible. Several small specimens of A.
palaensis and P. coarctatus (e.g., Fig. 15A) were originallyidentified as males, only to be confirmed to be subadultfemales on re-examination. As discussed above, males of A.
palaensis are very rare and, as such, records such as thoseof De Man (1929: 14), which recorded two males among
11 females, require confirmation.

CHARACTERS
Pinnotherid crabs are in most cases morphologicallyconservative, so the paucity of taxonomic characters has
always been one of the challenges of pinnotherid taxonomy.
Historically, carapace shape, MXP3 features, and proportionsof the ambulatory legs (P2–P5) have been the most importantcharacters used because the majority of species are known
only from females. Consequently, many important taxonomic
characters used for other brachyuran groups, such as the malepleon and gonopods, are unavailable for most pinnotherid
species and have not yet played a significant role in shaping
the pinnotherid system. In addition, as most pinnotherids,
particularly adult females, have poorly chitinised and weakly
calcified carapaces and appendages, they are more susceptible
to the vagaries of preservation than other, heavily chitinised
crabs. The degree of variability in many diagnostic charactersis often poorly understood, especially for pinnotherid species
represented by only a few specimens. Many taxonomic studies
use proportions of the MXP3 and P2–P5 to separate species,
recognising new taxa for slight differences without knowledgeof how much they may vary in life. As such, many of thecharacters that have been historically used as diagnostic need
to be tested against good series of specimens to evaluate
their taxonomic reliability. It helps when specimens are

Christensen & McDermott (1958, as Pinnotheres)characterised postlarval ontogenetic growth stages for Zaops
ostreum (Say, 1817), and Watanabe (2013, as A. alcocki)
reported an additional stage in male Arcotheres palaensis.
Although these stages might prove to have wider validity,
especially in males, the life histories of too few species of
pinnotherids, let alone males, are currently known. Althoughsome studies have examined staging (e.g., Manning, 1993b;
Komai et al., 2020), we do not attempt to correlate our
examined specimens with the stages defined by Christensen& McDermott (1958) and Watanabe (2013). That being said,137

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia
we certainly acknowledge that detailed life history studies
of more pinnotherids will be extremely useful in better
understanding their biology.

(Figs. 13, 14). For these reasons, several species need to besynonymised under A. palaensis as the only differentiating
character was the female carapace shape. In A. palaensis, theposterior margin is usually concave in fresh specimens (Fig.
3B, D–F), but is sometimes almost straight (Fig. 3A, H) togently convex (Fig. 3C). One caution here is that in manyof these specimens, the posterior margin becomes distinctly
more concave after preservation. There are patterns in somespecies. In female A. exiguus, for example, the posterior
carapace margin is usually straight to gently sinuous (e.g.,
Figs. 37A, C, 38A, B, 39D), but can sometimes be gently
concave (Fig. 39A). In some poorly preserved specimens,
however, the carapace can be badly deformed, making theposterior carapace margin artificially concave.

Carapace. The shape of the female carapace has been used asa major character in pinnotherid taxonomy, with the generalshape as well as structures of the anterolateral margins andfront often used as diagnostic features. Whether the dorsal
surface is inflated or flat has also been used as a species
character. While the carapace shape is a useful character ifwell preserved, the effects of suboptimal to poor preservationmust also be considered in identifying specimens. Because
the female carapace in most species is often poorly calcified
and weakly chitinised, it can be soft and pliable, even inlife. As such, depending on osmotic balance during thepreservation process, effects of highly concentrated alcohol
can rapidly dehydrate the specimen and cause deformation,
causing the cephalothorax to change shape shortly after
preservation. These effects are more serious if the specimen
is preserved after a recent moult, when the cuticle is evensofter. Likewise, poor initial preservation and/or long-term
storage in unbuffered formalin can cause the cuticle to become
frail and delicate. Also, if external osmotic conditions are
too different from that of the specimen, significant deformity
can result, either by swelling or shrivelling. In many female
specimens with thin and soft cuticle, the dorsal surface of thecarapace can collapse to differing degrees, with the lateraland posterior margins often folding in on themselves. In
such cases, the female pleon sometimes folds inwards orappears shrivelled.

In species such as A. exiguus and especially A. obesus,
the female carapace and even the pereopods are especiallypoorly chitinised, and after a few months in preservative,
they become very fragile, sometimes almost membranous.
This makes their study difficult.
The female carapace of Magnotheres globosus is interesting
given the relative thinness of the cuticle. In live specimens,
the carapace texture is crisp but firm. Within minutes ofbeing placed in 70% alcohol, the carapace surface often
starts to gently collapse, with folds or wrinkles forming
and the firmness disappearing rapidly. If first preserved instrong alcohol (85% or more), the carapace often becomesbrittle and fractures easily.
The carapace of males is a different matter. The cuticle
is invariably thicker and more strongly chitinised, and theshape and proportions do not deform or change even after
long preservation. As such, when available, male carapace
shapes can be used to distinguish species. A good case inpoint pertains to two morphologically similar species, A.
similis and A. placunicola. Adult male A. similis have around or slightly longitudinally ovate carapace (Figs. 31A, D,
32A) and the dorsal surface is inflated, appearing distinctly
convex (Fig. 31B). The carapace of A. placunicola, on theother hand, is always wider than long, appearing transversely
ovate (Figs. 60A, D, 62A), and the dorsal surface is only
gently convex, appearing almost flat in smaller specimens(Fig. 60B, E). These differences are consistent even forsmall specimens.

Beyond preservational effects, there can a substantial degreeof natural carapace variation in some species. The excellent
series of A. palaensis shows that female carapace shape alone
is not a reliable feature in this species. The female carapace
is typically transversely subovate with the front very low,barely protruding anteriorly, and the anterolateral margins
evenly convex (Figs. 5A, 8A, 11A, B, D, F, 12F, 13A, B,
C). The front, however, may be relatively more prominent
in some specimens, with the anterolateral margins gently
sloping posteriorly or evenly convex (Figs. 9A, 11E, G, H,
12B, C, E, G, H, 13E–H, 14A–G). Many female specimens
also have a carapace that appears more subquadrate, withthe anterolateral margins even appearing sinuous and theposterolateral margins more expanded laterally (Figs. 12G, H,
13D–H). One extreme example is seen in A. rayi. The typefemale specimen has a front that is in line with the truncate
anterolateral margins, so much so that the anterior margin of
the carapace is straight with the position orbits discernible
as two clefts (Fig. 46A). In a series of fresh specimens,
some have the frontal margin protruding anteriorly with theanterolateral margins gently convex (Figs. 47A, C, 48B, C,
49A), while others have the frontal margins at most weakly
produced (Fig. 48B). Similarly, the frontal margins of female
A. palaensis can vary from almost straight to gently convex or
sinuous (Figs. 5A, 8A, 13, 14). Even from one lot (e.g., ZRC
2021.821), the front may be sunken in (Fig. 3G) or anteriorly
produced (Fig. 3E, F). The posterior carapace margin also
varies markedly, from gently sinuous to deeply concave

Many of these preservational challenges relating to thecarapace shape are most apropos to the Indo-West Pacificand eastern Atlantic pinnotherids, where the fauna isdominated by pinnotherine species in which mature females
have a delicate, weakly chitinised carapace and pleon. As
a result, carapace and pleon shape must be used with care.
Conversely, the American pinnotherid fauna includes a muchlarger proportion of pinnixines, which are typically much
more stongly chitinised and for which carapace and pleon
shape are reliably preserved by traditional methods. Certainly,
soft-bodied pinnotherines also occur in the eastern Pacificand western Atlantic, but to a much lesser extent than in
the Indo-West Pacific and eastern Atlantic.

138

RAFFLES BULLETIN OF ZOOLOGY 2022
Epistome. The structure of the posterior margin of theepistome is often useful to separate species, but is usuallysimilar in close allies. It is useful for same-sex comparisons
only, as males generally have a narrower carapace thanfemales, which in turn affects epistome proportions. For
example, in A. similis and A. exiguus, the posterior margin isrelatively wider with the margins gently convex (Figs. 28D,
30K, 38C, 41C), but in A. palaensis, it is proportionately
narrower with the margins only gently concave (Figs. 4,
8C, 9C, 17O, 18M).

of Arcotheres boninensis. The proportions and symmetryof P2–P5 have become important features of pinnotherid
taxonomy, especially in the Indo-West Pacific. A key
character for Viridotheres Manning, 1996, for example, isthat the female P3 (especially the merus and dactylus) isthe longest of all the ambulatory legs (Manning, 1996). A
major diagnostic feature of Arcotheres is that the female
P4 is asymmetrical, with the longer leg being the longestof the ambulatory legs.
Compared to the carapace, the pereopods are relatively morerobustly chitinised and as such are less vulnerable to poor
preservation, although if preserved in acidic formalin and/or
previously dried, they also become soft and the proportions
and shape (especially the merus) may be affected. As theleg segment proportions are taxonomically effective, poor
preservation may result in incorrect ratios and shapes. In
cases where the specimens were previously desiccated andrehydrated, the appendages appear translucent and slightly
distorted; for these cases, the proportions and exact featuresneed to be interpreted more carefully.

Antennal article 1. A possibly phylogenetically informative
feature highlighted by Campos (2009: 42) is the protuberance
on the mesioventral margin of the epistome that he termed
the basal antennal article. Ng et al. (2019b: 349), in theirrevision of Orthotheres Sakai, 1969, discussed this characterat length, noting that the “basal antennal article” was antennal
article 1, which carries the opening of the green gland, thenephridiopore. While antennal article 1 is often immovably
fused to the epistome, the presence of the nephridiopore isalways indicative. Ng et al. (2019b: 349) observed: “The fusedarticles 2 and 3 are usually demarcated by shallow grooves
and article 1 is also immovably fused but positioned very low
on the epistome such that its lower margin intrudes into, andforms part of, the margin of the buccal cavity. This lower
margin of article 1 [as indicated by the nephridiopore] varies
from indistinct to bluntly angular, appearing as a short lobe
or protuberance, being most pronounced mesially. Amongthe species examined, the condition of this protuberanceshows no obvious taxonomic pattern within or between
genera, although it has a wider significance as a pinnotherid
synapomorphy based on our consideration of all Brachyura. In
Orthotheres s. str., the mesioventral angle of antennal article
1 is slightly projected and tubercle-like, with a small pore at
the tip, almost certainly the nephridiopore. This mesioventral
position of the presumed nephridiopore, readily observed inlarger specimens of Nepinnotheres and Arcotheres, may wellbe the normal position in most, if not all, pinnotherines (andpinnotherids).” Although we have found the nephridiopore
to be always present on the buccal margin in pinnotherids,
making it useful at high taxonomic levels, we have not foundthis feature to be useful in separating species or genera.

The extent of asymmetry is not well known for males,
notably in Arcotheres, mainly because they are generally
rare. Previously, even when males were described, they
were normally assumed to have symmetrical ambulatory
legs. Neither De Man (1921) nor Gordon (1936) discussed
male P2–P5 asymmetry; most papers do not discuss thematter. Ng (2018a: 478) noted that there was some degree ofasymmetry in walking legs of male A. placunicola, and that
the P4 dactylus on one side is slightly longer than the other.
In the present material, there is slight asymmetry in the maleP5 dactylus in A. placunicola (Fig. 62H versus Fig. 62L), andslight asymmetry in the P4 dactylus of A. similis (Fig. 32F
versus Fig. 32J); while in A. palaensis, the left P4 dactylus
is prominently longer than the right one (Fig. 22D versus
Fig. 22H). No asymmetry was observed in the pereopods
of male A. exiguus, A. rayi, or Pl. coarctatus, even in theP4 (Figs. 44A, F–I, 45C–F, 52A, F–I, 68A–H), although we
note that the former two species are also somewhat unusual
within the genus in other features (G1 and P5) as currently
composed. This means that asymmetry is present in at least
some male Arcotheres, albeit less distinctly than in females.
The present observation presents major challenges for thetaxonomy of some genera. While females with distinctly
asymmetrical ambulatory legs are easy to assign to genera
like Arcotheres, those with symmetrical legs are usuallyreferred to Pinnotheres or Nepinnotheres; what do we dowhen only males are known for some described species?
Which genus would they belong to if the differences in maleambulatory asymmetry is only slight (e.g., see Ahyong &Ng, 2021; discussion for Magnotheres, new genus)?

Chela. The structure of the chela is relatively consistent
within species for most taxa. Species such as Arcotheres
palaensis and Magnotheres globosus have relatively shortand stout adult female chelae (see Figs. 6F, 7B, 8D, 9D,
79D, 80D) with little variation. In A. exiguus, however, theproportions of the adult female chelae vary more than in
the aforementioned species, from relatively short (e.g., Fig.
45B) to longer (e.g., Figs. 36F, 37E, 38D, 41E).
Pereopods (P2–P5). De Man (1921: 14) was the first to
suggest that the asymmetry of the female ambulatory legsmight prove to be a constant character in at least somespecies of Pinnotheres (sensu lato). Gordon (1936: 164)
discussed this matter at length, noting that it was probably
common in the genus but not evident in small specimens.
Sakai (1939: 588, text-fig. 74) documented asymmetry of
the ambulatory legs, particularly of P4, as a normal feature

Why female ambulatory legs (notably the P4) are asymmetricalis not known and no studies have been done. It is interesting
nevertheless to note that most Arcotheres live in bivalve
species that have distinctly asymmetrical shells, and thelonger P4 on one side may be associated with helping the crabreach the more elongate part of the bivalve. It is certainly notalways easy for the whole crab to move around within the139

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia
bivalve due to its swollen carapace and size. Also noteworthy
is that in Plenotheres coarctatus, which is typically foundin bivalves of the family Geloinidae and Glauconomidaewith more symmetrical shells, the ambulatory legs showless pronounced asymmetry; and in Magnotheres globosus,
which occurs in the symmetrical Pinnidae, the ambulatory
legs show almost no asymmetry.

the propodus and dactylus is densely lined with long setaewith other surfaces largely glabrous (Figs. 41I, M, 42F, I,
49I, M, 66D, H), whereas in most other species, the setae
are proportionately shorter, less dense, and not largely
restricted to the flexor margins (e.g., in A. palaensis; Fig.
17D, J, N). The flexor margin of A. palaensis is densely
covered with long and short stiff setae, which gives it adistinct bottle-brush appearance (Fig. 7C). In similar species
like A. placunicola and A. similis, the setae are less dense
and appear less bristly.

An important character first highlighted by Gordon (1936)
is the presence, in some species, of small spinules on thesubdistal flexor margin of the male and female P5 dactylus.
These distoflexor spinules are distinct from the dense, short,
soft setae that may be found along the margins, sometimes
reaching the tip, being stiff and immobile. These spinules
are often small, semi-transparent, and not easy to observe,
so some degree of manipulation and adjusting of the lightsource is needed under high magnification. They may also
be abraded or broken off, so care must be taken to ascertain
this character. The row of spinules, when clearly present,
are graded with the distal ones longest, becoming smaller
proximally. The proximal spinules are often very small
and low, appearing granuliform and not easy to see. One
or two rows of spinules may be present, and when there
are two, the upper row is usually the more pronouncedwith the spinules better developed and more numerous,
but this can vary sometimes even in the same specimen(e.g., see Magnotheres globosus; Fig. 83D’, D”, H’, H”).
The length of the row varies between species, with one of
the rows relatively long in A. similis (Fig. 30D’, H’), but
shorter in A. placunicola (Fig. 59G’, K’, K”). This character
was used by Gordon (1936) to describe a new species, A.
spinidactylus (a synonym of A. similis; Fig. 30L) (although
she only recognised one row of spinules and did not noticethe second, shorter row present in her specimens), and hassince also been reported and used by other authors (e.g.,
see Griffin & Campbell, 1969; Dai et al., 1980; Takeda& Konishi, 1988; Ng, 2018a; Trivedi et al., 2018c, 2019;
Ahyong & Ng, 2020, 2021). The presence of these spinules isuseful to separate closely related species like A. placunicola
and A. palaensis, being distinct in the former and absent in
the latter. Two distinct rows of P5 distoflexor spinules arepresent in other Indo-West Pacific taxa like A. boninensis(Stimpson, 1858), A. pernicola (Bürger, 1895), and A.
purpureus (Alcock, 1900) (see Trivedi et al., 2019; Ahyong& Ng, 2021). Some taxa like Plenotheres coarctatus have
P5 distoflexor spinules that are relatively smaller, even forthe distalmost ones (Fig. 66J, K), but they can be relatively
large in A. similis (Fig. 30D’, H’, L). Arcotheres exiguus is
not considered to possess P5 distoflexor spinules, although
in a few specimens, one can discern a faint row of very low“prickles” or pointed granules, all of which are of similar
size (Fig. 42M); in most specimens these are absent or only
a few scattered, low “prickles” are visible. The same is true
for A. palaensis; there are sometimes some scattered, low
P5 distoflexor “prickles” or acute granules visible, but they
are not arranged in rows (Fig. 18F’).

Another series of characters that may hold taxonomic
potential is associated with the basis-ischium and merus of
P5 in males. The articulation between the P5 ischium andmerus is diagonal (oblique to the segment axis) in species
like A. exiguus, A. rayi, and Pl. coarctatus, but is more
perpendicular in other Arcotheres and M. globosus treated
here. The P5 basis-ischium is distinctly elongate in species
such as A. ocularius (Fig. 24G, K) and M. globosus (Fig.
83D, H), and slightly so in A. similis (Fig. 30D, H). In mostof the other taxa treated in this paper, the P5 basis-ischium
is more quadrate (e.g., Fig. 17D, H, J, N).
MXP3. The structure and proportions of the palp (carpus,
propodus, and dactylus) are important in diagnosing generaand species. While the point of articulation of the dactylus
on the propodus (whether proximal, median, or subdistal)
appears to be consistent, the length of the dactylus is notalways constant. Gordon (1936: 164, fig. 2a) first indicatedissues with variation when she noted that in one specimenof A. similis (as A. spinidactylus), the dactylus on one side
was distinctly shorter than that of the other side (Fig. 33A,
B). Ng (2018b) discussed this further for the holothurian
symbiont Trichobezoares villosissimus (Doflein, 1904), forwhich the dactylus was very short in several specimens. Ng(2018b: 621) suggested that a shorter dactylus may be due
to damage and that the character must be used carefully,
especially if there are few specimens. It is also important toalways check the condition of the dactylus on both MXP3
to ensure the proportions of the left and right structures are
the same. The structure of the fused ischiomerus is useful
in some cases, but seems to be more or less consistent inmembers of one genus. It has been used to support theestablishment of Trichobezoares Ng, 2018b, for Pinnotheres
villosissimus Doflein, 1904 (Ng, 2018b); and a new genus,
Plenotheres, for Pinnotheres coarctatus in the present study.
It is useful to note that some figures of the MXP3 dactylus
and propodus are not comparable between publications
because they may not have been drawn in the same plane;
the entire structure should be on a level surface, especiallythe articles of the palp. Because the palp is articulated, it isimportant to ensure that the carpus, propodus, and dactylus
are horizontal (as best as possible) before any figures are
made. If the palp is tilted anteriorly with the dactylus andpropodus not parallel, and more so if it is figured from theinner surface, the dactylus will appear to be longer than it
is and reach beyond the tip of the propodus, even when it
does not (e.g., see George & Noble, 1970: fig. 1). In one
specimen of Magnotheres globosus, for example, the dactylus

The degree and type of setation on the margins of P4 andespecially P5 also show some patterns. In species such as
A. exiguus, A. rayi, and Pl. coarctatus, the flexor margin of140

RAFFLES BULLETIN OF ZOOLOGY 2022
does not reach the tip of the propodus when drawn withthe structure level (Fig. 84F), but when drawn at a slightangle, the dactylus appears to overreach the propodus apex
(Fig. 84G). The same is true for the propodus, which may
appear shorter or longer depending on the orientation of thestructure when figured (e.g., Fig. 83J, K).

the pleon must always be checked for gonopods (see above
discussion). The pleon of early juvenile females can be nearindistinguishable from that of size-matched males (Fig. 15C).
Ng (2018a: 478) and Komai et al. (2020) noted that the shapeof the male somite 6 and telson change allometrically and,as such, when only singletons are available, the charactershould be used with caution, considering the maturity of thespecimen. Slight variation occurs even within species; inmale A. palaensis, the telson varies from near semicircular
to almost pentagonal (Fig. 22J, O). In A. placunicola, small
males have a triangular telson with convex margins (Fig.
62B), which become more subquadrate in shape in larger
specimens (Fig. 61B, C). Some differences, however, aresignificant; for example, the broadly trapezoidal male telson
in Magnotheres is diagnostic (Figs. 85C, 86J).

That being said, the shape of the MXP3 propodus is somewhat
variable, with the apex appearing rounded to truncate insome species. Whether the dactylus is inserted submedially,slightly proximal to, or slightly distal to the midlength ofthe ventral margin of the propodus can vary. Although it
is constant in most species, in at least Arcotheres exiguus
and Magnotheres globosus, it can be inserted slightly distal
to the midlength in the former, or slightly promixal in thelatter. The relative length of the dactylus can also vary;
in A. exiguus in particular, it is especially variable, fromshort to long (e.g., Figs. 36E, 41B, Q, 42A, L, 44B, 45A).
Whether this variation reflects normal polymorphism or isdue to damage and regrowth (as in T. villosissimus; see Ng,
2018b) cannot be determined at present.

Female pleon. The adult female pleon is almost never usedin Indo-West Pacific pinnotherid taxonomy, mainly because
there appears to be too much variation associated with growthand maturity. The pleon can change in shape substantiallyduring growth and bulge due to the large egg mass whenovigerous. For example, in A. similis, smaller females have aregularly broad triangular telson (Fig. 27B), which becomeswider and more subtruncate in large individuals (Fig. 28B);
in A. exiguus, the telson is broadly and unevenly fusiform
in smaller specimens (Fig. 37B), becoming transversely
ovate in large specimens (Fig. 39B). In A. rayi, on the other
hand, the adult female pleon shape changes little, even inlarge specimens (Figs. 46B, 47B). The shape of the adulttelson is generally consistent in shape, but not always. In
A. palaensis, for example, the adult female telson is usuallyquite wide and triangular with the lateral parts tapering,appearing fusiform (Figs. 5B, 8B, 9B). In some specimens,
however, the telson is distinctly narrower (Fig. 15D) or
appears almost flattened (Fig. 15E). In addition, in somespecimens, the telson is asymmetrical (e.g., see Fig. 15D),
differing on either side in shape and proportions. We areconfident that these represent intraspecific variations as mostother specimens from the same lots have the more typicalfemale telson condition. Some of the observed differences
may also be due to preservational effects on the soft cuticle
or poor condition prior to preservation, resulting in obvious
deformations. We fully concede, however, that the shapeof the adult female telson may prove to be taxonomicallyuseful for some taxa.

Eyes. When present, the cornea of species of Arcotheresis always prominent and well pigmented, being especially
distinct in fresh material (e.g., Figs. 2C, F, 4, 63C, D, 74C,
D). The eyes, when present (absent in A. latifrons (Bürger,1895)), although short and stout, fill the small orbit and arenever fused to the carapace in any species we have examined.
Whether the eyes are visible in dorsal view is a surprisingly
useful character in adult females of some species. In A.
palaensis, all female specimens unambiguously identified
with this species never have the cornea substantially visible
in dorsal view; in most cases, the eyes are not visible (Figs.
4A–D, 5A, 8A, 9A, 13, 14). Even when the eye can beextended anteriorly, only the edge of the cornea, at most,is visible in dorsal view. In contrast, the eyes in a few
female A. palaensis are distinctly visible in dorsal view(e.g., Figs. 6A–D, 19A, M). In old and poorly preserved
specimens, because of the poorly chitinised carapace that is
easily shrivelled by preservation or drying, the true condition
can be difficult to ascertain. The authors, however, haveexamined numerous live specimens of female A. palaensis
and freshly preserved specimens, and find the visibility ofthe eye in dorsal view a very reliable character state. This isregardless of whether the frontal margin is more prominentand protruding anteriorly, or the carapace more inflated. The
character has been shown to be reliable for A. ocularius fromFiji and Lombok, for which a large series of specimens isavailable with eyes that are always clearly visible in dorsal
view (Fig. 23; see Komai et al., 2020).

Gonopods. Gonopod structures are not often used inpinnotherid taxonomy, especially so in Arcotheres, wheremales are generally rarer. The general pattern for many
pinnotherid species, even across genera, is that the G1 isa gently curved structure, gradually tapering to a slenderdistal part which often has the tip gently deflected relative
to the main plane of the gonopod. That being said, G1
structures can sometimes be very diagnostic at the genus
and species levels. The case of A. exiguus is noteworthy;
the G1 structure (Figs. 44K, L, 45H, I) is consistent acrossits very wide range, from the western Indian Ocean to
the West Pacific. Plenotheres, for example, has the distal
part ornamented with a prominent dorsal lobe that is very
distinctive (Fig. 68N, P). Magnotheres, on the other hand,

Male pleon. The form of the male pleon has seldom beenused in Indo-West Pacific pinnotherid taxonomy, mainly
because of the rarity of males. As such, the degree ofvariation in this structure is not well known. As with other
Brachyura, however, the available data suggests that pleonalmorphology varies predictably in shape depending on the size
and maturity of the specimen, especially in the appearanceof somite 6 and the telson. Added to this, juvenile females
can so closely resemble males in external appearance that141

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia
villosulus (Guérin-Méneville, 1832) from many speciesof venerid clams (Meretrix casta (Gmelin, 1791), Callista
umbonella (Lamarck, 1818), Protapes cor (Sowerby, 1853),
Marcia recens (Holten, 1802), Gastrana multangula (Gmelin,1791), Psammotreta angulata (Linnaeus, 1767)) and arcid
cockles (Anadara antiquata (Linnaeus, 1758)) from Pakistan.
Jahangir et al.’s (2015: fig. 2B–E) figures of N. villosulus,
however, depict two or possibly three different species fromat least two genera. Moreover, from the variety of hosts
alone, it can be inferred that the authors probably had atleast two or three species of pinnotherids.

has the G1 very slender and long with the tip tapering (Fig.
86K). At the species level, the G1s of A. exiguus and A. rayi
are diagnostic with a distinct subterminal projection (Figs.
44K, L, 45H, I, 52N, O). The tip of the G1 is more slenderand gently upcurved in some species (e.g., in A. similis); thischaracter varies in relative length (Figs. 32L–N, 33E–H, J,K) and can be affected by age (see Ng, 2018a: 478) as well
as how well the specimen is preserved. In some specimens,
the tip appears slightly twisted or shorter because the cuticle
has slightly distorted.
The structure of the G2 endopod is relatively consistent,
always short with the distal part spatuliform to subspatuliform.
The presence of an exopod was only recently highlighted
by Ahyong et al. (2012), but our observations suggest thatit may be widely present in pinnotherids—it is present inall the species examined in this study. When present, theexopod is usually prominent, often as long as the endopod
itself, although in some extreme cases, it is several times thelength of the endopod, e.g., in Tacitotheres glaber (Bürger,1895) and several species of Viridotheres (see Ahyong etal., 2012: fig. 2I; Ahyong, 2019: fig. 1K; Ng et al., 2019b:
fig. 10L). The taxonomic value of this character will require
further study.

As such, host records, especially old records, must be treatedvery judiciously. In particular, historical changes in hosttaxonomy must be considered before accepting host records
at face value. For instance, there was some confusion withthe host identity of the poorly known species Arcotheres
pernicola (Bürger, 1895), supposedly from the mytilid Perna
Philipsson, 1788, and later shown to have come insteadfrom the ostreid oyster Magallana Salvi & Mariottini, 2016
(Trivedi et al., 2019). This is significant as A. pernicola isactually morphologically closer to oyster-dwelling species
such as A. boninensis (Stimpson, 1858) and A. purpureus(Alcock, 1900).

Host records. Some pinnotherids have been recorded
from a large number of hosts from different families (forreviews, see: Schmitt et al., 1973; Palacios Theil et al., 2016:supplementary material; De Gier & Becker, 2020). Manyof the old host records for “well known” or “widespread”
species should be considered doubtful and probably the resultof misidentification of the pinnotherid and/or the host, atleast until they can be substantiated. It is of course possiblethat some pinnotherid species are generalists and can livein many species of molluscs and other invertebrates, but
we are of the opinion that this is not as common as it has
been reported (e.g., Schmitt et al., 1973). Our studies showthat A. palaensis is only known from arcids, but according
to Schmitt et al. (1973: 65), it is also present in Placuna
Lightfoot, 1786 (Placunidae), and Mactra Linnaeus, 1767
(Mactridae), records which we dispute. Ng (2018a) argued
that Southeast Asian records from Placuna (originally from
Bürger, 1895: 373) are almost certainly incorrect and belong
to P. placunicola Ng, 2018a, instead. The record fromMactra was based on Chuang (1961: 188) and is likely to
be A. exiguus instead (see present study), especially since
the Mactridae is in the order Venerida. Many of the species
found in Southeast Asia have more narrow host preferences
than what the literature suggests. For example, Schmitt et al.
(1973: 87) also listed a large number of hosts for the East
Asian species, Arcotheres sinensis (Shen, 1932): Veneridae(Tapes Megerle von Mühlfeld, 1811, Paphia Röding, 1798,
Meretrix Lamarck, 1799), Mytylidae (Mytilus Linnaeus, 1758,
Volsella Scopoli, 1777), Pectenidae (Chlamys Röding, 1798),
and Ostreidae (Ostrea Linnaeus, 1758, Crassostrea Sacco,
1897). Shen (1932: 135) himself had reported his species froman oyster (Ostrea sp.) and a clam (Tapes variegatus Sowerby,
1852, currently, Venerupis aspera (Quoy & Gaimard, 1835)).
These seem doubtful and should all be rechecked. Morerecently, Jahangir et al. (2015) reported Nepinnotheres

Accurate host records are much more useful in assisting
identification of pinnotherid species than previously
recognised. In our study, we find that pinnotherids that
infest bivalves are usually host-specific, probably at leastat the family or superfamily level, although some bivalves
host more than one species of pea crab. In such cases, thepinnotherid species are usually very different in body form,
such as Xanthasia, Tridacnatheres, and Ostracotheres,
all of which infest tridacnid clams (Ahyong & Ng, 2005;
Ahyong, 2018). For example, as noted above, all reliable
records of A. palaensis are from arcoid clams, whilst A.
exiguus is known from various related venerids. The unusual
venerid, Gafrarium, hosts one species, A. obesus (Dana,
1852), with a wide range from Malaysia to Fiji (Komaiet al., 2020). Noteworthy is that Plenotheres coarctatus
occurs in two outwardly very different looking mangrove
bivalves in different families, Geloina coaxans (Gmelin,1791) (Cyrenidae) and Glauconome spp. (Glauconomidae
Gray, 1853), but the clams are phylogenetically related(both belong to the superfamily Cyrenoidea). Similarly,
Magnotheres globosus is known reliably only from fanshells of the family Pinnidae.
Host records are often confounded by the fact that pea crabs
are incidentally found by malacologists as part of their work,
and it has not always been possible to accurately associate
hosts with the crab symbiont. In large-scale expeditions,particularly when large amounts of material are collected,
some of the pinnotherids have no host records and/or therecords are doubtful because of errors made while processing
the volume of specimens collected. Also problematic witholder host records is that they often come from markets where
many bivalve species are sold together, and, unless the crabs
were directly removed from the shells by the collector, records
may be dubious. We have seen and collected pinnotherids
142

RAFFLES BULLETIN OF ZOOLOGY 2022
Table 1. List of species in Arcotheres and related genera.
Arcotheres Manning, 1993
Arcotheres alcocki (Rathbun, 1909) [Pinnotheres]= Pinnotheres parvulus De Man, 1887 (pre-occupied name)

Arcotheres atrinae (Sakai, 1939) (Pinnotheres)
Arcotheres boninensis (Stimpson, 1858) [Pinnotheres]
Arcotheres cyclinus (Shen, 1932) (Pinnotheres)
Arcotheres exiguus (Bürger, 1895) [Pinnotheres]= Pinnotheres winckworthi Gordon, 1936
= Pinnotheres vicajii Chhapgar, 1957
= Pinnotheres casta Antony & Kuttyamma, 1971
= Pinnotheres obscuridentata Dai & Song, 1986

Arcotheres lanensis (Rathbun, 1909) [Pinnotheres]
Arcotheres latifrons (Bürger, 1895) [Xenophthalmus]
Arcotheres nudifrons (Bürger, 1895) [Pinnotheres]
Arcotheres obesus (Dana, 1852) [Pinnothera (sic)]
Arcotheres ocularius Komai, Kawai & Ng, 2020
Arcotheres palaensis (Bürger, 1895) [Pinnotheres]= Pinnotheres arcophilus Bürger, 1895
= Pinnotheres rhombifer Bürger, 1895
= Pinnotheres latissimus Bürger, 1895
= Pinnotheres paralatissimus Dai & Song, 1986
= Arcotheres guinotae Campos, 2001

Arcotheres pernicola (Bürger, 1895) [Pinnotheres]
Arcotheres placunae (Hornell & Southwell, 1909) [Pinnoteres]
Arcotheres placunicola Ng, 2018
Arcotheres pollus Ahyong & Ng, 2020
Arcotheres purpureus (Alcock, 1900) [Pinnoteres (sic)]= Pinnotheres nigrans Rathbun, 1909

Arcotheres quadratus (Rathbun, 1909) [Pinnotheres]
Arcotheres rayi Ahyong & Ng, 2007
Arcotheres rotundatus (Bürger, 1895) [Pinnotheres]= Pinnotheres consors Bürger, 1895

Arcotheres shahi Trivedi, Campos & Vachhrajani, 2018
Arcotheres similis (Bürger, 1895) [Pinnotheres]= Pinnotheres modiolicola Bürger, 1895
= Pinnotheres kamensis Rathbun, 1909
= Pinnotheres spinidactylus Gordon, 1936

Arcotheres sinensis (Shen, 1932) [Pinnotheres]
Arcotheres tivelae (Gordon, 1936) [Pinnotheres]
Plenotheres, new genus
Plenotheres coarctatus (Bürger, 1895), new combination[Pinnotheres]
Magnotheres, new genus
Magnotheres globosus (Hombron & Jacquinot, 1846), newcombination [Pinnotheres]= Pinnotheres meleagrinae Hilgendorf, 1869 (nomen nudum)= Pinnotheres latus Bürger, 1895
= Pinnotheres tenuipes Borradaile, 1903 (pre-occupied name)= Pinnoteres borradailei Nobili, 1906a (replacement name)= Pinnoteres ridgewayi Southwell, 1911143

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia
Arcotheres palaensis (Bürger, 1895)(Figs. 1A–C, 2–22)

from the bottoms of aquaria and display tables of markets
where various bivalves were sold together and sometimes
deshelled for customers. In such circumstances, host records
can only be inferred at best.

Pinnotheres obesus — Miers, 1880: 314, pl. 4 fig. 4 (not Pinnotheraobesa Dana, 1852).
Pinnotheres palaensis Bürger, 1895: 372, 373, pl. 9 fig. 12, pl.
10 fig. 12 [type locality: Palau]; Tesch, 1918: 249 (list); De
Man, 1921: 260 (Amboina, Indonesia); 1929: 14, pl. 2 fig. 5,5a, b (Indramaju, Java, Indonesia); Perez, 1921: 59 (Amboina,
Indonesia) (parasitised by epicaridean); Gordon, 1936: 64(discussion); Estampador, 1937: 547 (list); Miyake, 1939: 221(list); Chuang, 1961: 189, pl. 94 fig. 1 (Singapore, Malaysia);
Silas & Alagarswami, 1967: 1205 (list); Serène, 1968: 94;
Schmitt et al., 1973: 64 (list).
Pinnotheres rhombifer Bürger, 1895: 374, pl. 9 fig. 15, pl. 10 fig.
14 [type locality: Ubay, Philippines]; Serène, 1968: 94. [Newsynonymy]
Pinnotheres latissimus Bürger, 1895: 373, pl. 9 fig. 13a, b, pl. 10
fig. 13 [type locality: Manila, Philippines]; Gordon, 1936: 176,
fig. 6; Serène, 1968: 94. [New synonymy]
Pinnotheres arcophilus Bürger, 1895: 371, pl. 9 fig. 10, pl. 10 fig.
10 [type locality: Ubay, Philippines]; Lanchester, 1900: 762(Singapore); Tesch, 1918: 248 (list); De Man, 1921: 260, pl.
8 figs. 1–7 (Lombok, Indonesia); Perez, 1921: 61 (Lombok,
Indonesia) (parasitism by epicaridean); Gordon, 1936: 164(discussion); Silas & Alagarswami, 1967: 1196 (list); Schmitt
et al., 1973: 39.
Pinnoteres palaensis — Balss, 1957: 1418 (list).
Pinnotheres paralatissimus Dai & Song, 1986: 55, 61, 62, text-fig.
2 [type locality: Beihai, Guangxi, China]; De Gier & Becker,
2020: tab. 1. [New synonymy]?Arcotheres alcocki — Takeda & Konishi, 1988: 138, figs. 1, 2f–h,
3; Watanabe, 2013: 47 (not Pinnotheres alcocki Rathbun, 1909).
Arcotheres guinotae Campos, 2001: 494, figs. 1, 2 [type locality:
Thailand]; Ng et al., 2008: 248; De Gier & Becker, 2020: tab.
1. [New synonymy]
Arcotheres palaensis — Pohle & Marques, 1998: 231 (part); Ahyong& Ng, 2007b: 200, fig. 9 (types); Ng et al., 2008: 248 (list);
Ng et al., 2017: 1094; Trivedi et al., 2018a: 197; De Gier &Becker, 2020: tab. 1; Komai et al., 2020: 222.
Arcotheres rhombifer — Ahyong & Ng, 2007b: 205, fig. 12; Ng
et al., 2017: 1094.
Arcotheres arcophilus — Ahyong & Ng, 2007b: 194, fig. 2; Ng
et al., 2008: 248; Ng et al., 2017: 1093; De Gier & Becker,
2020: tab. 1.
?Arcotheres sp. — Watanabe & Henmi, 2009: 217, figs. 2, 3.
Arcotheres acrophilus — Trivedi et al., 2018a: 197 [misspelling].

TAXONOMYFamily Pinnotheridae De Haan, 1833
Genus Arcotheres Manning, 1993aType species. Pinnotheres palaensis Bürger, 1895, byoriginal designation.
Diagnosis. MXP3 dactylus slender (digitiform), inserted
on proximal one-third or near midlength of ventral marginof propodus, variable length; propodus linear, usually
spatuliform, 2.5–3.0 times longer than wide; inner marginof widest part of ischiomerus rounded to gently angular;
exopod relatively slender with proximal part of exopod notespecially wide. Female P4 asymmetrical, with one side
distinctly longer (usually merus, propodus, and dactylus);
longer P4 and both P5 dactyli longer, different in structure
than those of P2 and P3; P5 dactylus elongate, linear;ischiomeral articulation perpendicular to segment axis. Malepleon broadly triangular, telson subhexagonal, subpentagonal
to broadly triangular or semicircular, widest basally near
midlength. G1 relatively stout, arcuate, with or without
subdistal ventral projection; G2 with exopod.
Remarks. Arcotheres must be attributed to Manning (1993a)even though the name was first used in 1880 by Nauck in
an unpublished note and later cited by Bürger (1895: 361),
although he did not recognise the genus. Manning (1993a:
524) diagnosed the genus in a short note, attributed theauthorship to Bürger (1895) and assigned Pinnotheres
palaensis Bürger, 1895, as the type species (see also Manning,
1993a: 127, 128). Campos & Manning (2001) clarified thenomenclature of the genus (see also Ng et al., 2008: 252).
Arcotheres is a large genus with over 30 species, with
Ahyong & Ng (2021) having recently transferred another fiveto the genus. Ng et al. (2017) questioned the monophyly of
Arcotheres (see also Ahyong & Ng, 2007b) given the diversity
in form in the large genus, and singled out A. borradailei(Nobili, 1906a) and A. ridgewayi (Southwell, 1911) as being
different in several features of the pereopods. Both species
are here synonymised with Magnotheres globosus (Hombron& Jacquinot, 1846) (see later). The carapace, MXP3, andpereopods of A. coarctatus (Bürger, 1895) are also quite
different from those of typical Arcotheres, so A. coarctatusis also referred here to its own genus, Plenotheres, newgenus. As redefined in this study, Arcotheres now contains
23 valid species (Table 1).

Type material. Lectotype: female (10.2 × 8.4 mm) (SMFZMG 948a), Palau, coll. C. Semper. Paralectotypes: 3
poorly preserved females (10.3 × 8.3–10.8 × 8.9 mm; 1 withepicaridean) (SMF-ZMG 948b), same data as lectotype; 1
female (10.0 × 7.0 mm, shrivelled) (SMF-ZMG 947), samedata as lectotype. Philippines: 4 females (9.5 × 7.4–11.8
× 8.3 mm, shrivelled) (SMF-ZMG 177) (paralectotypes of
Pinnotheres palaensis Bürger, 1895), Ubay, from “Placunasella”, coll. C. Semper; 1 ovigerous female (8.5 × 6.8 mm)(SMF-ZMG 179a) (simultaneous lectotype of Pinnotheres
rhombifer Bürger, 1895, and neotype of Pinnotheres
latissimus Bürger, 1895), Ubay, from Pectunculus aurifluus;
1 ovigerous female (7.4 × 5.7 mm) (SMF-ZMG 179b)(paralectotype of Pinnotheres rhombifer Bürger, 1895), same

144

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 1. Original figures of selected species. A, A. palaensis (Bürger, 1895), after Bürger (1895: pl. 9 fig. 12); B, A. guinotae Campos,
2001, after Campos (2001: fig. 1A); C, A. arcophilus (Bürger, 1895), after Bürger (1895: pl. 9 fig. 10); D, A. modiolicola (Bürger, 1895),
after Bürger (1895: pl. 9 fig. 9); E, Pl. coarctatus (Bürger, 1895), after Bürger (1895: pl. 9 fig. 7); F, M. latus (Bürger, 1895), after Bürger
(1895: pl. 9 fig. 16).

Other material examined. Peninsular Malaysia: 1 female(8.1 × 6.6 mm) (ZRC 2017.1252), Peninsular Malaysia or
Singapore, from practical class, coll. 30 August 1967; 1
ovigerous female (7.3 × 5.8 mm), 1 non-ovigerous female(carapace length 6.0 mm; with bopyrid on right side ofcarapace), 1 non-ovigerous female (carapace length 5.7 mm;with bopyrid on left side of carapace) (ZRC 1987.537–539),
in Tegillarca granosa, from markets, coll. P. K. L. Ng, 29
November 1986; 1 female (6.1 × 5.2 mm) (ZRC 1993.119),

data as lectotype; 1 female (5.3 × 4.7 mm) (SMF-ZMG 172a)(lectotype of Pinnotheres arcophilus Bürger, 1895), Ubay,
Philippines, from Arca, C. Semper, 1863–1864; 1 male (2.8
× 2.5 mm), 1 shrivelled female (5.1 × 4.5 mm) (SMF-ZMG
172b) (paralectotypes of Pinnotheres arcophilus Bürger,1895), same data as lectotype. Thailand: 1 female (6.4 ×
5.8 mm) (MNHN-B9498) (holotype of Arcotheres guinotae
Campos, 2001) [photographs examined], from Barbatia sp.,coll. R. Serène.
145

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 2. Arcotheres palaensis (Bürger, 1895), colour in life, in situ in Anadara and Tegillarca (Arcidae). A, ovigerous female (13.3 × 11.2mm) (ZRC 2017.1281), Peninsular Malaysia; B, ovigerous female (8.7 × 8.0 mm) (ZRC 2020.1), Peninsular Malaysia; C, D, ovigerous
female (11.6 × 10.3 mm) (ZRC 2020.11), Peninsular Malaysia; E, F, female (7.9 × 6.6 mm) (ZRC 2018.255), Peninsular Malaysia. A,
D, E, in situ; B, dorsal view; C, ventral view. A, host A. cf. rhombea (Born, 1778); B–F, host T. granosa (Linnaeus, 1758). Photographs:
Paul Y. C. Ng.

146

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 3. Arcotheres palaensis (Bürger, 1895), colour in life, overall dorsal view. A, ovigerous female (8.7 × 8.0 mm) (ZRC 2020.1), Peninsular
Malaysia; B, female (9.5 × 8.2 mm) (ZRC 2020.1), Peninsular Malaysia; C, female (9.3 × 8.3 mm) (ZRC 2020.1), Peninsular Malaysia; D,ovigerous female (11.6 × 10.3 mm) (ZRC 2020.11), Peninsular Malaysia; E, female (6.3 × 5.0 mm) (ZRC 2021.821), Peninsular Malaysia;
F, ovigerous female (7.9 × 6.4 mm) (ZRC 2021.821), Peninsular Malaysia; G, female (6.4 × 4.8 mm) (ZRC 2021.821), Peninsular Malaysia;
H, female (7.8 × 6.6 mm) (ZRC 2021.0801), Peninsular Malaysia. Photographs: Paul Y. C. Ng, P. K. L. Ng.

147

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 4. Arcotheres palaensis (Bürger, 1895), ovigerous female (11.6 × 10.3 mm) (ZRC 2020.11), Peninsular Malaysia, colour in life. Twosubventral views showing eyes, antennules, antennae, and MXP3.

in Tegillarca granosa, from markets, coll. P. K. L. Ng,
November 1987; 1 ovigerous female (8.5 × 7.2 mm)(ZRC 1992.8378), in Tegillarca granosa, from markets in
Singapore, coll. P. K. L. Ng, January 1992; 1 female (9.6
× 7.5 mm) (ZRC 1999.975), probably Peninsular Malaysia,
from food with cockles (Clementi Town Centre, Singapore),coll. D. C. J. Yeo, 27 May 1999; 1 soft female (ca. 9.0 mm
carapace width) (ZRC 2000.2613), Batu Pahat, Johor, in
Tegillarca granosa, mangroves, coll. H. K. Tan, 16 August
1983; 1 ovigerous female (13.3 × 11.2 mm) (ZRC 2017.1281),
probably Malaysia, in Tegillarca cf. rhombea, Chinatownmarket, Singapore, coll. 14 January 2018; 1 ovigerous female(7.9 × 6.6 mm) (ZRC 2018.255), 1 ovigerous female (8.6
× 6.5 mm) (AM P105905), probably Peninsular Malaysia,
in Tegillarca granosa, from Chinatown market, Singapore,coll. P. K. L. Ng, 25 February 2018; 1 ovigerous female(7.5 × 5.8 mm) (ZRC 2018.770), from food, probably from
Tegillarca sp., coll. B. Y. Lee, 6 May 2018; 1 ovigerous
female (8.4 × 6.6 mm) (ZRC 2018.759), probably Peninsular
Malaysia, in Tegillarca granosa, from Chinatown market,
Singapore, coll. P. Y. C. Ng, 4 November 2018; 1 ovigerous
female (ZRC 2019.1039), Johor, in Tegillarca granosa,
from Chinatown market, Singapore, coll. P. K. L. Ng, 27
July 2019; 9 females (3 photographed live) (ZRC 2020.1),
Kandang Kerbau market, Singapore, in Tegillarca granosa,
coll. 5 January 2020; 1 ovigerous female (ZRC 2021.379),
Kandang Kerbau market, Singapore, in Tegillarca granosa,
coll. P. K. L. Ng, 12 January 2020; 1 ovigerous female (ZRC
2020.2), Kandang Kerbau market, Singapore, in Tegillarca
granosa, coll. 12 January 2020; 10 females (smallest 4.1 ×
4.1 mm) (ZRC 2020.9), Kandang Kerbau market, Singapore,in Tegillarca granosa, coll. P. K. L. Ng, 29 March 2020;
3 ovigerous females (10.2 × 7.8 mm, 10.3 × 7.9 mm, 11.6
× 10.3 mm) (ZRC 2020.11), in Tegillarca granosa, from
Chinatown market, Singapore, coll. P. K. L. Ng, 16 April
2020; 1 female (7.8 × 6.6 mm) (ZRC 2021.0801), from
Kandang Kerbau market, in Tegillarca granosa, coll. P. K.
L. Ng, 5 December 2021; 1 ovigerous female (8.9 × 7.8mm) (ZRC 2021.814), in Tegillarca nodifera, from Yew Tee
Market, Singapore, coll. H. H. Tan, 12 December 2021; 1
ovigerous female (7.9 × 6.4 mm), 2 females (6.3 × 5.0 mm,
6.4 × 4.8 mm) (ZRC 2021.0821), in Tegillarca granosa,

from Kandang Kerbau Market, Singapore, from Peninsular
Malaysia, coll. P. K. L. Ng, 26 December 2021; 1 female(7.4 × 6.7 mm) (ZRC 2022.42), in Tegillarca granosa, fromYew Tee Market, Singapore, coll. H. H. Tan, 23 January
2022. Indonesia: 2 ovigerous females (ZRC 1999.2404),
Indonesian markets, in Tegillarca granosa, coll. C. M.
Yang, 3 August 1983; 1 ovigerous female (7.1 × 5.7 mm),
1 non-ovigerous female (7.7 × 6.2 mm), 1 ovigerous female(carapace length 7.1 mm; with bopyrid) (ZRC 1993.120–122),
Tegillarca granosa, from markets, coll. P. K. L. Ng, 8
December 1987; 1 ovigerous female (5.1 × 4.3 mm) (ZRC
1999.1202), between Pulau Berang and Pulau Ileu, Pulau
Lingga, Riau Islands, coll. J. B. Sigurdsson, 28 March 1993;
1 female (8.8 × 7.8 mm) (ZRC 2018.764), Teluk Kodek,
Lombok, Indonesia, unknown host, coll. S. Dwiono, 2014;
1 female (carapace length 6.7 mm; with bopyrid) (ZRC
2018.762), Teluk Kodek, Lombok, Indonesia, in Anadara
antiquata, coll. S. Dwiono, 12 November 2013; 1 male (3.6
× 3.6 mm) (ZRC 2019.1788), in Tegillarca granosa, from
Kandang Kerbau market, Singapore, coll. P. K. L. Ng, 29
December 2019; 2 ovigerous females (ZRC 2021.355), fromsupermarket, in Tegillarca granosa, coll. P. Y. C. Ng, 26
March 2021. Singapore: 2 females (ZRC 2014.217), coll.
P. K. L. Ng, 2005; 10 females (ZRC 1965.11.24.17–26),
from market, probably from cockles, coll. August 1934; 1
female (6.7 × 5.0 mm) (ZRC 2017.1038), Tanah Merah Ferry
Terminal Beach, near Changi Point, intertidal, in Barbatia sp.,coll. S. K. Tan et al., 12–15 January 2017; 3 females (largest
9.5 × 7.7 mm) (ZRC 1965.11.24.30–32), Pulau Senang, in“Barbatia sp. J”, coll. November 1934; 1 female (7.0 ×
5.6 mm) (ZRC 1965.11.24.33), Raffles Lighthouse, inside
Barbatia decussata, coll. 1930s. Borneo: 1 female (8.7 × 8.1mm) (ZRC 2017.1260), off Kuching, Sarawak, from large
Tegillarca sp. from mudflats, from seafood market, coll. P.
K. L. Ng, June 2015. Thailand: 3 females (3.3 × 3.4 mm,
5.0 × 4.2 mm, 11.1 × 9.8 mm) (ZRC 2018.767), Bangkok
market, from Tegillarca granosa, coll. P. Y. C. Ng, 8 April
2018. Japan: 17 females (2.3 × 2.0–9.6 × 6.5 mm), 1 male
(4.2 × 3.7 mm) (NSMT-Cr 9363), Mukaishima, Hiroshima
Prefecture, Seto Inland Sea, in Barbatia virescens, coll. K.
Konishi, 18 June 1981; 6 females (4.5 × 3.8–8.6 × 6.1 mm)(ZRC 2019.1879), Kawachi, Kumamoto, Kyushu, in Barbatia
148

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 5. Arcotheres palaensis (Bürger, 1895). A, B, lectotype female (10.2 × 8.4 mm) (SMF-ZMG 948a), Palau. A, overall dorsal view;
B, ventral view of cephalothorax.

virescens, coll. T. Watanabe, 15 October 2007. Others: 1
spent female (9.6 × 7.9 mm), 1 ovigerous female (9.3 × 8.2mm) (NHM 1860.6), “Indo-Malayan Seas”.

in adults; mobile, completely filling orbit (Figs. 3, 5A, 8A,
C, 9A, C, 13, 14). Epistome with median part triangular,
lateral margins gently concave (Figs. 8C, 9C, 17O, 18M).

Description. Carapace and pereopods well chitinised,
usually firm in post-hard stage. Female: Carapace subovate
to subhexagonal, wider than long; dorsal and lateral surfacessmooth, glabrous; appearing domed in frontal view; front
sometimes projecting anteriorly beyond orbits, margin gently
sinuous to gently convex; anterolateral margin thickened,blunt but defined, sometimes subparallel with frontal marginor sloping posteriorly to various degrees, forming rounded
angle with posterolateral margin (Figs. 3, 5A, 8A, 9A,
11–14). Eyes small, not or slightly visible in dorsal view

MXP3 outer surface with scattered short setae; ischiomerus
completely fused, subrhomboidal, maximum length 1.5–1.8
times maximum width, inner margin usually rounded,occasionally slightly angular at widest point; carpus short;propodus about 3 times as long as high, subspatulate,distinctly longer than carpus, tip rounded to subtruncate;dactylus slender, inserted at proximal one-third to near
midlength of propodus, tip not reaching propodal apex;
exopod relatively slender, about two-thirds length ofischiomerus, flagellum 2-segmented (Fig. 16).
149

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 6. Arcotheres palaensis (Bürger, 1895). A, B, female (5.3 × 4.7 mm) (SMF-ZMG 172a) (lectotype of Pinnotheres arcophilus Bürger,1895), Philippines; C–F, ovigerous female (5.1 × 4.3 mm) (ZRC 1999.1202), Indonesia. A, C, overall dorsal view; B, D, dorsal view ofcarapace; E, frontal view of cephalothorax; F, outer view of right chela.

Chela not prominently elongate, dactylus about two-thirds
palm length; palm relatively slender, narrower proximallythan distally; outer surface of palm, fingers (except for
distal part) almost glabrous, with only scattered short setae;
ventral margin of palm gently convex to sinuous, sometimes
with low bulge near midlength, extending onto mesial
surface; dactylus occlusal margin with distinct subproximal
tooth; pollex occlusal margin with 1 low proximal tooth, 1
submedian tooth, and minute denticles; tips of fingers sharp,
hooked (Figs. 8D, 9D, 15B).

distinctly the longer; P2 and P3 dactyli short, subequal,
tip gently hooked, half propodus length or less; longer P4
dactylus elongate, slender, almost straight, sparsely setose
distally; P5 merus 3.6–4.0 times longer than wide, P5 dactylus
longest, margins lined with short and long setae, denser onventral margin, distoflexor margin without rows of spinules,
at most sometimes with 3–8 scattered low spinules (Figs.
17A–N, 18A–L).
Pleon extending to buccal region, covering bases of P2–P5;
telson gently recessed into concave distal margin of somite
6 (Figs. 5B, 6B, 9B).

P2–P5 dorsally, ventrally unarmed; outer surface covered
with scattered, very short setae or glabrous; ventral margins
of propodus and dactylus slightly more setose; relative
lengths of meri P4>P3>P2>P5; left (sometimes right) P4

Male: Carapace almost circular, slightly wider than long;
dorsal surface almost smooth, not prominently inflated,
150

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 7. Arcotheres palaensis (Bürger, 1895), female (6.4 × 5.8 mm) (MNHN-B9498), Thailand (holotype of A. guinotae Campos, 2001).
A, overall dorsal view; B, left cheliped; C, ventral view of distal part of P5 dactylus. Photographs: A, B, Arthur Anker; C, Sébastien
Soubzmaigne.

lateral surfaces with setae; front projecting anteriorly, margin
sinuous to almost straight (Figs. 20A, B, 21A, B). Eyes
distinctly visible in dorsal view (Figs. 20A, 21A). MXP3
as in female (Fig. 22A). Anterior thoracic sternum wide,
sternites 1, 2 fused, partially sunken into buccal cavity; suture
between sternites 2 and 3 shallow; sternites 3, 4 completely
fused, separated only by shallow grooves. Chela relatively
stout, shorter than in female (Figs. 20A, B, 21A, C); ventral
margin of palm gently to distinctly sinuous, sometimes withlow bulge near midlength. P2–P5 dorsally, ventrally unarmed;
outer surface covered with short setae; P3 and P4 carpus andpropodus with long natatory setae; left and right meri equal,
relative lengths of meri P4>P3>P2>P5; dactyli of P2–P4
progressively longer; left P4 dactylus slightly longer, more
slender than right side; P4 and P5 dactylus subequal to thatof P3, covered with short setae (Fig. 22B–I). Pleon slender,
triangular, widest at somite 3, lateral margins of somite 4gently concave to almost straight; somite 6 trapezoidal or
subquadrate; telson ovate to semicircular, slightly wider thanlong, widest near midlength (Fig. 22J, O). G1 relatively
stout, arcuate, curved outwards, tip gently curved upwards
or straight (Fig. 22K–M, O). G2 short, with spatuliform tip;
exopod present as long curved structure (Fig. 22N).
Variation. Preservational effects notwithstanding, there isconsiderable variation in the shape of the carapace (Figs. 13,14), from transversely ovate with the front barely protruding
in dorsal view (Figs. 5A, 6A, 11A, D, 12F, 13A–C), to morerounded with a distinctly protruding frontal margin (Figs.
11E, G, H, 12A–C, E, G, H, 13E–H, 14); and in some, thecarapace is almost subtrapezoid (Fig. 14E–H). The good seriesof specimens demonstrates that there are many intermediate
conditions that make precise carapace shape unreliable as adiagnostic character in A. palaensis. There is some variation151

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 8. Arcotheres palaensis (Bürger, 1895), ovigerous female (8.5 × 7.2 mm) (ZRC 1992.8378), Peninsular Malaysia. A, overall dorsal
view; B, ventral view of cephalothorax; C, frontal view of cephalothorax; D, outer view of right chela.

is deeply concave, appearing as a broad indentation. In thelargest female of A. palaensis examined (13.3 × 11.2 mm;
ZRC 2017.1281), however, the anterior thoracic sternum isalso broad and medially shallow; and as such, this charactermay vary to some degree. In the smallest female examined(3.3 × 3.4 mm; ZRC 2018.767; Fig. 15A), the pleon resembles
that of males, being elongate with a semicircular telsonin the length of the MXP3 dactylus, being relatively shorterin some specimens (e.g., Fig. 17D, G); in most specimens,
however, the dactylus almost reaches the tip of the propodus(e.g., Fig. 17H), but never beyond. In most adult female A.
palaensis examined (including small specimens of about 5
mm in carapace width), the distal margin of fused thoracic
sternites 1–4 is proportionately narrower and the median part
152

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 9. Arcotheres palaensis (Bürger, 1895), ovigerous female (6.7 × 5.0 mm) (ZRC 2017.1038), Singapore. A, overall dorsal view; B,
ventral view of cephalothorax; C, frontal view of cephalothorax; D, outer view of right chela.

153

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 10. Arcotheres palaensis (Bürger, 1895), female (7.3 × 5.2 mm) (CAS), China (holotype of Pinnotheres paralatissimus Dai & Song,1986); after Dai & Song (1986: fig. 2(1–7)). A, overall dorsal view; B, left MXP3; C, outer view of right chela; D–G, right P2–P5 dactyli;
H, left P4 dactylus. Scales = A, 2.0 mm; B, D–H, 0.5 mm; C, 1.0 mm.

(Fig. 15C). The male pleon varies slightly in shape, with
the Philippine paralectotype male of P. arcophilus (2.8 ×
2.5 mm; SMF-ZMG 172b) being slightly more hexagonal(Fig. 22O), whereas that of the Indonesian male is somewhat
more rounded (Fig. 20J). Their G1 morphologies, however,agree in all key aspects (Fig. 22K, O).

identification. Moreover, our observations based on thelarge sample size enabled reassessment of the taxonomy ofnominal species of Arcotheres similar to A. palaensis, manyof which are synonymised herein, as well as clarificationof the identity of most published records in the region.
Unfortunately, we have not been able to obtain additional
specimens of A. palaensis from the type locality in Palau;
despite a recent survey there to sample topotypic material of
Brachyura (e.g., see Ng, 2019), the molluscs sampled did notyield any pinnotherids. Arcotheres palaensis is present in theadjacent Philippines (through the types and synonymisationof Pinnotheres latissimus and Pinnotheres rhombifer withit, their type localities being there; see later). We also havespecimens from Borneo and Thailand, which connects the
distribution of the species to Malaysia and Singapore. The
records from Japan are also probably A. palaensis, thoughthese may warrant further study (see later).

Colour. The colour is variable, from pale to dirty white topinkish or yellowish when alive, with the eyes pigmented
red to black (Figs. 2–4). In the freshly collected male (ZRC
2019.1788), the carapace is dirty white with numerous small
brown flecks and the ventral surfaces are white (Fig. 20).
Host. Known with certainty only from the Arcidae Lamarck,
1809. Arcotheres palaensis has been found in Anadara
antiquata (Linnaeus, 1758), Tegillarca granosa (Linnaeus,
1758), Tegillarca cf. rhombea (Born, 1778), Tegillarca
nodifera (Martens, 1860), Barbatia decussata (Lamarck,1819), Barbatia virescens (Reeve, 1844), and Barbatia
sp. The one record from Tucitona aurifluus (Reeve, 1843)(previously in Pectunculus) (Glycymerididae Dall, 1908)(supposedly for two synonyms, Pinnotheres rhombifer
Bürger, 1895, and Pinnotheres latissimus Bürger, 1895)
needs to be confirmed. Noteworthy, however, is that both
Arcidae and Glycymerididae belong to the superfamily
Arcoidea Lamarck, 1809. As discussed earlier, records of“A. palaensis” from Placuna Lightfoot, 1786 (Placunidae),and Mactra Linnaeus, 1767 (Mactridae) (cf. Schmitt et al.,
1973: 65), almost certainly belong to other species.

Ahyong & Ng (2007b: 205–206) synonymised Pinnotheres
latissimus Bürger, 1895, under Pinnotheres rhombifer Bürger,
1895, commenting that there were no major charactersseparating them. The species had already been transferred
to Arcotheres by Campos & Manning (2001). The goodseries of specimens of A. palaensis on hand now showsthat both P. rhombifer and P. latissimus are its juniorsynonyms. No other characters apart from small differences
in carapace proportions purportedly separate A. palaensis
from A. rhombifer and A. latissimus, differences that are
fully encompassed by the range in variation observed here.
As all the synonymised species by Bürger were described in
the same paper (Bürger, 1895), the names are considered assimultaneously published under the zoological Code (ICZN,
1999). Under Article 24 of the Code (ICZN, 1999), we here
regard Pinnotheres palaensis Bürger, 1895, as having priority
over P. latissimus Bürger, 1895, and P. rhombifer Bürger,
1895, when they are regarded as synonyms.

Remarks. The large series of A. palaensis available forstudy, particularly from Singapore and Malaysia, has enabled
documentation of the range of morphological variationin this species. The morphological range observed in the
Singapore–Malaysia material alone fully encompasses thatof the lectotype from Palau, giving confidence in species

154

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 11. Arcotheres palaensis (Bürger, 1895), overall dorsal view. A, ovigerous female (13.3 × 11.2 mm) (ZRC 2017.1281), Peninsular
Malaysia; B, female (7.0 × 5.6 mm) (ZRC 1965.11.24.33), Singapore; C, female (9.6 × 7.5 mm) (ZRC 1999.975), Peninsular Malaysia;
D, female (8.1 × 6.6 mm) (ZRC 2017.1252), Peninsular Malaysia; E, female (7.3 × 5.8 mm) (ZRC 1987.537), Peninsular Malaysia; F,ovigerous female (7.1 × 5.7 mm) (ZRC 1993.120), Peninsular Malaysia; G, female (8.4 × 7.3 mm) (ZRC 1965.11.24.17), Peninsular
Malaysia; H, ovigerous female (7.9 × 6.6 mm) (ZRC 2018.0255), Peninsular Malaysia.

155

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 12. Arcotheres palaensis (Bürger, 1895), overall dorsal view. A, female (8.7 × 8.1 mm) (ZRC 2017.1260), Sarawak; B, ovigerous
female (7.5 × 5.8 mm) (ZRC 2018.770), Peninsular Malaysia; C, ovigerous female (8.7 × 8.0 mm) (ZRC 2020.1), Peninsular Malaysia;
D, female (7.7 × 6.2 mm) (ZRC 1993.121), Indonesia; E, female (8.8 × 7.8 mm) (ZRC 2018.764), Lombok, Indonesia; F, female withbopyrid (carapace length 6.7 mm) (ZRC 2018.762), Lombok, Indonesia; G, female (8.0 × 5.8 mm) (ZRC 2019.1879), Japan; H, female(8.6 × 6.1 mm) (ZRC 2019.1879), Japan.

156

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 13. Arcotheres palaensis (Bürger, 1895), dorsal view of carapace. A, ovigerous female (13.3 × 11.2 mm) (ZRC 2017.1281), Peninsular
Malaysia; B, female (8.9 × 6.5 mm) (ZRC 1965.11.24.27), Singapore; C, ovigerous female (8.5 × 7.2 mm) (ZRC 1992.8378), Peninsular
Malaysia; D, female (9.3 × 8.3 mm) (ZRC 2020.1), Peninsular Malaysia; E, ovigerous female (8.7 × 8.0 mm) (ZRC 2020.1), Peninsular
Malaysia; F, ovigerous female (7.1 × 5.7 mm) (ZRC 1993.120), Peninsular Malaysia; G, female (7.7 × 6.2 mm) (ZRC 1993.121), Indonesia;
H, female (8.8 × 7.8 mm) (ZRC 2018.764), Lombok, Indonesia.

157

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 14. Arcotheres palaensis (Bürger, 1895), dorsal view of carapace. A, female (9.5 × 8.2 mm) (ZRC 2020.1), Peninsular Malaysia;
B, female (6.1 × 5.2 mm) (ZRC 1993.0119), Peninsular Malaysia; C, female (7.5 × 5.8 mm) (ZRC 2018.770), Peninsular Malaysia; D,ovigerous female (7.9 × 6.6 mm) (ZRC 2018.0255), Peninsular Malaysia; E, female (8.0 × 5.8 mm) (ZRC 2019.1879), Japan; F, female(8.6 × 6.1 mm) (ZRC 2019.1879), Japan; G, female (9.5 × 7.7 mm) (ZRC 1965.11.24.30), Singapore; H, ovigerous female (6.7 × 5.0mm) (ZRC 2017.1038), Singapore.

158

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 15. Arcotheres palaensis (Bürger, 1895). A–C, juvenile female (3.3 × 3.4 mm) (ZRC 2018.767), Thailand; D, female (8.2 × 7.0 mm)(ZRC 2020.9), Singapore; E, ovigerous female (6.6 × 5.2 mm) (ZRC 1965.11.24.32), Singapore. A, overall dorsal view; B, outer view ofleft chela; C, juvenile female pleon; D, E, female pleonal somites 5 and 6 and telson. Scales = 1.0 mm.

Arcotheres arcophilus is also certainly a junior synonymof A. palaensis. The types of A. arcophilus (a female and amale) are in poor condition (Figs. 6A, B, 19A), and appear
to differ from the lectotype of A. palaensis only in that: thefemale carapace is relatively more hexagonal in shape with arelatively more distinct front (Figs. 6A, B, 19A), the anterior
carapace margins of females are relatively less divergent(Figs. 6A, B, 19A), the eyes (for both sexes) are clearlyvisible in dorsal view (Figs. 6A, B, 19A), and the female
ambulatory meri are slightly more slender (Fig. 19A) (seealso Ahyong & Ng, 2007b: fig. 2A, C). One small ovigerous
female specimen (5.1 × 4.3 mm) from the Riau Islands inIndonesia (ZRC 1999.1202) closely resembles the lectotype
female in these characters (Figs. 6C–E, 19D–M); while a
recently collected male (3.6 × 3.6 mm; ZRC 2019.1788)
agrees very well with the paralectotype male of A. arcophilus(SMF-ZMG 172b), notably in the structures of the ambulatory
legs, pleon, and G1 (Figs. 20–22).

arcophilus in possessing a more projecting frontal marginand in the less divergent anterior part of the lateral carapace
margins. One female specimen (8.7 × 8.0 mm; ZRC 2020.1;
Figs. 3A, 12C, 13E) of A. palaensis has a carapace closely
resembling that of the type female A. arcophilus (Fig. 6A,B), but its eyes are completely hidden by the carapace rim in
dorsal view, the ambulatory meri are slightly more slender,
the longer P4 propodus is somewhat shorter, and the chela
is relatively more elongate. Eight other female specimensof A. palaensis from that lot (ZRC 2020.1) show a rangeof carapace shape variation (Figs. 3B, C, 11D, 12D, 14A).
Most specimens of A. palaensis, however, have the sameeye, leg, and chela features. Another female A. palaensis
from Malaysia (ZRC 2018.770) has a carapace shapethat is superficially like that of the type and other female
specimen of A. arcophilus (ZRC 1999.1202), but the anterior
parts of the lateral margins diverge more strongly andthe eyes are underneath the anterior margins, completely
invisible in dorsal view (Figs. 11B, 14C). The differencesin the ambulatory legs are not significant and can easily be
accounted for by variation. In the lectotype female of A.
arcophilus, the MXP3 dactylus does not reach the apex of

Carapace proportions and the anterior outline, however,
have already been shown to be very variable in A. palaensis.
Some specimens of A. palaensis superficially resemble A.
159

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 16. Arcotheres palaensis (Bürger, 1895), outer view of left MXP3. A, female (9.6 × 7.5 mm) (ZRC 1999.975), Peninsular Malaysia;
B, ovigerous female (8.5 × 7.2 mm) (ZRC 1992.8378), Peninsular Malaysia; C, female (7.3 × 5.8 mm) (ZRC 1987.537), Peninsular
Malaysia; D, female (6.1 × 5.2 mm) (ZRC 1993.119), Peninsular Malaysia; E, female (8.1 × 6.6 mm) (ZRC 2017.1252), Peninsular
Malaysia; F, female (7.7 × 6.2 mm) (ZRC 1993.121), Indonesia; G, ovigerous female (6.7 × 5.0 mm) (ZRC 2017.1038), Singapore; H, I,
ovigerous female (10.3 × 7.9 mm) (ZRC 2020.11), Peninsular Malaysia; J, female (8.8 × 7.8 mm) (ZRC 2018.764), Lombok, Indonesia.
Scales = 0.5 mm.

160

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 17. Arcotheres palaensis (Bürger, 1895). A–H, female (7.7 × 6.2 mm) (ZRC 1993.121), Indonesia; I–O, ovigerous female (6.7 × 5.0mm) (ZRC 2017.1038), Singapore. A–D, left P2–P5, respectively; E–H, right P2–P5, respectively; I, J, left P4 and P5, respectively; K–N,
right P2–P5, respectively; O, posterior margin of epistome. Scales = 0.5 mm.

161

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 18. Arcotheres palaensis (Bürger, 1895). A–D, female (6.1 × 5.2 mm) (ZRC 1993.119), Peninsular Malaysia; E–H, M, ovigerous
female (8.5 × 7.2 mm) (ZRC 1992.8378), Peninsular Malaysia; I, J, soft female (ca. 9.0 mm carapace width) (ZRC 2000.2613), Peninsular
Malaysia; K, L, female (9.5 × 7.7 mm) (ZRC 1965.11.24.30), Singapore. A, B, left P4 and P5, respectively; C, D, right P4 and P5,
respectively; E, F, left P4 and P5, respectively; F’, distal part of left P5 dactylus (subventral view, soft setae omitted); G, H, right P4
and P5, respectively; I, J, right P4 and P5, respectively; K, L, right P4 and P5, respectively; M, posterior margin of epistome. Scales =A–M, 0.5 mm; F’, 0.25 mm.

162

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 19. Arcotheres palaensis (Bürger, 1895). A–C, female (5.3 × 4.7 mm) (SMF-ZMG 172a) (lectotype of Pinnotheres arcophilus Bürger,1895), Ubay, Philippines; D–M ovigerous female (5.1 × 4.3 mm) (ZRC 1999.1202), Indonesia. A, overall dorsal view; B, right MXP3;
C, right chela; D, left MXP3; E–H, left P2–P5, respectively; I–L, right P2–P5, respectively; M, frontal margin of carapace showing eyesin dorsal view. Scales = A–L, 0.5 mm; M, 1.0 mm.

163

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 20. Arcotheres palaensis (Bürger, 1895), male (3.6 × 3.6 mm) (ZRC 2019.1788), Indonesia, colour in life. A, overall dorsal view;B, frontal view; C, ventral view. Photographs: Paul Y. C. Ng.

above, approaching the condition in the type material of A.
arcophilus. In any case, A. ocularius can be distinguished byother characters of the carapace, pleon, and MXP3 (Komaiet al., 2020). Under Article 24 of the Code (ICZN, 1999),
we here also regard Pinnotheres palaensis Bürger, 1895, as
having priority over P. arcophilus Bürger, 1895, when they
are regarded as synonyms.

the propodus (Fig. 19B), but this character varies substantiallyin A. palaensis, with the dactylus sometimes almost reaching
the tip of the propodus (Fig. 16).
The one character of A. arcophilus that needs discussionis the visibility of the eyes of female specimens in dorsal
view. The type specimens of A. arcophilus have part ofthe eyes visible in dorsal view (Figs. 6A, B, 19A), as does
the more recent Indonesian female (ZRC 1999.1202; Figs.
6C, D, 19M). The eyes in all other females we referred to
A. palaensis are concealed under the frontal margins or, at
most, scarcely visible in dorsal view. Komai et al. (2020)
had used the dorsally exposed eyes as one of the diagnostic
features of A. ocularius from Fiji and Lombok, but thisfeature was consistent in a large series of specimens ofvarious sizes. In the case of A. arcophilus, however, thetypes and recent specimen from Indonesia are all relatively
small (and the types are poorly preserved), and it would be
more parsimonious to consider the dorsally exposed eyesof A. arcophilus as being at one end of a continuum ofvariation within the species, present at smaller sizes prior to
maximum swelling of the adult carapace. It is not uncommon,in smaller A. palaensis, for the carapace margins lateral to
the front to be sufficiently recessed that the eyes lie at theedge of the carapace, albeit remaining concealed in dorsal
view; more rarely, however, are the eyes just visible from

Miers (1880) recorded Pinnotheres obesus with doubt from
“Indo-Malayan Seas”, but Gordon (1936: 176) re-examined
and identified them as P. latissimus instead (see also Schmitt
et al., 1973: 51). Komai et al. (2020: 230), in revising P.
obesa s. str., commented that Miers’ (1880) material was
actually A. exiguus based on a personal communication by
the present authors, but this is incorrect. Miers’ (1880) poorly
preserved specimens (NHM 1860.6), re-examined here, areactually referable to A. palaensis, the chelae and ambulatory
legs agreeing well with this species.
Tesch (1918) described Pinnotheres onychodactylus fromthree females collected from an unspecified host on a reef
at 18-m depth, from between Gisser and Ceram islands in
the Moluccas, Indonesia, the largest female being 8.8 ×
6.8 mm. Tesch (1918: 259) comments: “This species muchresembles P. rhombifer Bürger, which, perhaps, is nothing
but P. palaensis Bürger, from which P. rhombifer is only
164

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 21. Arcotheres palaensis (Bürger, 1895), male (3.6 × 3.6 mm) (ZRC 2019.1788), Indonesia. A, overall dorsal view; B, frontal viewof cephalothorax; C, outer view of right chela.

distinguished by having the penultimate pair of legs shorter
than the last pair. In the ‘Siboga’ specimens the carapace,the breadth of which is 1⅓ times its length, agrees withthese of the two named species of Bürger; it is very muchvaulted, with bulging hepatic and branchial regions, without
sculpture, completely naked and very thin. The front isthickened, not prominent, and the eyes are not visible fromabove.” His figures and description, which note the diagnostic
ambulatory dactylar structures, strongly suggest that his
species is A. palaensis s. str. Unfortunately, he figures thelegs (especially the P4) as symmetrical (Tesch, 1918: pl. 17
fig. 5), with the left and right P4 dactyli the same size andform, and the MXP3 dactylus also exceeding the tip of the
propodus (Tesch, 1918: pl. 17 fig. 5A). Until the types can bere-examined to ascertain the accuracy of the P4 and MXP3
characters, it may be better to treat it as a distinct species
for the moment. In any case, as discussed earlier, we regard
A. rhombifer as a subjective junior synonym of A. palaensis.
As we do not know whether the P4 is asymmetrical, we
leave the species in Pinnotheres s. lato for the time being,
although it is likely to be an Arcotheres.

have no doubt of his identification. De Man (1921) also
figured a female Arcotheres specimen (as a Pinnotheres)from Ambon and another from Lombok, referring themto A. palaensis and A. arcophilus, respectively (they are
regarded as synonyms here). De Man based this mainly on
the structure propodus, MXP3 dactylus, and the form of thechela, following Bürger’s (1895) original descriptions andfigures. He noted that the carapace was in bad condition
in both specimens and could not help the taxonomy. He
discussed at length the asymmetry in the ambulatory legs
and considered the possibility of it being anomalous or eventhe result of bopyrid infection. Ambulatory leg asymmetry
was revisited in a later paper (De Man, 1929). Perez (1921:61), following on his correspondence with De Man (1921),
briefly commented on this problem and followed De Man’s
identifications of the Ambon and Lombok specimens. De
Man’s (1921) figures for the two species are excellent andhis figures of the third maxillipeds and chelipeds in particular
agree very well with those of A. palaensis (and A. arcophilus)
here. The MXP3 dactylus of his “A. arcophilus” is short,terminating some distance from the tip of the propodus, andthe chela is short and stout (De Man, 1921: pl. 8 figs. 1, 2),while in A. palaensis, the MXP3 dactylus is slightly longer,
almost reaching the tip of the propodus, and the chela isrelatively more elongate and slender (De Man, 1921: pl. 8
figs. 8, 9; cf. present Figs. 8D, 9D, 16). De Man’s (1921)

De Man’s (1921) description and figure of a female specimen
from Ambon, Indonesia, as well as his record of 11 females
and two males from Java (De Man, 1929), from a speciesof “Arca”, match those of A. palaensis very well and we165

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 22. Arcotheres palaensis (Bürger, 1895). A–N, male (3.6 × 3.6 mm) (ZRC 2019.1788), Indonesia; O, male (2.8 × 2.5 mm) (SMFZMG 172b) (paralectotype of Pinnotheres arcophilus Bürger, 1895), Philippines. A, left MXP3; B–E, left P2–P5, respectively; F–I, rightP2–P5, respectively; J, pleon; K, left G1 (ventral view); L, M, left G1 (slightly different dorsal views); N, left G2; O, pleonal somites
3–5, telson, and left G1. Scales = A, K–N, 0.2 mm; B–J, 0.5 mm; O, 0.1 mm.

166

RAFFLES BULLETIN OF ZOOLOGY 2022
record of “A. arcophilus” is also almost certainly from an
Anadara or Tegillarca (then known as Arca).

closely resembles A. palaensis, and necessary comparisonsshould be made to determine if they are related.

Pinnotheres paralatissimus Dai & Song, 1986, was describedfrom two females collected from an unspecified host in
Beihai, Guangxi, southern China (Dai & Song, 1986: 55,
text-fig. 2; Fig. 10A–H). Despite two separate searches in2018 and 2019, the types could not be found in the Institute ofZoology, Chinese Academy of Sciences (Academia Sinica),
where they were stated to have been kept (Meng Kai, Ng
Ngan Kee, Lee Bee Yan, pers. comm.). The description andfigures, however, are identical with what is here recognised
as A. palaensis s. str.

The Japanese records of “A. alcocki” are not that species.
Ahyong & Ng (2007b: 193–194) observed that the recordof “A. alcocki” by Takeda & Konishi (1988) is a different
species close to A. sinensis (Shen, 1932), or perhaps evenundescribed. We have since examined their specimens fromthe Hiroshima area as well as a series of females from nearby
Kumamoto (Figs. 12G, H, 14E, F), and they agree best with
A. palaensis as recognised here. As such, the records by
Watanabe & Henmi (2009) and Watanabe (2013) should
now also be referred to A. palaensis. The male reported
by Takeda & Konishi (1988) is not in good condition but
agrees with the present male from Indonesia (Figs. 20–22)
in most features. The carapace is slightly more roundedin the Japanese male (not evident from the figure) and its
frontal margin is weakly emarginate (carapace wider witha straighter frontal margin in the Indonesian male; Figs.
20A, 21A). The male pleon of the Japanese male, however,is unusual in being proportionately wider, with strongly
concave margins, and with the telson relatively broader
and subcircular in shape (Takeda & Konishi, 1988: fig. 3c)(versus pleon less wide, margins weakly concave, and telson
sub-hexagonal to semicircular in the Indonesian male; Fig.
22J, O). The differences in carapace and pleon features may
be due to size-associated variation—the Japanese male, at
4.2 × 3.7 mm, is larger than the Indonesian specimen (3.6
× 3.6 mm). In A. placunicola, however, smaller males havea relatively wider pleonal somite 6 and a somewhat more
rectangular telson, with somite 6 becoming proportionately
longer and the telson semicircular in larger specimens (Ng,2018a: 478, fig. 4B, C); their G1s, however, agree in relative
stoutness. Takeda & Konishi (1988: fig. 3d, e) figured thetip of the G1 as having a small spine, but this proved to
be an uneven evaginated fold caused by preservation; thegonopods are otherwise indistinguishable between Japaneseand Malaysian specimens. The Japanese records, however,are the northernmost and only records of the species
from temperate-subtropical waters, all others being fromthe tropics. It cannot be excluded that the morphologicaldifferences between the Japanese and Malaysian males
possibly indicate more than one species; more work should
be done to confirm this. The Japanese taxon, however, iscertainly not A. alcocki s. str. (see later for this species).

Campos (2001) described A. guinotae from one female(6.4 × 5.8 mm; MNHN-B9498), collected from somewherein Thailand from an unidentified species of Barbatia. He
compared it with A. similis, A. spinidactylus, A. alcocki, andA. tivelae (Gordon, 1936), noting that “the acute and sinuous
anterolateral margin of the carapace in A. guinotae n. sp.,
is not observed in all other known congeners” (Campos,
2001: 497). We have examined photographs of the typespecimen, courtesy of A. Anker (Fig. 7), who also kindly
confirmed various key characters for us. The carapace shapeof A. guinotae is somewhat unusual (Figs. 1B, 7A; Campos,
2001: fig. 1A), but we have on hand a female (6.7 × 5.0 mm;
ZRC 2017.1038) collected from a Barbatia in Singapore,
which is comparable in size to the holotype female of A.
guinotae and agrees with it in all features, including thecarapace shape (Figs. 9, 14H, 16G, 17K–O). Our comparisonsof the large series of A. palaensis, including specimens
collected from Barbatia, found that the carapace shapevariation fully encompasses that reported for A. guinotae.
We have specimens that have carapace shape of intermediate
conditions that transition into the more ovate form, also
found in A. palaensis (Figs. 11–14). The female telson of
A. guinotae is proportionately narrower (Campos, 2001: fig.
1C) than those in most adult female specimens (e.g., Figs.
5B, 8B, 9B), which we consider intraspecific variation. We
have a few specimens of A. palaensis in which the female
telson (Fig. 15D, E) is similar to that of the holotype of A.
guinotae. Other specimens from the same lot or site havethe more typically wide female telsons. There are no othercharacters that can separate the two species and the excellent
description and figures by Campos (2001) leave no doubt.
As such, we synonymise A. guinotae under A. palaensis.

We have on hand female specimens from Lombok, two of
which were collected from specimens of Anadara antiquata(ZRC 2018.764, ZRC 2018.762) and belong to two separate
species. Two specimens, one of which is infected by abopyrid (carapace length 6.7 mm; ZRC 2018.762), match A.
palaensis in all diagnostic features (Fig. 12F). The second
specimen (9.4 × 8.4 mm; ZRC 2019.1025) is referable to
A. ocularius Komai, Kawai & Ng, 2020 (Fig. 23B), as the
MXP3 has a proportionately longer MXP3 dactylus, the front
is more pronounced anteriorly, the eyes are clearly visible
in dorsal view, and the female anterior thoracic sternum(fused sternites 1–4) is relatively broader with the medianpart shallow rather than deeper (Komai et al., 2020). Our
morphological identification of A. ocularius is corroborated

The Indian record of “Arcotheres alcocki” by Lalitha Devi
(1981: 216, figs. 1 (right), 2, 3) from Tegillarca granosa in
Kakinada Bay in the Bay of Bengal may be A. palaensis aswell. The figures of the female specimen are not all given
in sharp detail (Lalitha Devi, 1981: fig. 2), but neverthelessagree well with A. palaensis. The figured male agrees very
well with male specimens examined here, including in theform of the pleon and G1 (Lalitha Devi, 1981: fig. 3). The
available evidence of “Arcotheres alcocki” presented by
Lalitha Devi (1981) points to A. palaensis, but her material
should be re-examined to determine if this is indeed thecase. Trivedi et al. (2018a) recently described a species
from southern India, A. shahi, ostensibly from oysters, that167

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia(CBM-ZC 15959); 1 male (4.8 × 5.4 mm) (ZRC 2020.7),
coll. 16 August 2010; 1 ovigerous female (10.7 × 9.1 mm)(CBM-ZC 15960), coll. 6 September 2011; 7 ovigerous
females (8.6 × 7.2–10.7 × 9.5 mm) (CBM-ZC 15961), August
2018; 5 ovigerous females (9.4 × 8.0–10.7 × 9.3 mm) (ZRC
2020.8), August 2018 (all from same locality as holotype);
1 ovigerous female (CBM-ZC 19562), Ucikumi, near Kumi
Village, coll. 22 February 2019.

by unpublished molecular comparisons currently underway
in collaboration with L. M. Tsang.
In the ZRC there is one lot with a male and a female
specimen initially identified as A. palaensis, collected in 1983
from Tegillarca (as Anadara) in mangroves in Batu Pahat,
Peninsular Malaysia (ZRC 2000.2613). These specimens had
been loaned to Charles Pregenzer for his comparative studies(unpublished) and Gerhard Pohle for his phylogenetic work(see Pohle & Marques, 1998). The female specimen (ca. 9.0
mm carapace width) is in poor condition now, soft and witha deformed carapace, but is clearly A. palaensis as presently
defined (Fig. 18I, J). The small male (3.7 × 3.3 mm), whichis relatively hard and in good condition, however, cannot
be this species even though it is labelled as being from thesame host and location, because the P5 dactylus has a rowof spinules (Fig. 60H) which A. palaensis lacks. In the shapeand flatness of the carapace (when viewed frontally; Fig.
60D, E), it closely resembles A. placunicola. In addition,
the G1 is more similar in shape and features to that of A.
placunicola, including the obliquely directed distal tip (Figs.
60G, 62M, N). We are confident this small male is actually
A. placunicola and is here referred to that species with a new
catalogue number (ZRC 2021.802). Although it cannot be
excluded that the small male entered a Tegillarca instead of
Placuna host, we consider it more likely that the specimen
was mislabelled or misplaced. If this is the case, we do notknow how the specimens of the two species got mixed in
one lot, nor whether it was collection error or otherwise. It
is important to note, however, that the male A. palaensis
obtained by the authors from Tegillarca and Anadara has
a clearly more elongate carapace, with the dorsal surface
prominently convex and the front clearly more projected
anteriorly (Figs. 20A, B, 21A, B).

Other material examined. Fiji: 4 poorly preserved females(4.2 × 3.3–6.0 × 4.6 mm) (ZRC 2019.1878), intertidal,
associated with Anadara sp., coll. K. Kawai, August 2018.
Indonesia: 1 female (9.4 × 8.4 mm) (ZRC 2019.1025), Teluk
Kodek, Lombok, intertidal, in Anadara antiquata (Linnaeus,
1758) (Arcidae), coll. S. Dwiono, 12 November 2013.
Description. Carapace and pereopods well chitinised.
Female: Carapace subcircular, wider than long; dorsal andlateral surfaces smooth, glabrous; appearing domed in frontal
view; front projecting anteriorly beyond orbits, margin gently
convex to almost straight; anterolateral margin smooth,subparallel with frontal margin or gently sloping posteriorly,
forming rounded angle with posterolateral margin (Fig. 23).
Eyes clearly visible in dorsal view in adults; mobile, filling
orbit (Fig. 23). Epistome with median part low, triangular,
lateral margins wide, gently concave (Komai et al., 2020:
figs. 7D, 8B).
MXP3 outer surface with scattered short setae; ischiomerus
completely fused, subrhomboidal, inner margin rounded togently angular at widest point; carpus short; propodus about
3 times as long as high, subspatulate, longer than carpus,
tip rounded; dactylus digitiform, inserted submedially on
flexor margin of propodus, tip reaching end of or slightly
overreaching propodal apex; exopod relatively slender, about
two-thirds length of ischiomerus, flagellum 2-segmented(Fig. 24A, B).

Distribution. Western Pacific to the eastern Indian Ocean,from Palau and Japan to the South China Sea including
the Philippines, Indonesia, Borneo, Malaysia, Singapore,
Thailand, and eastern India.

Chela short, stout, dactylus about two-thirds palm length;
palm relatively slender, medially widest; outer surfaces ofpalm, fingers (except for distal part) almost glabrous, with
only scattered short setae; ventral margin of palm gently
convex, with low bulge near midlength; dactylus occlusal
margin with 1 subproximal tooth; pollex occlusal margin
with 1 low sharp proximal tooth and denticles; tips of fingers
sharp, hooked (Fig. 24C).

Arcotheres ocularius Komai, Kawai & Ng, 2020
(Figs. 23, 24)
Arcotheres ocularius Komai, Kawai & Ng, 2020: 232, figs. 7–17
[type locality: Fiji]; De Gier & Becker, 2020: tab. 1.

Type material. Holotype: ovigerous female (11.8 × 10.2 mm)(CBM-ZC 15952), Kumi Village, Viti Levu, Fiji, 17°53.21′S,
178°36.07′E, intertidal, with Anadara sp. (Bivalvia: Arcidae),
16 August 2010, coll. K. Kawai. Paratypes: 1 female (8.9
× 8.2 mm), 3 ovigerous females (9.6 × 8.4 mm, 10.9 × 9.7mm, 12.3 × 11.9 mm) (ZRC 2019.1877), coll. 16 August
2010; 2 females (5.5 × 5.0 mm, 6.9 × 6.4 mm), 17 ovigerous
females (8.5 × 7.4–13.4 × 9.6 mm) (CBM-ZC 15953), coll.
16 August 2010; 1 male (6.2 × 6.8 mm) (CBM-ZC 15954);
1 female (5.2 × 5.2 mm) (CBM-ZC 15956), coll. 16 August
2010; 1 male (4.6 × 5.0 mm) (CBM-ZC 15957), coll. 16
August 2010; 2 males (3.2 × 3.2 mm, 3.3 × 3.4 mm) (CBMZC 15958), coll. 16 August 2010; 1 male (5.8 × 5.8 mm)

P2–P5 dorsally, ventrally unarmed; outer surface covered with
scattered, very short setae or glabrous; ventral margins ofpropodus and dactylus slightly more setose; merus relatively
short, stout, relative lengths of meri P4>P3>P2>P5; left
(sometimes right) P4 distinctly the longer; P2 and P3 dactyli
short, subequal, tip gently hooked, half length of propodus;
longer P4 dactylus elongate, almost straight, sparsely setose,
longer than P5 dactylus; P5 merus 4.1–4.4 times longer thanwide; P5 dactylus shorter than longer P5 dactylus, longer
than shorter P4 dactylus, margins lined with short and longsetae, denser on ventral margin, distoflexor margin without
rows of spinules (Fig. 24D–K).

168

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 23. Arcotheres ocularius Komai, Kawai & Ng, 2020, overall dorsal view. A, paratype ovigerous female (10.9 × 9.7 mm) (ZRC
2019.1877), Fiji; B, female (9.4 × 8.4 mm) (ZRC 2019.1025), Indonesia.

Pleon extending to buccal region, covering bases of P2–P5;
telson gently recessed into concave distal margin of somite
6 (Komai et al., 2020: fig. 8D, E).

to sinuous (Komai et al., 2020: figs. 12, 13A, 15A). Eyes
distinctly visible in dorsal view (Komai et al., 2020: figs.
12, 13A, 15A). MXP3 as in female except dactylus justreaching tip of propodus (Komai et al., 2020: figs. 13C,
16A). Anterior thoracic sternum wide, sternites 1, 2 fused,partially sunken into buccal cavity; suture between sternites
2 and 3 shallow; sternites 3, 4 completely fused, separated

Male: Carapace almost circular, slightly longer than wide;
dorsal surface almost smooth, gently inflated, lateral surfaces
with setae; front projecting anteriorly, margin gently convex
169

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 24. Arcotheres ocularius Komai, Kawai & Ng, 2020. A, C–K, paratype female (10.9 × 9.7 mm) (ZRC 2019.1877), Fiji; B, female(9.4 × 8.4 mm) (ZRC 2019.1025), Indonesia. A, B, left MXP3; C, outer view of right chela; D–G, left P2–P5, respectively; H–K, rightP2–P5, respectively. Scales = A, B, 0.5 mm; C–K, 1.0 mm.

only by shallow grooves (Komai et al., 2020: figs. 13D, 15B).
Chela relatively stout, shorter than in female (Komai et al.,2020: figs. 14A, 16B). P2–P5 dorsally, ventrally unarmed;
outer surface covered with short setae; P3 and P4 carpus andpropodus with long natatory setae; left and right meri equal,
relative lengths of meri P4>P3>P2>P5; dactyli of P2–P4
progressively longer; left P4 dactylus slightly longer, more

slender than right side; P4 and P5 dactylus subequal to thatof P3, covered with short setae (Komai et al., 2020: figs.
14C–G, 15A, 16D–H). Pleon slender, triangular, widest at
somite 3, lateral margins of somite 4 gently sinuous to almost
straight; somite 6 subtrapezoidal; telson subhexagonal, muchwider than long (Komai et al., 2020: figs. 13D, H, 15F). G1
relatively stout, arcuate, curved outwards, tip subtruncate,170

RAFFLES BULLETIN OF ZOOLOGY 2022
Arcotheres similis (Bürger, 1895)(Figs. 25–33)

without elongate process (Komai et al., 2020: figs. 13I, J,
15G, H). G2 short, with spatuliform tip; exopod slightly
longer than endopod (Komai et al., 2020: figs. 13K, 15I).

Pinnotheres similis Bürger, 1895: 373, 374, pl. 9 fig. 14 [typelocality: Ubay, Philippines]; Tesch, 1918: 250 (list); Silas &Alagarswami, 1967: 1210 (list); Serène, 1968: 94; Schmitt etal., 1973: 86.
Pinnotheres modiolicola Bürger, 1895: 370, pl. 9 fig. 9, pl. 10 fig.
9 [type locality: Philippines, from Modiolus philippinarium];
Lanchester, 1900: 761 (Singapore); Estampador, 1937: 5 (list).
[New synonymy]
Pinnotheres kamensis Rathbun, 1909: 110; 1910: 335, figs. 18, 19;
Tesch, 1918: 249, 252; Suvatti, 1938: 69; 1950: 159; Silas &Alagarswami, 1967: 1200, 1223, 1225; Schmitt et al., 1973:
50; Naiyanetr, 1980: 42; 1998: 104; 2007: 11; De Gier &Becker, 2020: tab. 1.
Pinnotheres modiolicolus — Tesch, 1918: 249 (list); Silas &Alagarswami, 1967: 1203 (list); Serène, 1968: 94; Schmitt etal., 1973: 57; Dev Roy, 2013: 156, 157.
Pinnotheres spinidactylus Gordon, 1936: 169, figs. 1, 2; Serène,
1968: 94; ?Griffin & Campbell, 1969: 157, figs. 7, 8; Stephenson
et al., 1970: 492; Schmitt et al., 1973: 88; Jones, 1990: 202;
Davie, 2002: 428, fig. a, b, 434.
Arcotheres spinidactylus — Campos, 2001: 494; Ahyong & Brown,
2003: 10; Ng et al., 2008: 248; Ng et al., 2017: 1094; Trivedi
et al., 2018a: 197, 248; De Gier & Becker, 2020: tab. 1.
Arcotheres similis — Ahyong & Ng, 2007b: 207, fig. 14; Ng etal., 2008: 248; Ng et al., 2017: 1094; Trivedi et al., 2018a:
197; De Gier & Becker, 2020: fig. 8B, tab. 1.
Arcotheres modiolicola — Ahyong & Ng, 2007b: 199, fig. 7; Ng
et al., 2008: 248; Ng et al., 2017: 1094; Trivedi et al., 2018a:
197; De Gier & Becker, 2020: tab. 1.

Variation. Komai et al. (2020: 241) noted that the carapace
shape varied from broadly ovate (Komai et al., 2020: fig.
7A) to almost circular (Komai et al., 2020: fig. 7B, C). The
condition of the dorsally exposed eyes and dactylus reachingend of or slightly overreaching MXP3 propodus, however,were always reliable.
Colour. “Entirely whitish; female ovary, visible throughintegument, orange; cornea of eye red” (Komai et al., 2020:
241).
Host. Known from Anadara sp. in Fiji and Anadara antiquata(Linnaeus, 1758) in Lombok (Arcidae Lamarck, 1809).
Remarks. Komai et al. (2020) discussed the taxonomy of A.
ocularius at length, differentiating it from the morphologicallysimilar A. palaensis s. str. in the more strongly chitinised
carapace and pereopods, eyes that are clearly visible indorsal view in adult females, the prominently projecting
front, the MXP3 dactylus being always long and reachingor overreaching the propodus apex, the male telson being
subpentagonal or subhexagonal, and the G1 with a subtruncate
tip (Figs. 23, 24A, B; Komai et al., 2020: figs. 7A–D, 8C,
F, G, 10B, E, 13D, H–J, 15F–H, 16A). They also notedthat unpublished genetic studies by Tsang Ling Ming fromthe Chinese University of Hong Kong show both species
to be distinct. Arcotheres ocularius is known from Fiji and
Lombok, Indonesia, but not recorded from the main Sunda
Shelf where A. palaensis is common. Both species, however,are present in Lombok.

Type material. Holotype: female (8.3 × 6.4 mm) (SMF-ZMG
956), Ubay, Philippines, coll. C. Semper. Philippines: 1
damaged ovigerous female (8.0 × 6.5 mm) (SMF-ZMG 168),from Modiolus philippinarum, coll. C. Semper (holotypeof Pinnotheres modiolicola Bürger, 1895). Thailand: 1
juvenile male (1.8 × 1.6 mm) (NHMD CR-9396), west of
Koh Kam, Gulf of Thailand, 9.1 m, coll. Th. Mortensen, 6
February 1900 (holotype of Pinnotheres kamensis Rathbun,1909). Singapore: 1 ovigerous female (6.7 × 5.5 mm)(NHM 1936.6.19.2), Siglap, from Modiolus philippinarum,coll. R. Winckworth (holotype of P. spinidactylus Gordon,
1936); 1 male (3.3 × 3.3 mm), 4 ovigerous females (5.8 ×
4.7 mm, 6.7 × 5.8 mm, 5.7 × 4.9 mm, 6.3 × 5.3 mm) (NHM
1936.6.19.3–7), Siglap, from Modiolus philippinarum, coll.
R. Winckworth (paratypes of P. spinidactylus Gordon, 1936).

Komai et al. (2020: 244) observed that in the figure of
the lectotype of A. arcophilus in Ahyong & Ng (2007b:
fig. 2A) (present Figs. 6A, 19A), the “female P4 merus is
proportionately longer in A. ocularius” than in A. arcophilus.
We have re-examined the lectotype of A. arcophilus (atpresent synonymised under A. palaensis) and the P4 merus
is actually similar to A. ocularius and more like the condition
in a more recent female of A. palaensis from Indonesia (Figs.
6C, 19E–L). In the protruding front and dorsally visible eyesin adult females, A. ocularius resembles some specimensof A. palaensis, but the carapace of A. ocularius is always
more strongly chitinised, the MXP3 dactylus always longand overreaching the propodus apex, the longer female P4
merus relatively longer, the male telson subpentagonal or
subhexagonal, and the G1 with a subtruncate tip (Figs. 23,
24A, B; Komai et al., 2020: figs. 7A–D, 8C, F, G, 10B, E,
13D, H–J, 15F–H, 16A).

Other material examined. Singapore: 2 males (4.0 × 4.3
mm, 4.8 × 5.0 mm) (NHM 1936.6.19.14–15), Siglap, coll.
R. Winckworth; 1 female (9.3 × 7.8 mm) (ZRC 2017.1034),
from Modiolus philippinarum, intertidal area, off Changi
beach, next to Carpark 7, near ferry terminal, coll. S. K.
Tan et al., 8 June 2016; 3 ovigerous females (7.1 × 5.9mm, 7.6 × 6.4 mm, 8.3 × 6.7 mm) (ZRC 2017.1035), from
Modiolus philippinarum, intertidal area, off Changi beach,next to Carpark 7, near ferry terminal, coll. S. K. Tan etal., 3 August 2012; 1 male (5.5 × 5.8 mm), 5 females, 1juvenile female (ZRC 2017.1264), 1 female (AM P102287),
from Modiolus philippinarum, intertidal area, off Changi

Distribution. Western Pacific, from Fiji to southeastern
Indonesia.

171

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 25. Arcotheres similis (Bürger, 1895). A, ovigerous female (9.3 × 7.8 mm) (ZRC 2017.1034), Changi, Singapore; B, C, two females(ZRC 2017.1264) in situ on Modiolus philippinarum (Hanley, 1843) (Mytilidae); D, ovigerous female (7.9 × 6.3 mm) (ZRC 2017.1037),
Tuticorin, India; E, female (6.5 × 5.2 mm) (ZRC 2013.1429), in situ on Modiolus philippinarum (Hanley, 1843) (Mytilidae), Pulau Ubin,
Singapore. Photographs: A–C, E, Tan Heok Hui; D, R. Ravinesh.

beach, next to Carpark 7, near ferry terminal, coll. S. K.
Tan et al., 26 June 2017; 1 female (6.5 × 5.2 mm) (ZRC
2013.1429), in Modiolus philippinarum, Pulau Ubin, coll. S.
K. Tan et al., 6 March 2012; 1 female (8.6 × 6.8 mm) (ZRC
2017.1036), Tanah Merah Besar beach, coll. R. U. Gooding,

3 April 1966; 4 shrivelled females (largest ca. 7.6 × 6.3 mm)(ZRC 1965.11.24.42–45), from “Pinna sp.”, coll. February
1938; 4 females (ZRC 1965.11.24.42–45), “in Pinna”,
Pulau Senang, Southern Islands, coll. 1930s; 2 females (8.8
× 8.1 mm, 5.8 × 5.1 mm) (ZRC 2018.785), 1 male (4.8 ×
172

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 26. Arcotheres similis (Bürger, 1895), colour in life. A, B, ovigerous female (8.4 × 7.1 mm) (ZRC 2021.79), Pulau Ubin, Singapore;C, D, male (5.5 × 5.5 mm) (ZRC 2021.79), Tuticorin, India. Photographs: Paul Y. C. Ng.

4.8 mm), 2 females (9.5 × 7.7 mm, 8.8 × 7.7 mm) (AMP105906), from Modiolus philippinarum, intertidal area, off
Changi beach, next to Carpark 7, near ferry terminal, coll.
S. K. Tan et al., 25 July 2018; 2 males (4.7 × 4.9 mm, 4.5
× 4.3 mm), 3 females (larger 9.2 × 7.7 mm, 6.3 × 4.7 mm),
1 ovigerous female (6.3 × 5.2 mm) (ZRC 2018.1369), from
Modiolus philippinarum, intertidal area, off Changi beach,next to Carpark 7, near ferry terminal, coll. S. K. Tan et al.,25 December 2018 [largest pair male 4.7 × 4.9 mm, female
9.2 × 7.7 mm from 100.4 mm long Modiolus]; 5 males, 13
females (ZRC 2018.1389), from Modiolus philippinarum,
intertidal area, off Changi beach, next to Carpark 7, near
ferry terminal, coll. S. K. Tan et al., 25 December 2018;
9 males, 19 ovigerous females, 6 females (2 females withbopyrids under pleon) (ZRC 2019.534), from Modiolus
philippinarum, intertidal area, off Changi beach, next to
Carpark 7, near ferry terminal, coll. S. K. Tan et al., 22 April
2019; 6 males, 1 juvenile female, 25 ovigerous females (ZRC
2019.578), from Modiolus philippinarum, intertidal area, off
Changi beach, next to Carpark 7, near ferry terminal, coll.
S. K. Tan et al., 4 July 2019; 1 male, 22 females (ZRC
2019.579), from Modiolus philippinarum, intertidal area,
off Changi beach, next to Carpark 7, near ferry terminal,coll. S. K. Tan et al., 5 July 2019; 1 ovigerous female (ZRC
2021.16), station DW57, probably from Modiolus sp., eastof Pulau Tekong, 01°25.342′N, 104°04.775′E–01°24.949′N,
104°05.080′E, coll. Comprehensive Marine Biodiversity
Survey, 22 October 2012; 1 male (5.5 × 5.5 mm), 10
females (1 photographed: 8.4 × 7.1 mm) (ZRC 2021.79),
in Modiolus philippinarum, Changi Bay, ex Red Cliff Shoalarea, Singapore, coll. S. K. Tan et al., 13 January 2021; 3
females (1 dismembered, poor condition) (ZRC 2013.574),

no host record, East Coast Park, Bedok Jetty, Singapore, coll.
S. Y. Chan, 31 July 2012. Peninsular Malaysia: 3 females(ZRC 2017.1282), no other data. Borneo: 1 female (7.3 ×
6.0 mm; soft) (ZRC 1965.11.24.47), in Vulsella vulsella, on
deep sea cable, 45 fathoms, South China Sea, near Sarawak,
4°26′42″N, 112°16′55″E, coll. 16 October 1933. Indonesia:
3 males (largest 5.7 × 5.6 mm), 16 females (largest 8.5 × 7.0mm) (ZRC 2019.1026), in sand at Ekas, Lombok, probably
discarded by artisanal shell collectors, Indonesia, coll. D. L.
Rahayu, 14 May 2007. India: 5 females (7.1 × 5.8 mm, 7.3
× 6.0 mm, 7.9 × 6.3 mm, 8.0 × 6.7 mm, 8.1 × 6.7 mm) (ZRC
2017.1037), from Modiolus philippinarum, Tuticorin, Tamil
Nadu, coll. R. Ravinesh, March 2017. Uncertain locality: 5
females (6.3 × 4.8 mm, 7.4 × 5.6 mm, 8.6 × 6.9 mm, 9.6 ×
7.5 mm, 9.9 × 7.9 mm) (ZRC 1965.11.24.9–13), unidentified
location, possibly from South Island, Cocos-Keeling Islands,
Indian Ocean Territory, label badly damaged.
Description. Carapace and pereopods well chitinised.
Female: Carapace subcircular, wider than long; dorsal andlateral surfaces smooth, glabrous; appearing domed in frontal
view; front produced slightly anteriorly beyond orbits, margin
gently concave to gently sinuous; anterolateral marginsubparallel with frontal margin or gently sloping posteriorly
to varying degrees, forming rounded angle with posterolateral
margin (Figs. 27A, C, D, 28A, C, D, 29). Eyes usually justvisible in dorsal view in adults; filling orbit (Figs. 27A, 29C,
D). Epistome with median part triangular, lateral margins
gently concave (Figs. 27D, 28D, 30K).
MXP3 outer surface with scattered short setae; ischiomerus
completely fused, subovate, inner margin usually rounded173

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 27. Arcotheres similis (Bürger, 1895), female (6.7 × 5.5 mm) (NHM 1936.6.19.2), Singapore (holotype female of Pinnotheres
spinidactylus Gordon, 1936). A, overall dorsal view; B, ventral view of cephalothorax; C, dorsal view of carapace; D, frontal view of
cephalothorax; E, outer view of right chela.

174

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 28. Arcotheres similis (Bürger, 1895), ovigerous female (9.3 × 7.8 mm) (ZRC 2017.1034), Changi, Singapore. A, overall dorsal view;
B, ventral view of cephalothorax; C, dorsal view of carapace; D, frontal view of cephalothorax.

even at widest point; carpus short; propodus about 3 times
as long as high, subspatulate, distinctly longer than carpus,
tip rounded to subtruncate; dactylus slender, inserted slightly
proximal to midlength of propodus, tip not reaching propodal
apex; exopod relatively slender, about two-thirds length ofischiomerus, flagellum 2-segmented (Figs. 30J, 33C, D).

Chela not prominently elongate, dactylus about two-thirds
palm length; palm relatively slender, proximally narrower
than distally or subequal; outer surfaces of palm, fingers(except for distal part) almost glabrous, with only scatteredshort setae; ventral margin of palm gently sinuous to almost
straight; dactylus occlusal margin with distinct subproximal175

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 29. Arcotheres similis (Bürger, 1895). A, female (8.1 × 6.7 mm) (ZRC 2017.1037), Tuticorin, India; B, female (8.0 × 6.7 mm) (ZRC
2017.1037), Tuticorin, India; C, female (7.4 × 5.6 mm) (ZRC 1965.11.24.10), Cocos-Keeling Islands (?); D, female (6.3 × 4.8 mm) (ZRC
1965.11.24.9), Cocos-Keeling Islands (?). A, B, overall view of carapace; C, D, dorsal view of carapace.

tooth; pollex occlusal margin with 1 low proximal tooth anddenticles; tips of fingers sharp, hooked (Figs. 27E, 30I).

Male: Carapace longitudinally ovate, slightly longer thanwide; dorsal surface appears punctate to smooth, gently
inflated, dorsal surfaces convex, lateral surfaces with setae;
front projecting anteriorly, margin sinuous to almost straight(Figs. 31A, B, 32A). Eyes distinctly visible in dorsal view
(Figs. 31A, 32A). MXP3 similar to that of female, althoughischiomerus with slightly more angular inner distal margin(Fig. 32C). Anterior thoracic sternum wide, sternites 1, 2
fused, partially sunken into buccal cavity; suture between
sternites 2 and 3 shallow; sternites 3, 4 completely fused,
separated only by shallow grooves. Chela relatively stout,
shorter than in female (Fig. 31C). P2–P5 legs dorsally,
ventrally unarmed; outer surface covered with short setae;
P3 and P4 carpus and propodus with long natatory setae; left
and right meri equal, relative lengths of meri P4>P3>P2>P5;
dactyli of P2–P4 progressively longer, flexor margin withsetae and row of small spinules; right P4 dactylus slightly
longer, more slender than left side; P4 and P5 dactylus
subequal or longer to that of P3, covered with short setae;
P5 dactylus with 2 rows of distoflexor spinules, one row
sometimes low, indistinct (Fig. 32D–K’). Pleon slender,
triangular, widest at somite 3 with distinctly convex lateral
margins, lateral margins of somite 4 gently concave; somite
6 subtrapezoidal; telson semicircular with convex margins,
wider than long (Fig. 32B). G1 relatively stout, arcuate,

P2–P5 merus–propodus dorsally, ventrally unarmed; outersurface covered with scattered, very short setae or glabrous;
ventral margins of propodus and dactylus slightly moresetose; merus relatively longer, more slender, relative
lengths of meri P4>P3=P2>P5; right (sometimes left) P4
distinctly the longer; P2 and P3 dactyli subequal, weakly
curved, exceeding two-thirds length of propodi, with orwithout flexor row of spinules; longer P4 dactylus weakly
falciform, slightly shorter than propodus, with or without
flexor row of spinules; P5 merus 4.6–5.2 times longer thanwide; P5 dactylus longer than longer P4 dactylus, longer thanpropodus, margins lined with short and long setae, denser
on ventral margin, distal part lined with 2 distinct rows of
distoflexor spinules, upper row with 5–10 spinules, lower
row with 8–31 spinules, increasing in size distally, lower
row always with distinctly more spinules than upper row
in adults (Fig. 30A–H’).
Pleon extending to buccal region, covering bases of P2–P5;
telson gently recessed into concave distal margin of somite
6 (Figs. 27B, 28B).

176

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 30. Arcotheres similis (Bürger, 1895). A–K, ovigerous female (9.3 × 7.8 mm) (ZRC 2017.1034), Singapore; L, ovigerous female(6.7 × 5.5 mm) (NHM 1936.6.19.2), Singapore (holotype of Pinnotheres spinidactylus Gordon, 1936). A–D, left P2–P5, respectively; D’,distal part of dactylus of left P5 showing stiff subdistal spinules (subventral view, soft setae omitted); E–H, right P2–P5, respectively;
H’, L, distal part of dactylus of right P5 showing stiff subdistal spinules (subventral view, soft setae omitted); I, outer view of left chela
and carpus; J, outer view of left MXP3 (setae omitted); K, posterior margin of epistome. Scales = A–H, I, 1.0 mm; J, K, 0.5 mm; D’,
H’ = 0.25 mm; L = 0.10 mm.

177

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 31. Arcotheres similis (Bürger, 1895). A–C, male (5.5 × 5.8 mm) (ZRC 2017.1264), Singapore; D, male (3.3 × 3.3 mm) (NHM
1936.6.19.3), Singapore (paratype of Pinnotheres spinidactylus Gordon, 1936). A, D, overall dorsal view; B, frontal view of cephalothorax;
C, outer view of right chela.

Colour. In life, the colour is light brown to whitish brownoverall, with the pereopods relatively lighter in colour (Figs.
25, 26).

curved outwards, tip produced to differing lengths as shortprojection in line with main axis (Figs. 32L–N, 33E–H, J,
K). G2 short, with spatuliform tip; exopod curved, two-thirds
endopod length (Figs. 32O, 33I).

Host. Almost always from Modiolus philippinarum (Hanley,
1843) (Mytilidae Rafinesque, 1815). The one exception isa specimen (ZRC 1965.11.24.47) recorded from Vulsellavulsella (Linnaeus, 1758) (Pteriidae Gray, 1847). This record
from Vulsella, however, is highly unusual not only becauseof the disparate taxonomic and phylogenetic position of thehosts (Combosch et al., 2017), but because it was taken at 45

Variation. The eyes of A. similis are usually partially visible
in dorsal view (Fig. 27A), although sometimes completely
hidden (Figs. 28A, 29).

178

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 32. Arcotheres similis (Bürger, 1895), male (5.5 × 5.8 mm) (ZRC 2017.1264), Singapore. A, dorsal view of carapace; B, pleon (setae
omitted); C, left MXP3 (setae denuded); D–G, left P2–P5, respectively (setae omitted); H–K, right P2–P5, respectively; G’, distal partof dactylus of left P5 showing stiff subdistal spinules (dorsal view, soft setae omitted); K’, distal part of dactylus of right P5 showing
stiff subdistal spinules (dorsal view, soft setae omitted); L, left G1 (ventral view); M, N, left G1 (dorsal view, setae omitted); O, left G2.
Scales = A, D–K, 1.0 mm; B, C, L–O, 0.5 mm; G’, K’, 0.25 mm.

of P. modiolicola, the minimal original description of P.
kamensis, and the confused identity and mixed material of
P. similis reported by Gordon (1936) (see Ahyong & Ng,
2007b; Ng, 2018a). Griffin & Campbell (1969: 161) had
already noted the strong similarities between P. modiolicola
and P. spinidactylus, and Ahyong & Ng (2007b: 207), in
redescribing A. similis, observed that it may be synonymouswith A. spinidactylus, although the number of distoflexorrows of spinules on the P5 dactylus seemingly differed (onerow, based on Gordon’s 1936 account). Clarification of theidentity of these four nominal species has only been possible
following restudy of their respective type specimens. Reexamination of Gordon’s (1936) material of P. spinidactylus

fm (82 m) depth on an undersea cable, significantly deeper
than any known record of A. similis, which is otherwise
known only from intertidal and shallow subtidal depths. The
accuracy of the Vulsella host record requires corroboration;
it may represent an atypical vagrant association given theunusual habitat.
Remarks. We consider Pinnotheres similis Bürger, 1895,
Pinnotheres modiolicola Bürger, 1895, Pinnotheres kamensis
Rathbun, 1909, and Pinnotheres spinidactylus Gordon, 1936,
to all represent the same species, Arcotheres similis. The
confusion has stemmed from the combination of incomplete
descriptions by Bürger (1895), the badly damaged type female179

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 33. Arcotheres similis (Bürger, 1895). A, B, ovigerous female (6.7 × 5.5 mm) (NHM 1936.6.19.2), Singapore (holotype of Pinnotheres
spinidactylus Gordon, 1936); C, holotype female (8.3 × 6.4 mm) (SMF-ZMG 956 Go 950a), Philippines (holotype of Pinnotheres similis
Bürger, 1895); D, female (7.3 × 6.0 mm; soft) (ZRC 1965.11.24.47), Sarawak; E, F, male (3.3 × 3.3 mm) (NHM 1936.6.19.3), Singapore;
G–I, male (4.8 × 5.0 mm) (NHM 1936.6.19.14), Singapore; J, K, male (5.7 × 5.6 mm) (ZRC 2019.1026), Lombok, Indonesia; L, male
(3.3 × 3.3 mm) (NHM 1936.6.19.3), Singapore (paratype of P. spinidactylus Gordon, 1936). A, left MXP3 (inner view); B, palp of left
MXP3 (outer view); C, D, right MXP3 (outer view); E, G, J, left G1 (ventral view); F, distal part of left G1 (ventral view); H, K, distal
part of left G1 (dorsal view); I, left G2; L, male pleonal somites 5, 6 and telson. A, B, E, F, after Gordon (1936: figs. 1c, 2a); C, after
Ahyong & Ng (2007: fig. 14C). Scales = D, G–K, 0.5 mm; L, 0.2 mm.

Pinnotheres similis Bürger, 1895, and Pinnotheres
modiolicola Bürger, 1895, were described in the same
paper (Bürger, 1895) so both names must be consideredas simultaneously published under Article 24 of the ICZN
(1999). As first revisers, we here select Pinnotheres similis
Bürger, 1895, to have priority over P. modiolicola Bürger,
1895, when they are regarded as synonyms. We select P.
similis to have priority because it is the more widely citedspecies and because its holotype is still in good condition,
whereas that of P. modiolicola is badly damaged (Ahyong& Ng, 2007b).

revealed that two rows of distoflexor spinules are present
on the P5 dactylus in both sexes (Fig. 30L), as in A. similis(Figs. 30D’, H’, 32G’, K’). Ahyong & Ng (2021) showedthat A. similis is a senior synonym of A. spinidactylus andA. kamensis. Likewise, our reassessment of A. modiolicola
finds it to be indistinguishable from A. similis. Note that
Ahyong & Ng (2007b) incorrectly indicated the presenceof a single row of P5 dactylar spines, rather than two. The
present reappraisal, which has examined types of all four
nominal species, indicates they are conspecific.

180

RAFFLES BULLETIN OF ZOOLOGY 2022
Gordon (1936: 169) described Pinnotheres spinidactylus on
the basis of five females and one male from Siglap, Singapore,
all from the mytilid Modiolus philippinarum (Hanley, 1843).
She compared her new species with Pinnotheres latus Bürger,
1895 (now in Magnotheres, new genus), Pinnotheres parvulus
Stimpson, 1858, the Chinese Pinnotheres sinensis Shen,
1932 (now in Arcotheres), Pinnotheres tsingtaoensis Shen,
1932 (now in Nepinnotheres, see Remarks for Plenotheres,new genus), and Pinnotheres gordoni Shen, 1932. The
excellent series of specimens from Singapore agrees withthe descriptions and figures of Gordon (1936) very well.
The G1 figured by Gordon (1936: fig. 2c) agrees well in
general shape with that of the present male except that the
distal tip is straight (versus gently curved inwards; Fig. 33E,
F). In any case, the distal tip varies in length as well, fromtypically short (Fig. 32L–N) to sometimes long (Fig. 33G,
H, J, K). Gordon’s (1936) P. spinidactylus is referable to
A. similis, but her record of P. similis from Singapore isreferable to A. placunicola (cf. Ng, 2018a).

and the difference in pleon shape may be a function of its
immaturity, but we are of the opinion that it is actually an
immature female, especially since the authors describe it
as having “two pairs of biramous pleopods”. The G1 ofa 3.3 × 3.3-mm male (NHM 1936.6.19.3) from Singapore
was already developed and clearly visible, so Griffin &Campbell’s “male” specimen and its pleon is similar to thatof larger males, except that somite 6 is relatively shorter withthe lateral margins of somite 6 gently concave (Fig. 33L).
We also note that in the immature female of A. palaensis, thelateral margins of pleonal somite 6 are prominently convex(Fig. 15C), unlike that of similarly sized males which havethe margins gently convex to sinuous (Fig. 22J, O). Theseobservations suggest that Griffin & Campbell’s male of cw
3.1 mm is almost certainly a juvenile female instead. Without
re-examination of the pleopods, however, we cannot becertain, but on the basis of the other female characters and its
host, we provisionally include Griffin & Campbell’s (1969)
record in the synonymy of P. similis; this Australian record
will need to be confirmed with fresh collections, especiallysince Queensland is some distance from the nearest confirmed
records in Southeast Asia. Australian records of A. similis
by Ward (1967) and Ahyong & Brown (2003) are actuallyreferable to M. globosus, new combination (see account ofM. globosus below; Ahyong, 2020b); that of Rathbun (1924)
requires verification (Ahyong, 2020b).

Of the males available to Gordon (1936: 169), only thesmallest male (3.3 × 3.3 mm; NHM 1936.6.19.3) was referredto the type series (Fig. 31D). She noted that the “two other[non-type] male specimens, measuring about 4.4 and 5.4
mm in length respectively [NHM 1936.6.19.14–15], havethe second and third legs fringed with setae on the propodus
and carpus much as in P. tsingtaoensis Shen (1932, p. 149).
The abdomen, however, and the dactylus of the external
maxillipede are more like those of P. gordoni Shen (p. 152),
but the first pleopod is heavily setose on both sides andis straight, not curved, at the apex” (Gordon, 1936: 171).
We examined all three males and they belong to the same
species. The differences noted by Gordon (1936) can all be
accounted for by size-associated change, the paratype malebeing smallest and yet to exhibit fully adult features. Fully
adult male specimens have the G1 distinctly more setose(Figs. 32L, M, 33G, H), and ambulatory propodi and carpithat are also lined with long setae along the flexor margins(Fig. 31A). Gordon (1936: fig. 1c) did not remove the G1sfrom the males when she compared the G1, and their shapeis actually very similar (Fig. 33E–H), except that the distal
projection is relatively longer and the subdistal hump is
relatively higher in the larger male (Fig. 33G, H; cf. Fig.
33E, F).

George & Noble (1970: 392) recorded Pinnotheres
modiolicolus from Karwar and Kodibag in southwestern
India, but it is probably misidentified (see also Trivedi et al.,2018b: 61). They recorded the species from Marcia opima(Gmelin, 1791) (as a Katelysia Römer, 1857) (Veneridae)
and Mactra violacea Gmelin, 1791 (Mactridae), both fromthe order Venerida. Although the MXP3 figured (George& Noble, 1970: fig. 1.4) agrees with that of A. similis as
defined here (Figs. 30J, 32C, 33C, D), the G1 is different,with the tip truncated (George & Noble, 1970: fig. 1.5, 1.6)
and without the recurved slender part as is typical for the
species (Figs. 32L–N, 33G, H, J, K). The G1 differences,
together with the fact that they were from venerid clams andnot mytilids, indicates that their record is not correct. The
Karwar and Kodibag specimens will need to be re-examined
to determine their identity.
Dai et al. (1980: 131, fig. 3) reported “Pinnotheres
spinidactylus” from the mytilid Mytilus crassitesta Lischke,
1868, from Hainan Island. The figures in her paper (see also
Dai et al., 1986: 393, fig. 208 (1–7); Dai & Yang, 1991: 424,
fig. 208 (1–7)) superficially agree with A. similis. We have,
however, examined photographs of the Hainan specimens in
CAS, and while the carapace and pereopod features seemto agree with A. similis, there are two problems: the malepleon and G1 (neither figured by the above authors) arevery different from those of A. similis s. str., and indicatethat it is a different species. The male pleon of the Hainan
specimens has somite 6 and the telson proportionately moreelongate, and most significantly, the G1 is much longer andless curved, with the distal part very elongate and graduallytapering to a subtruncate tip. They are also not referable to
Arcotheres boninensis (Stimpson, 1858) or A. purpureus

Griffin & Campbell (1969: 157, figs. 7, 8) recorded“Pinnotheres spinidactylus” from two females and one malecollected from the mytilid, Modiolus, in Moreton Bay in
Queensland, Australia, commenting that although the leg
proportions appeared to differ slightly, the carapace shape,
front, and presence of distoflexor rows of spinules on the
P5 dactylus are diagnostic for the species. Their figures
agree closely with A. similis as characterised here except
for the pleon of a 3.1-mm cw male, figured with somite 6
proportionately broader and the lateral margins prominently
convex rather than gently concave (compared to the mature
5.5-mm cw male) (Fig. 32B versus Griffin & Campbell,
1969: fig. 8B), and gonopods described as “immature, twopairs of biramous appendages” (Griffin & Campbell, 1969:
159). The Griffin & Campbell (1969) male is a juvenile181

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia& Kutyamma, 1971), in Marcia cf. cordata, west coast of
Tamil Nadu, India, coll. R. Ravinesh, 2016.

(Alcock, 1900), which have a markedly wider, more rounded
carapace and relatively shorter P2–P5 (Ahyong & Ng, 2021).
Nevertheless, Dai et al. (1980, 1986) and Dai & Yang (1991)
recorded A. boninensis from China, and their figures conformto what we know about the species. The Chinese specimensof “Pinnotheres spinidactylus” will need to be restudied toascertain their true identity.

Other material examined. Peninsular Malaysia: 6 spentfemales (4.6 × 4.2–7.0 × 6.3 mm), 1 juvenile female (3.8
× 3.6 mm) (ZRC 2018.772), Chinatown market, Singapore,from Marcia recens, coll. P. K. L. Ng, 25 February 2018;
1 male (4.9 × 4.8 mm) (ZRC 2018.773), from veneridbought from Sheng Siong Supermarket, Singapore, coll. 27
September 2018; 2 males (3.4 × 3.4 mm, 3.7 × 3.7 mm), 1
ovigerous female (4.8 × 3.9 mm), 1 spent female (4.5 × 4.1mm), 1 ovigerous female (4.3 × 4.0 mm) (ZRC 2018.774),
in Dosinia sp., from local market, Singapore, coll. S. K. Tan,
10 December 2013; 1 male (4.7 × 4.6 mm), 1 ovigerous
female (6.9 × 5.6 mm), 3 spent females (5.4 × 4.6–8.2 × 7.4mm), 1 ovigerous female (4.5 × 4.0 mm) (ZRC 2018.775),
in Marcia sp., from market, Singapore, coll. S. K. Tan, 10
December 2013; 1 ovigerous female (ZRC 2018.1068),
from clam, Yew Tee Market, Singapore, coll. H. H. Tan,
9 September 2018; 1 male (4.3 × 4.2 mm), 1 ovigerous
female (6.9 × 6.1 mm) [photographed], 1 ovigerous female(7.9 × 7.3 mm) [photographed], 8 ovigerous females, 6spent females, 2 juvenile females (2.1 × 1.6 mm, 1.6 × 1.5mm) (ZRC 2019.516), from Marcia recens, from market,
Singapore, coll. P. Y. C. Ng, March 2019; 1 ovigerous
female (ZRC 2020.352), from Marcia recens, Yew Tee
Market, Singapore, coll. H. H. Tan, 2 June 2019; 1 male
(4.3 × 4.2 mm), 2 non-ovigerous females (5.7 × 4.9 mm, 5.7
× 5.1 mm), 2 ovigerous females (5.7 × 4.9 mm, 7.0 × 5.0mm) (ZRC 2020.14), in Marcia recens, from Sheng Shiong
Supermarket, Singapore, coll. S. K. Tan, 23 March 2020; 2
ovigerous females (ZRC 2020.66), in Marcia recens, from
Chinatown market, Singapore, coll. P. K. L. Ng, 4 June
2020. Singapore: 4 ovigerous females (6.2 × 5.2–7.0 × 6.3mm) (ZRC 2018.776), from kelong (artisanal fish traps), in“Lala clams” (probably Marcia recens), coll. B. Y. Lee,
25 March 2018; 2 ovigerous females (ZRC 2020.390), in“Lala clams” (Marcia sp.), in seafood dish, Ah Hua Kelong
Restaurant, Singapore, coll. M. S. Foo, 1 November 2020;
2 males (5.6 × 5.4 mm, 5.4 × 5.2 mm) (ZRC 2020.419),
from Meretrix meretrix, from Sheng Shiong Supermarket
in Bukit Batok, Singapore, coll. W. Xu, 12 February 2017;
1 ovigerous female (ZRC 2021.10), from clam in seafood
restaurant, Singapore, coll. students, 2005. Philippines: 1
female (ZRC 2021.17), in Ruditapes philippinarum, fromplate of noodles at seafood restaurant, purportedly from Korea
but probably from Philippines or adjacent area, coll. S. K.
Tan, 16 December 2020. India: 3 males (4.3 × 4.1 mm, 4.5
× 4.2 mm, 4.7 × 4.4 mm), 1 non-ovigerous female (8.0 ×
6.7 mm), in Marcia cf. cordata, Tamil Nadu, India, coll. R.
Ravinesh, 2016 (ZRC 2018.768); 1 juvenile female (4.0 × 3.6mm) (ZRC 2018.769), in Protapes cf. rhamphodes, Tamil
Nadu, coll. R. Ravinesh, 2016; 1 female (ZRC 2018.498),
male (8.3 × 5.5 mm), Meretrix aurora, Mumbai, Maharashtra
state, India, coll. S. Gosavi, 27 September 2018.

Distribution. Western Pacific to the central Indian Ocean,
including Australia, the Philippines, Indonesia, Singapore,
Malaysia, Thailand, eastern India, and possibly the CocosKeeling Islands.
Arcotheres exiguus (Bürger, 1895)(Figs. 34–45, 50A–G)
Pinnotheres exiguus Bürger, 1895: 377, pl. 9 fig. 19, pl. 10 fig. 30[type locality: Samar Island, Philippines]; Serène, 1968: 93.
Pinnotheres winckworthi Gordon, 1936: 177, fig. 7; Serène, 1968:
95. [New synonymy]
Pinnotheres vicajii Chhapgar, 1957: 505, 506, pl. 12 fig. n–q;
Chhapgar, 1958: 253, fig. 2; Silas & Alagarswami, 1967: 1212;
Schmitt et al., 1973: 81; Kumari & Rao, 1974: 165, figs. 1–19;
Ng et al., 2008: 251; Pati et al., 2012: 384; Dev Roy, 2013:
156, 157. [New synonymy]
Pinnotheres sp. — Silas & Alagarswami, 1967: 1163, text-figs.
1, 2, pls. 1–3.
Pinnotheres gracilis — George & Noble, 1970: 392, fig. 1.1–1.3[not Pinnotheres gracilis Bürger, 1895].
Pinnotheres casta Antony & Kutyamma, 1971: 60, figs. 1, 2; Schmitt
et al., 1973: 81; Dev Roy, 2013: 156, 157. [New synonymy]
Pinnotheres obscuridentata Dai & Song, 1986: 56, 62, text-fig. 3[type locality: Longmen, Guangxi, China]; De Gier & Becker,
2020: tab. 1. [New synonymy]
Arcotheres exiguus — Campos & Manning, 2001: 168; Ahyong& Ng, 2007b: 195, fig. 4; Ng et al., 2008: 248; Ng et al.,
2017: 1093.
Arcotheres winckworthi — Ng et al., 2008: 248; Ng et al., 2017:1093.
Arcotheres alcocki — Pati et al., 2015: 1067, fig. 1; Dev Roy &Rath, 2017: 103 (not Pinnotheres alcocki Rathbun, 1909).
Nepinnotheres vicajii — Ng & Kumar, 2015: 265; Trivedi et al.,2018b: 62.
Arcotheres casta — Ng & Kumar, 2015: 265; Ng et al., 2017:1094; Kazmi et al., 2018: 136, fig. 5; Trivedi et al., 2018a:
197; Trivedi et al., 2018b: 61 (list); Trivedi et al., 2020: 505.
Viridotheres gracilis — Dev Roy, 2013: 156, 157; Trivedi et al.,2018b: 62 (not Pinnotheres gracilis Bürger, 1895).
Arcotheres vicajii — Trivedi et al., 2020: 497–507, figs. 1–8; De
Gier & Becker, 2020: tab. 1.

Type material. Lectotype: female (5.6 × 4.6 mm), Samar
Island, Philippines, coll. C. Semper, 1876. Peninsular
Malaysia: 1 ovigerous female (9.3 × 7.6 mm) (NHM
1936.6.19.12), Penang, Malay Peninsula, from Protapes
gallus, coll. R. Winckworth, March 1933 (holotype of
Pinnotheres winckworthi Gordon, 1936); 1 ovigerous
female (8.8 × 7.3 mm) (NHM 1936.6.19.13), Penang, Malay
Peninsula, from Protapes gallus, coll. R. Winckworth, March
1933 (paratype of Pinnotheres winckworthi Gordon, 1936);
3 males (4.3 × 4.2–5.1 × 4.9 mm), 6 ovigerous females (6.3
× 5.4–7.7 × 6.7 mm). India: 1 ovigerous female (8.2 × 6.9mm) (ZRC 2021.784) (neotype of Pinnotheres casta Antony

Description. Female: Carapace and pereopods poorly
chitinised, soft. Carapace subcircular to subhexagonal, wider
than long; dorsal and lateral surfaces smooth, glabrous;
appearing domed in frontal view; front slightly projecting
182

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 34. Arcotheres exiguus (Bürger, 1895), colour in life and in situ in Marcia recens (Holten, 1802) (Veneridae), Peninsular Malaysia. A,
male (4.3 × 4.2 mm) and ovigerous female (6.3 × 5.4 mm) (ZRC 2018.772) (placed together for photograph but originating from different
shells); B, C, ovigerous female (6.3 × 5.4 mm) (ZRC 2018.772); D–F, male (4.3 × 4.2 mm) (ZRC 2018.772). Photographs: Paul Y. C. Ng.

anteriorly beyond orbits, margin gently sinuous to gently
convex; anterolateral margin usually subparallel with frontal
margin or sloping posteriorly to various degrees, forming
rounded angle with posterolateral margin (Figs. 34B, 35B,
E, 36C, 37A, C, D, 38A–C, 39A, C–E, 41A, N). Eyes
small, not or just visible in dorsal view in adults; mobile,completely filling orbit (Fig. 41A, N). Epistome with medianpart triangular, lateral margins gently concave (Figs. 38C,
39C, 41C).

length of ischiomerus, flagellum 2-segmented (Figs. 36E,
41B, Q, 42A, L).
Chela relatively short to not prominently elongate, dactylus
half to two-thirds palm length; palm relatively slender,
proximally narrower than distally; outer surfaces of palm,
fingers (except for distal part) almost glabrous, with only
scattered short setae; ventral margin of palm gently concave
to sinuous; dactylus occlusal margin with distinct subproximal
tooth; pollex occlusal margin with 1 low proximal tooth, 1
submedian tooth, and minute denticles; tips of fingers sharp,
hooked (Figs. 36F, 37E, 38D, E, 41E, 42B).

MXP3 outer surface with scattered short setae; ischiomerus
completely fused, subrhomboidal, inner margin rounded toangular at widest point; carpus short; propodus about 3 times
as long as high, subspatulate, distinctly longer than carpus,
tip rounded to subtruncate; dactylus slender, varying lengths,
inserted slightly proximal to midlength to near proximal
one-third of propodus, tip not reaching or just reaching
propodal apex; exopod relatively slender, about two-thirds

P2–P5 dorsally, ventrally unarmed; outer surface covered with
scattered, very short setae or glabrous; ventral margins ofpropodus and dactylus slightly more setose; merus relatively
longer, more slender, relative lengths of meri P4>P3>P5>P2;
left (sometimes right) P4 distinctly the longer; P2 and P3

183

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 35. Arcotheres exiguus (Bürger, 1895), colour in life. A, B, ovigerous female (7.9 × 7.3 mm) (ZRC 2019.516), in situ in Marcia
recens (Holten, 1802) (Veneridae), Peninsular Malaysia; C, male (4.3 × 4.2 mm) (ZRC 2019.516), Peninsular Malaysia; D, male (4.5 × 4.2mm) (ZRC 2018.768), India; E, F, ovigerous female, from Meretrix aurora Hornell, 1917 (Veneridae), India (specimens not examined).
Photographs: A–C, Paul Y. C. Ng; D–F, R. Ravinesh.

dactyli short, subequal, tip gently hooked, half propodus
length; longer P4 dactylus elongate, broadly falciform,
distinctly longer than half propodus length, slightly longer
than P5 dactylus; P5 merus 4.1–4.5 times longer than wide;P5 dactylus shorter to longer (in specimens > 6.5 mm cl) thanpropodus extensor margin, length/height > 5.0 in specimens> 6.0 mm cl, margins lined with short and long setae, denser
on ventral margin, distoflexor margin without distinct rowsof spinules, sometimes with a row or patches of very low,
spinule-like structures which are all similarly sized (Figs.
36C, D, 41F–M, O, P, 42C–K, M, 50A–G).

Pleon extending to buccal region, covering bases of P2–P5;
telson gently recessed into concave distal margin of somite
6 (Figs. 34C, 35F, 37B, 39B, 41D).
Male: Carapace and pereopods well chitinised, firm. Carapace
almost circular, slightly wider than long; dorsal surface
almost smooth, not prominently inflated, lateral surfaces
with setae; front projecting anteriorly, margin gently sinuous
to almost straight (Figs. 34D, 35C, D, 43A, B, 44A). Eyes
distinctly visible in dorsal view (Figs. 34D, 35C, D, 43A,
B, 44A). MXP3 as in female (Figs. 44B, 45A). Anterior

184

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 36. Arcotheres exiguus (Bürger, 1895). A, B, after Bürger (1895: pl. 9 fig. 19, pl. 10 fig. 30); C–F, lectotype female (5.6 × 4.6 mm)(USNM 32432), Samar, Philippines, after Ahyong & Ng (2007: fig. 4). A, C, overall dorsal view; B, E, right MXP3; D, left P5 propodus
and dactylus; F, right chela. Scale = B, 1.5 mm; C, 2.0 mm; E, 1.0 mm.

thoracic sternum wide, sternites 1, 2 fused, partially sunken
into buccal cavity; suture between sternites 2 and 3 shallow;
sternites 3, 4 completely fused, separated only by shallow
grooves. Chela relatively stout, shorter than in female(Figs. 34F, 44D, 45B). P2–P5 dorsally, ventrally unarmed;
outer surface covered with short setae; P3 and P4 carpus
and propodus with long natatory setae; left and right meriequal, relative lengths of meri P4>P3>P2>P5; dactyli of
P2–P4 progressively longer; P4 and P5 dactylus subequal tothat of P3, covered with short setae (Figs. 44F–I, 45C–F).

Pleon slender, triangular, widest at somite 3, lateral margins
of somite 4 gently concave to almost straight; somite 6
trapezoidal; telson semicircular, wider than long (Figs. 44J,
45G). G1 relatively stout, arcuate, curved outwards, with
distinct subdistal dorsal projection, tip rounded (Figs. 44K,
L, 45H, I). G2 short, with spatuliform tip; exopod about
half endopod length (Figs. 44M, N, 45J).
Variation. The length of the MXP3 dactylus varies
somewhat, from falling short of (Fig. 42A, C) to reaching185

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 37. Arcotheres exiguus (Bürger, 1895), ovigerous female (9.3 × 7.6 mm) (NHM 1936.6.19.12), Peninsular Malaysia (holotype of
Pinnotheres winckworthi Gordon, 1936). A, overall dorsal view; B, ventral view of cephalothorax; C, dorsal view of carapace; D, frontal
view of cephalothorax; E, outer view of right chela.

186

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 38. Arcotheres exiguus (Bürger, 1895). A, C–E, ovigerous female (7.7 × 6.7 mm) (ZRC 2018.772), Peninsular Malaysia; B, female(8.2 × 7.4 mm) (ZRC 2018.775), Peninsular Malaysia. A, B, overall dorsal view; C, frontal view of cephalothorax; D, outer view of rightchela; E, outer view of fingers of left chela.

as far as, but not beyond, the propodus (Figs. 36E, 41B).
In the lectotype female of A. exiguus, the MXP3 dactylus
is relatively long (Fig. 36E; Ahyong & Ng, 2007b: fig.
4C). The degree of variation highlights the importanceof a large series of specimens for accurate morphologicalcharacterisation. The MXP3 dactylus figured by Trivedi et al.
(2020: figs. 6L, 7A) for two Indian specimens is inaccurate
as the structure is actually much shorter (present Figs. 42C,
45A), albeit still within the known variation in this species.
The general features of adults vary little: the frontal margin
is invariably slightly protruding in dorsal view, whereas
the lateral margins are usually divergent but may be gently
convex and subparallel (Figs. 36C, 37A, 38A, B, 39A, D, E,
41A, N). The P5 distoflexor margin is usually not armed with
spinules, although it may be possible to sometimes discerna row of very low, spinule-like structures (e.g., Fig. 42M).
These structures, however, are unlike those in A. similisor A. placunicola because they are much weaker and all
similarly sized, with the distal ones not elongate. Trivedi
et al. (2020: fig. 4I) figured a P5 distoflexor margin (as A.
vicajii, here treated as a junior synonym of A. exiguus) thathas several prominent spinules, but these are not arranged
in a row and actually much smaller than drawn. As with A.
rayi, the slenderness of the P5 dactylus on A. exiguus varies
allometrically, becoming more slender with increasing bodysize. However, at any given size, the P5 dactylus is more
slender in A. exiguus than in A. rayi (Table 2; Fig. 50).
Colour. The colour is rather variable, with females usually
pale yellowish- to cream-white, but the gonads, whenmature, are bright orange (Figs. 34A–C, 35A, B). In somespecimens, however, the overall carapace and appendages
are pale yellow as well (Fig. 35E, F), the colour possibly
being a consequence of host metabolism or food. Males arewhite to orange, with numerous small black spots (Figs.
34D–F, 35C, D).
Host. Almost always from Protapes gallus (Gmelin, 1791),
Protapes cf. rhamphodes (Oliver & Glover, 1996), Marcia
recens (Holten, 1802), Marcia cf. cordata (Forskål, in
Niebuhr, 1775), Meretrix aurora Hornell, 1917, Meretrix
meretrix (Linnaeus, 1758), Dosinia sp., and Ruditapes
philippinarum Adams & Reeve, 1850 (all Veneridae
Rafinesque, 1815). Kazmi et al. (2018: 136) reported the
Pakistani host as Mercenaria Schumacher, 1817, but thisis a coldwater genus not known from Asia, so the authors
were probably dealing with a different venerid.
Remarks. Pinnotheres exiguus Bürger, 1895, transferred to
Arcotheres by Campos & Manning (2001), was described187

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 39. Arcotheres exiguus (Bürger, 1895). A–C, ovigerous female (8.2 × 6.9 mm) (ZRC 2021.784), India (neotype of Pinnotheres casta
Antony & Kutyamma, 1971); D, non-ovigerous female (8.0 × 6.7 mm) (ZRC 2017.1037), India; E, juvenile female (4.0 × 3.6 mm) (ZRC
2018.769), India. A, D, E, overall dorsal view; B, ventral view of cephalothorax; C, frontal view of cephalothorax.

188

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 40. Arcotheres exiguus (Bürger, 1895), female (6.8 × 4.8 mm) (CAS), China (holotype of Pinnotheres obscuridentata Dai & Song,1986); after Dai & Song (1986: text-fig. 3). A, overall dorsal view; B, left MXP3; C, outer view of right chela; D–G, right P2–P5 dactyli.
Scales = A, 2.0 mm; B–G, 0.5 mm.

formally naming it Pinnotheres casta. On the basis of thecharacters, this is clearly a species of Arcotheres. The
present specimens from southern India (ZRC 2018.768, ZRC
2018.769, ZRC 2021.784) agree very well with the accounts
by Silas & Alagarswami (1967) and Antony & Kutyamma(1971) of P. casta. The figure of the overall female habitus
by Antony & Kutyamma (1971: fig. 1A) is schematic, but
those of the MXP3 and ambulatory legs clearly show theasymmetry and leg proportions (Antony & Kutyamma,
1971: figs. 1B, 2) (see also Silas & Alagarswami, 1967:
text-fig. 2). Silas & Alagarswami (1967) also had malesof the species and figured their G1 and MXP3 (Silas &Alagarswami, 1967: text-fig. 1(4–6)). Interestingly, Chhapgar(1957) had described Pinnotheres vicajii from western India,
but the identity of his species was always in doubt because
the original description was very brief and the figures too
schematic. Ng & Kumar (2015) transferred the species to
Nepinnotheres on the basis of Chhapgar’s (1957) figuresof the ambulatory legs, which appeared to be symmetrical.
Trivedi et al. (2020) re-examined the holotype male and fresh
specimens of P. vicajii from the type locality and showedthat P. casta was its junior subjective synonym.

from a small female (5.6 × 4.6 mm) from the Philippines,
with the lectotype fixed by Ahyong & Ng (2007b: 195)
(Fig. 36A, B). The lectotype (Fig. 36C–F) is now in poor
condition, but agrees well, in all its MXP3 and ambulatory
leg characters, with the type material of A. winckworthi(Gordon, 1936) and the series of specimens on hand; thetwo species are herein synonymised. Arcotheres exiguus isdistinctive in the genus in its combination (in females) of theelongated P5 dactylus (as long as or longer than the propodus
and twice as long as the dactyli of the P2, P3, and shorter
P4), which lacks a row of distinct distoflexor spinules on the
distal part and is strongly setose only on the flexor margins,
being glabrous or sparsely setose on the other surfaces. As
discussed earlier (under Characters), the setation of the P5
dactylus of A. exiguus (as well as A. rayi) contrasts withthat of A. palaensis, A. ocularius, and A. similis, with theflexor margin distinctly more setose. Males of A. exiguus and
A. rayi are unusual among congeners, of which males are
known for having the articulation between the P5 ischiumand merus diagonal rather than perpendicular to the segment
axis, and a triangular subdistal lobe on the G1 (absent inothers). In these respects, males of A. exiguus and A. rayi
resemble male Plenotheres coarctatus. Arcotheres exiguus ismost similar to A. rayi; distinguishing features are discussed
under the account of the latter.

Both sexes of P. vicajii (and P. casta) agree very well with
A. exiguus (Fig. 42) and we regard them as conspecific.
The structure of the G1, in particular, is diagnostic (Figs.
44K, L, 45H, I; cf. Trivedi et al., 2020: figs. 3D, 7H, I). As
discussed by Trivedi et al. (2020), the types of Pinnotheres
casta Antony & Kutyamma, 1971, are no longer extant. In
view of the complex taxonomy of the various species now
synonymised under A. exiguus, to stabilise the taxonomyof these taxa, we here select an ovigerous female (8.2 ×
6.9 mm; ZRC 2021.784) from the western shore of Tamil
Nadu, India (Fig. 39A–C), as the neotype of P. casta; it
agrees with the type description and figures in almost all

Females of Arcotheres exiguus are ovigerous by 4.1 mm cl(ZRC 2018.775). The smallest male examined (cl 3.4 mm;
ZRC 2018.774) is already mature.
Silas & Alagarswami (1967) described and figured in detailan unidentified species of Pinnotheres obtained from thevenerid clam Meretrix casta (Gmelin, 1791) from Cochin,western India. Antony & Kutyamma (1971) subsequently
collected the same species from the same host in Cochin,189

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 41. Arcotheres exiguus (Bürger, 1895). A–M, ovigerous female (9.3 × 7.6 mm) (NHM 1936.6.19.12), Penang (holotype of Pinnotheres
winckworthi Gordon, 1936); N–Q, ovigerous female (8.8 × 7.3 mm) (NHM 1936.6.19.13), Penang (paratype of Pinnotheres winckworthi
Gordon, 1936). A, N, carapace; B, Q, left MXP3; C, frontal view of cephalothorax; D, anterior thoracic sternum; E, right chela; F–I, left
P2–P5; J–M, right P2–P5; O, left P4, dactylus, and propodus; P, right P4, dactylus, and propodus. Scale = A, N, 2.5 mm; B, Q, 0.625
mm; C–M, O, P, 1.25 mm.

190

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 42. Arcotheres exiguus (Bürger, 1895). A–I, ovigerous female (8.2 × 6.9 mm) (ZRC 2021.784), India (neotype of Pinnotheres casta
Antony & Kutyamma, 1971); J, K, non-ovigerous female (8.0 × 6.7 mm) (ZRC 2018.768), India; L, juvenile female (4.0 × 3.6 mm) (ZRC
2018.769), India; M, ovigerous female (4.5 × 4.0 mm) (ZRC 2018.775), Peninsular Malaysia. A, L, left MXP3 (setae omitted); B, leftchela and carpus; C–F, left P2–P5, respectively (P3 carpus, propodus, and dactylus missing); G–I, right P2, P4, and P5, respectively; J, K,
left and right P4, respectively; M, distal part of right P5 dactylus (ventral view, soft setae omitted). Scales = A–L, 0.5 mm; M, 0.25 mm.

191

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 43. Arcotheres exiguus (Bürger, 1895), male (5.1 × 4.9 mm) (ZRC 2018.772), Peninsular Malaysia. A, overall dorsal view; B, frontal
view of cephalothorax; C, outer view of right chela.

aspects. The neotype is from a location south of Kochi in
the Indian state of Kerala, the original type locality, but isstill in the southwestern part of India. Trivedi et al. (2020)
redescribed the species at length, as A. vicajii.

repository, Institute of Zoology in the Chinese Academy of
Sciences (Academia Sinica) (Dai & Song, 1986), despite two
separate searches in 2018 and 2019 (Meng Kai, Ng Ngan
Kee, Lee Bee Yan, pers. comm.). The account by Dai &Song (1986: 56) is somewhat confusing, indicating that only
the holotype female was available, yet providing two setsof carapace measurements, perhaps corresponding to morethan one specimen or multiple points of measurement of
the same specimen (carapace width 5.3–7.0 mm and length
3.9–4.8 mm). Their figured holotype specimen measures 6.8
× 4.8 mm according to the figure scale (Dai & Song, 1986:
fig. 3-1; Fig. 40A).

Pinnotheres obscuridentata Dai & Song, 1986, describedfrom one female (no host recorded) collected in Longmen,
Guangxi, southern China (Fig. 40), is here provisionallyconsidered to be a junior synonym of A. exiguus. Dai & Song(1986: 56, 62) commented that a distinguishing feature wasthe absence of a sub-basal tooth on the dactylar finger of thechela, but it seems more likely that it is simply broken oreroded; all other species of Arcotheres possess the dactylar
tooth. In almost all other features as understood at present,
it agrees well with A. exiguus. The MXP3 has a relatively
short propodus with a short dactylus, and the chela is shortand stocky (Dai & Song, 1986: fig. 3-2, 3-3); all charactersof A. exiguus and the proportions of the ambulatory legs also
agree (Dai & Song, 1986: fig. 3-1, 3-4–3-7). The carapaceof P. obscuridentata is proportionally slightly wider thantypical A. exiguus (at least as figured by Dai & Song, 1986:
fig. 3-1), but the carapace in this species is weakly chitinised
and if poorly preserved can appear to vary substantially(Figs. 36A, 37A, 38A, B, 39A, D, E, 41A, N). The concave
posterior margin of the carapace of P. obscuridentata, as
figured by Dai & Song (1986), is a result of contractionof the soft carapace cuticle and slight anterior tilt of thespecimen when figured, which in A. exiguus, is usuallyalmost straight but occasionally concave (Fig. 39A). The
holotype of P. obscuridentata could not be found in the typeThe record of “Pinnotheres gracilis” by George & Noble(1970: 392) from Karwar in India is very likely A. exiguus.
They did not illustrate the whole specimen, but the G1
figured (George & Noble, 1970: fig. 1.2, 1.3) agrees very
well with that of A. exiguus (Figs. 44K, L, 45H, I). The
MXP3 (figured from the inner view), however, depicts a
relatively long dactylus that extends beyond the tip of the
propodus (George & Noble, 1970: fig. 1.1); in A. exiguus,
the dactylus is always shorter (Figs. 36E, 41B, 42A, C,
44B, 45A). The MXP3 figured shows its internal (rather
than external) surface and does not appear to have been
illustrated with the palp articles in the same plane, making
the propodus appear shorter than it actually is. The G1 as
figured is almost identical to that known for A. exiguus.
Therefore, we provisionally refer their record to A. exiguus. It
is noteworthy that George & Noble (1970) recorded the crabfrom the venerid clam Marcia opima (Gmelin, 1791) (as a192

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 44. Arcotheres exiguus (Bürger, 1895), male (5.1 × 4.9 mm) (ZRC 2018.772), Peninsular Malaysia. A, overall dorsal view; B, right
MXP3; C, frontal view of cephalothorax; D, anterior thoracic sternum; E, right chela; F–I, right P2–P5; J, pleon; K, L, left G1 (ventral anddorsal views, respectively); M, N, left G2 (ventral and dorsal views, respectively). Scale = A, 2.0 mm; B, K–N, 0.5 mm; C–J, 1.0 mm.

Katelysia). Pinnotheres gracilis s. str. is now in Viridotheres
Manning, 1996 (see Ahyong & Ng, 2007b: 220; Trivedi etal., 2018b: 62), and occupies Solen sp. (Adapedonta).

almost certainly referable to Amusiotheres hanumantharaoi(Devi & Shyamasundari, 1989), known from the Bay of
Bengal (see Devi & Shyamasundari, 1989; Ng & Ho, 2016a).

The record of “Pinnotheres gracilis” by Lalitha Devi (1981:
fig. 1 (left)) from Kakinada Bay, Bay of Bengal, was fromthe pectenid scallop Amusium pleuronectes (Linnaeus, 1758)
and is misidentified. The photograph of the specimen is notclear, but considering the host and pereopod proportions, it is

Arcotheres exiguus is superficially very close to A. cyclinus(Shen, 1932), especially in the form of the carapace,pereopods, and slender chelae (Shen, 1932: figs. 80, 81c).
The species was described from China from the venerid clam
Cyclina sinensis (Gmelin, 1791). The one major difference193

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 45. Arcotheres exiguus (Bürger, 1895), male (4.5 × 4.2 mm) (ZRC 2018.768), India. A, left MXP3 (setae omitted); B, left chela andcarpus; C–F, right P2–P5, respectively; G, male pleon; H, left G1 (dorsal view); I, left G1 (ventral view); J, left G2 with exopod. Scales= 0.5 mm.

is the proportionately longer MXP3 dactylus, which extends
beyond the tip of the propodus (Shen, 1932: fig. 81a); thedactylus extends no further than the propodus tip in A.
exiguus (Figs. 36E, 41B, 42A, C, 44B, 45A).

be a composite taxon, four names are available that could
be resurrected from synonymy.
Distribution. South China Sea, including the Philippines, toSingapore, Malaysia, India, and the northern Arabian Sea.

With clarification of the identity of A. exiguus anddocumentation of its geographic range, it is now evident
that the species ranges widely from the South China Sea
westwards to the northern Arabian Sea. Our scutiny ofmaterial from across the range has not revealed distinctions
that would suggest that more than one species is present,
as might be suspected on the basis of its wide distribution.
Evidently, the distribution of A. exiguus appears to trackthat of its hosts. Nevertheless, should A. exiguus prove to

Arcotheres rayi Ahyong & Ng, 2007b(Figs. 46–49, 50H–L, 51, 52)
Arcotheres rayi Ahyong & Ng, 2007b: 203, fig. 11 [type locality:
Mariveles or Bohol, Philippines]; Ng et al., 2008: 248; Clark& Schram, 2009: 450; Ng et al., 2017: 1094; Trivedi et al.,2018a: 54, 56; Trivedi et al., 2019: 590, tab. 1; De Gier &Becker, 2020: tab. 1.

194

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 46. Arcotheres rayi Ahyong & Ng, 2007, holotype female (11.3 × 8.7 mm) (SMF-ZMG 952), Philippines. A, overall dorsal view;
B, ventral view of cephalothorax; C, outer view of left chela.

Type material. Holotype: female (11.3 × 8.7 mm) (SMFZMG952), Mariveles or Bohol, Philippines, coll. Semper(paralectotype of Xenophthalmus latifrons Bürger, 1895).

MXP3 outer surface with scattered short setae; ischiomerus
completely fused, subrhomboidal, inner margin usuallyrounded, at widest point; carpus short; propodus about 3
times as long as high, subspatulate, distinctly longer thancarpus, tip rounded to subtruncate; dactylus slender, inserted
medially on propodus, tip underreaching or just reaching
propodal apex; exopod relatively slender, about two-thirds
length of ischiomerus, flagellum 2-segmented (Fig. 49B).

Other material examined. Peninsular Malaysia: 3 males(4.2 × 3.8–4.4 × 4.0 mm), 8 ovigerous females (5.4 × 5.1–8.4
× 7.0 mm), 5 spent females (5.6 × 4.0–6.9 × 5.2 mm), 10
females with bopyrids (5.4 × 3.8–10.4 × 6.2 mm) (ZRC
2013.1416), Bentong market, Pahang, in Paphia sp., coll.
M. Low, 10 May 2010.

Chela not prominently elongate, dactylus half to two-thirds
palm length; palm relatively slender, proximally narrower
than distally; outer surfaces of palm, fingers (except for
distal part) almost glabrous, with only scattered short setae;
ventral margin of palm gently convex to sinuous; dactylus
occlusal margin with distinct subproximal tooth; pollex
occlusal margin usually with 1 low proximal tooth (sometimes
obsolete), 1 submedian tooth, and minute denticles; tips offingers sharp, hooked (Figs. 46C, 47E, 49E).

Description. Female: Carapace and pereopods poorly
chitinised, soft. Carapace subcircular to subhexagonal,
wider than long; dorsal and lateral surfaces smooth,glabrous; appearing domed in frontal view; front in line with(larger specimens) or slightly produced anteriorly (smaller
specimens) beyond transverse anterolateral margins, margin
gently convex; anterior margin with (larger specimens) orwithout (smaller specimens) shallow notch above orbit
flanking front (Figs. 46A, 47A, C, D, 48A–C, 49A). Eyes
small, not or just visible in dorsal view in adults; mobile,completely filling orbit (Figs. 46A, 47A, C, D, 48A–C, 49A,
C). Epistome with median part triangular, lateral margins
gently concave (Fig. 49C).

P2–P5 dorsally, ventrally unarmed; outer surface covered with
scattered, very short setae or glabrous; ventral margins ofpropodus and dactylus slightly more setose; merus relatively
longer, more slender, relative lengths of meri P4>P3>P5>P2;
left (sometimes right) P4 distinctly the longer; P2 andP3 dactyli short, subequal, tip gently hooked; P4 longer
195

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 47. Arcotheres rayi Ahyong & Ng, 2007, female (6.9 × 5.2 mm) (ZRC 2013.1416), Peninsular Malaysia. A, overall dorsal view; B,
ventral view of cephalothorax; C, dorsal view of carapace; D, frontal view of cephalothorax; E, outer view of left chela.

196

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 48. Arcotheres rayi Ahyong & Ng, 2007. A, D, ovigerous female (7.6 × 5.8 mm) (ZRC 2013.1416), Peninsular Malaysia; B, C, E,
female (6.2 × 4.4 mm) (ZRC 2013.1416), Peninsular Malaysia. A, B, overall dorsal view; C, dorsal view of carapace; D, E, right P5.

dactylus elongate, weakly falciform, distinctly longer thanhalf propodus length, longer than P5 dactylus; P5 merus
3.9–4.5 times longer than wide; P5 dactylus shorter thanpropodus extensor margin, longer than P2 and P3 dactyli,
length/height < 5.0, margins lined with short and long setae,
denser on ventral margin, distoflexor margin without rowsof spinules (Figs. 49F–M, 50H–L).

Pleon extending to buccal region, covering bases of P2–P5;
telson gently recessed into concave distal margin of somite
6 (Figs. 46B, 47B, 49D).
Male: Carapace and pereopods well chitinised, firm. Carapace
almost circular, slightly wider than long; dorsal surface
almost smooth, not prominently inflated, lateral surfaces with197

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 49. Arcotheres rayi Ahyong & Ng, 2007, ovigerous female (7.6 × 5.8 mm) (ZRC 2013.1416), Peninsular Malaysia. A, overall dorsal
view; B, right MXP3; C, frontal view of cephalothorax; D, anterior thoracic sternum; E, right chela; F–I, left P2–P5; J–M, right P2–P5.
Scale = A, 2.0 mm; B, 0.5 mm; C–M, 1.0 mm.

setae; front projecting anteriorly, margin gently sinuous toalmost straight (Figs. 51A, 52A). MXP3 as in female but
dactylus shorter (Fig. 52B). Anterior thoracic sternum wide,
sternites 1, 2 fused, partially sunken into buccal cavity;
suture between sternites 2 and 3 shallow; sternites 3, 4
completely fused, separated only by shallow grooves. Chelarelatively stout, shorter than in female (Figs. 51C, 52D).
P2–P5 dorsally, ventrally unarmed; outer surface covered
with short setae; P3 and P4 carpus and propodus with long
natatory setae; left and right meri equal, relative lengths ofmeri P4>P3>P2>P5; dactyli of P2–P4 progressively longer;
right P4 dactylus slightly longer, more slender than left side;

P4 and P5 dactylus subequal to that of P3, covered withshort setae (Fig. 52E–L). Pleon slender, triangular, widest
at somite 3, lateral margins of somite 4 gently concave;
somite 6 trapezoidal; telson semicircular, wider than long
(Fig. 52M). G1 relatively stout, arcuate, curved outwards,
with short triangular subdistal dorsal projection, tip rounded(Fig. 52N, O). G2 short, with spatuliform tip; exopod about
two-thirds endopod length (Fig. 52P).
Variation. The carapace shape varies somewhat, the mostimportant being the prominence of the front, whose variationis broadly size-related. In the holotype of A. rayi, the largest
198

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 50. Right female P5. A–G, Arcotheres exiguus (Bürger, 1895), Peninsular Malaysia: A, juvenile (3.8 × 3.6 mm) (ZRC 2018.772); B,
ovigerous (4.5 × 4.1 mm) (ZRC 2018.775); C, non-ovigerous (4.6 × 4.2 mm) (ZRC 2018.772); D, spent (5.4 × 4.6 mm) (ZRC 2018.775);
E, ovigerous (6.8 × 6.1 mm) (ZRC 2019.516); F, ovigerous (7.7 × 6.7 mm) (ZRC 2018.772); G, ovigerous (8.9 × 7.6 mm) (ZRC 2018.775).
H–L, A. rayi Ahyong & Ng, 2007, Peninsular Malaysia: H, juvenile (5.4 × 5.8 mm) (ZRC 2013.1416); I, non-ovigerous (5.6 × 4.3 mm)(ZRC 2013.1416); J, non-ovigerous (8.1 × 5.2 mm) (ZRC 2013.1416); K, spent (6.9 × 5.2 mm) (ZRC 2013.1416); L, ovigerous (8.4 ×
7.0 mm) (ZRC 2013.1416). Scale = 1.0 mm.

Host. Known only from Paphia sp. (Veneridae Rafinesque,1815).

known specimen of the species, the front does not protrude
in advance of the general anterior margin, but is demarcatedon either side by a shallow notch above each orbit (Fig.
46). Other specimens, however, have a front that protrudes
anteriorly beyond the general anterior margin, some retaining
the notch on either side of the front (Fig. 48B, C), but mostothers without (Figs. 47, 48A, 49). The length of the longer
P5 dactylus varies allometrically, being proportionately
shorter in small specimens (Table 2; Fig. 50H–L).

Remarks. Arcotheres rayi is morphologically very close to
A. exiguus, but the good series of specimens has permitted
evaluation of intraspecific variability, enabling more reliable
recognition of interspecific differences. Females of A. rayi,
in which the carapace front is demarcated by a notch in
the anterior margin above each orbit (Figs. 46, 48B, C, E),
are readily distinguished from A. exiguus, which does nothave the anterior carapace notches (Figs. 33B, C, 35B, E,
199

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 51. Arcotheres rayi Ahyong & Ng, 2007, male (4.2 × 3.8 mm) (ZRC 2013.1416), Peninsular Malaysia. A, overall dorsal view; B,
frontal view of cephalothorax; C, outer view of right chela.

36–41). Female A. rayi, however, with the anteriorly produced
front but without the flanking notches (Figs. 47A, C, 48A),
are indistinguishable from A. exiguus based on carapace
features, although the two species can still be separated by
the proportional differences in the P5 dactylus length andstoutness. The proportional length and stoutness of the P5
dactylus in A. rayi and A. exiguus increases allometrically,
but when similarly sized females of A. exiguus and A. rayi
are compared, the P5 dactylus is always proportionally shorterand stouter in A. rayi (see Figs. 48D, E, 50H–L versus Fig.
50A–G; Table 2). The length:height ratio of the female P5
dactylus is always less than 5.0 in A. rayi, but exceeds 5.0
in specimens of A. exiguus larger than 6.0 mm cl. Similarly,
the female P5 dactylus in A. rayi is always shorter thanthe extensor margin of the corresponding propodus, but aslong as or slightly longer in A. exiguus exceeding 6.5 mm
cl, sometimes 5 mm. Males of A. rayi and A. exiguus are
similar but easily distinguishable by the subdistal projection
on the G1, which is always proportionately longer in A.
exiguus (Figs. 44K, L, 45H, I) than in A. rayi (Fig. 52N, O).

Table 2. Length/height proportions of P5 dactylus of Arcotheres
exiguus and A. rayi.
A. exiguus

A. rayiP5 length/heightP5 length/height

4.0–4.5

3.83–4.01

3.05

4.5–5.5

4.66–5.63

2.97–3.51

5.5–6.5

5.20

3.9

6.5–7.5

5.97–6.32

4.02

7.5–8.5

5.70–6.09–

8.5–9.5–

4.69

cl (mm)mm (smallest ovigerous female 5.4 mm cl; ZRC 2013.1416).
The smallest male examined (cl 3.8 mm; ZRC 2013.1416)
is mature.
Distribution. Philippines and Peninsular Malaysia.

Arcotheres rayi appears to mature at a similar size to A.
exiguus. Females of A. rayi are mature by at least cl 4.0

200

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 52. Arcotheres rayi Ahyong & Ng, 2007, male (4.2 × 3.8 mm) (ZRC 2013.1416), Peninsular Malaysia. A, overall dorsal view; B,
right MXP3; C, frontal view of cephalothorax; D, right chela; E–H, left P2–P5; I–L, right P2–P5; M, pleon; N, O, right G1 (dorsal andventral views, respectively); P, right G2. Scale = A, 2.0 mm; B, N–P, 0.5 mm; C–M, 1.0 mm.

201

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia
Arcotheres alcocki (Rathbun, 1909)(Figs. 53, 54)

Male: Not known.
Variation. Not known.

Pinnotheres parvulus — Bürger, 1895: 375, pl. 9 fig. 18, pl. 10
fig. 17 [not P. parvulus Stimpson, 1858].
Pinnotheres alcocki Rathbun, 1909: 114 (part; replacement name)[type locality: Burias, Philippines, by lectotype designation(Ahyong & Ng, 2007b)]; 1910: 304; Serène, 1968: 93.
Arcotheres alcocki — Ahyong & Ng, 2007b: 193, 194, fig. 1; Ng
et al., 2008: 248; Ng et al., 2017: 1094; Trivedi et al., 2018a:
197; De Gier & Becker, 2020: tab. 1.

Colour. Not known.
Host. The lectotype female did not have any host record(Ahyong & Ng, 2007b: 193). The present specimen wasfrom a clam, Marcia sp. (Veneridae).
Remarks. The taxonomy of this poorly known species was
discussed at length by Ahyong & Ng (2007b). Rathbun(1909, 1910) argued that specimens identified as “Pinnotheres
parvulus”, recorded by Bürger (1895) from the Philippines,
De Man (1887: 105; 1888: 383) from the Mergui Archipelago
and Indonesia, and Alcock (1900), presumably from Indian
waters, were not conspecific with Pinnotheres parvulus
Stimpson, 1858, and as such, she proposed a new name forthem, Pinnotheres alcocki. No types were selected, rendering
all of the material referred by these authors to P. parvulus
as syntypes of P. alcocki (see Schmitt et al., 1973). Ahyong& Ng (2007b) studied one of Bürger’s (1895) “P. parvulus”
specimens reported from Burias, Philippines, which they
designated as the lectotype of Pinnotheres alcocki Rathbun,
1909. They redescribed and figured the specimen andtransferred the species to Arcotheres. Ahyong & Ng (2007b)
argued that the syntype specimens from Mergui (De Man,
1887) were not conspecific with the Philippine lectotype,but were uncertain of their correct identity. As a result of
the present restudy of Mergui material, these records andthose of Alcock (1900) and Gordon (1936) are clarified hereas Magnotheres globosus. De Man’s (1888) records of “P.
parvulus” from Indonesia remain to be verified.

Material examined. 1 female (9.6 × 7.0 mm) (ZRC
2016.272), Pasir Panjang food terminal, Singapore,from Marcia recens, coll. J. C. Mendoza, 5 April 2016.
Description. Carapace and pereopods well chitinised.
Female: Carapace transversely ovate-subhexagonal, wider
than long; dorsal and lateral surfaces smooth, glabrous;
appearing gently domed in frontal view; front barely
projecting anteriorly beyond orbits, margin gently convex;
anterolateral margin gently sloping posteriorly to various
degrees, forming rounded angle with posterolateral margin(Figs. 53A, C, D, 54A). Eyes small, partially visible in dorsal
view in adults; filling orbit (Figs. 53C, 54A). Epistome
with median part triangular, lateral margins gently concave(Fig. 54C).
MXP3 outer surface with scattered short setae; ischiomerus
completely fused, subrhomboidal, inner margin rounded,even at widest point; carpus short; propodus about 3 times
as long as high, subspatulate, distinctly longer than carpus,
tip rounded; dactylus slender, inserted at slightly proximal
to midlength of propodus, tip not reaching propodal apex;
exopod relatively slender, about two-thirds length ofischiomerus, flagellum 2-segmented (Fig. 54B).

The present female specimen (ZRC 2016.272) of A. alcocki
agrees very well with the lectotype in all key aspects.
Ahyong & Ng (2007b: 194) indicated that A. alcocki s. str.
was morphologically very close to A. sinensis (Shen, 1932)
from Chinese waters, and that the two may be synonyms.
Detailed comparison of these two species is being undertaken
as part of a separate study. We note, however, that several
reports of Arcotheres sinensis (as Pinnotheres) from Tamil
Nadu, India (Sethuramalingam & Khan, 1991: pl. 24 fig. i;
Ravichandran & Kannupandi, 2007: 334; Kannappan et al.,2012: 45; see also Trivedi et al., 2018b: 61), cannot presently
be determined as the original descriptions and/or figures are
not sufficiently informative. Sethuramalingam & Khan (1991)
and Ravichandran & Kannupandi (2007) did not record
hosts, whereas Kannappan et al.’s (2012) specimens camefrom a gastropod (Turritella attenuata Reeve, 1849, family
Turritellidae Lovén, 1847), and from his figure (Kannappan
et al., 2012: fig. 2) their species appears to be somethingquite different from A. alcocki or A. sinensis.

Chela not prominently elongate, dactylus about two-thirds
palm length; palm relatively slender, proximally narrower
than distally; outer surfaces of palm, fingers (except for distalpart) almost glabrous, with only scattered short setae; ventral
margin of palm gently sinuous; dactylus occlusal margin
with distinct subproximal tooth; pollex occlusal margin
with 1 low proximal tooth, 1 submedian tooth, and minutedenticles; tips of fingers sharp, hooked (Figs. 53E, 54E).
P2–P5 dorsally, ventrally unarmed; outer surface covered with
scattered, very short setae or glabrous; ventral margins ofpropodus and dactylus slightly more setose; merus relatively
short, stout, relative lengths of meri P4>P3>P2>P5; rightP4 longer than left; P2 and P3 dactyli short, subequal, tip
gently hooked, half propodus length; right P4 dactylus
longer, weakly falciform, shorter than propodus, shorter thanP5 dactylus; P5 merus 4.3–4.4 times longer than wide; P5
dactylus longest, longer than propodus, margins lined withshort and long setae, denser on ventral margin, distoflexor
margin with 2 rows of 7–12 spinules (Fig. 54F–M).

The precise provenance of the present specimen (ZRC
2016.272) is not known, as Marcia clams, although occurringin Singapore, are also imported as seafood; most imported
stocks come from Peninsular Malaysia or Indonesia.

Pleon extending to buccal region, covering bases of P2–P5;
telson gently recessed into concave distal margin of somite
6 (Figs. 53B, 54D).
202

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 53. Arcotheres alcocki (Rathbun, 1909), spent female (9.6 × 7.0 mm) (ZRC 2016.272), Singapore. A, overall dorsal view; B, ventral
view of cephalothorax; C, dorsal view of carapace; D, frontal view of cephalothorax; E, outer view of left chela.

203

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 54. Arcotheres alcocki (Rathbun, 1909), spent female (9.6 × 7.0 mm) (ZRC 2016.272), Singapore. A, overall dorsal view; B, right
MXP3; C, front view of cephalothorax; D, anterior thoracic sternum; E, right chela; F–I, left P2–P5; J–M, right P2–P5. Scale = A, 2.0mm; B, 0.5 mm; C–M, 1.0 mm.

Distribution. South China Sea, from the Philippines andprobably to the vicinity of Peninsular Malaysia or Indonesia.

Pinnotheres obesus –– Silas & Alagarswami, 1967: 1204 (list);
Serène, 1968: 94; Schmitt et al., 1973: 60 (part) (list anddiscussion).
Pinnotheres obesus –– Ng et al., 2008: 250 (list).
Arcotheres obesus — Komai et al., 2020: 223, figs. 1–6; De Gier
& Becker, 2020: tab. 1.

Arcotheres obesus (Dana, 1852)(Figs. 55, 56)
Pinnothera obesa Dana, 1852: 380 [type locality: Kumi Village,
Viti Levu, Fiji, by neotype designation (Komai et al., 2020)];
1855: pl. 24 fig. 3; Woodward, 1886: 177 (list).

Type material. Neotype: female (5.5 × 4.4 mm) (CBM-ZC
15945), Kumi Village, Viti Levu, Fiji, intertidal, associated204

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 55. Arcotheres obesus (Dana, 1852), female (8.9 × 6.9 mm) (ZRC 2019.1876), Fiji. A, overall dorsal view; B, dorsal view of carapace;C, frontal view of cephalothorax; D, outer view of right chela.

Description. Carapace and pereopods very poorly chitinised.
Female: Carapace subovate, wider than long; dorsal andlateral surfaces smooth, glabrous; appearing gently domed in
frontal view; front not clearly projecting anteriorly beyond
orbits, margin gently sinuous to gently convex, appearing
almost contiguous with convex anterolateral margin, forming
rounded angle with posterolateral margin (Komai et al., 2020:
fig. 55A–C). Eyes small, not visible in dorsal view in adults;
filling orbit (Komai et al., 2020: fig. 55A–C). Epistome withmedian part triangular, lateral margins concave (Komai etal., 2020: fig. 3B).

with Gafrarium pectinatum, coll. K. Kawai, 22 February
2019.
Other material examined. Fiji: 3 ovigerous females(6.7 × 5.4 mm, 6.6 × 5.6 mm, 5.9 × 4.6 mm) (CBM-ZC
15949–15951), same data as neotype, in G. tumidum; 3females (5.3 × 4.2–6.1 × 5.1 mm), 1 ovigerous female (5.4
× 4.8 mm), 2 males (4.2 × 3.6 mm, one damaged) (CBM-ZC
15946), Kumi Village, Viti Levu, intertidal, in Gafrarium
pectinatum, coll. K. Kawai, 16 August 2010; 2 females (5.3
× 4.0 mm, 9.2 × 7.2 mm) (CBM-ZC 15947, 15948), 2 soft
females (8.8 × 6.2 mm, 8.9 × 6.9 mm), 2 males (1.9 × 1.7mm, 2.6 × 2.2 mm) (ZRC 2019.1876), Waiganake, Viti Levu,
intertidal, associated with Gafrarium pectinatum (Linnaeus,
1758), coll. K. Kawai, 22 August 2011; 1 ovigerous female(6.2 × 4.9 mm) (ZRC 2020.6), associated with Gafrarium
pectinatum, Telau, Viti Levu, coll. K. Kawai, 22 February
2019. Peninsular Malaysia: 2 soft females (7.2 × 5.4 mm,
5.7 × 4.7 mm) (ZRC 2000.2281), Pulau Tinggi, Johor, inGafrarium dispar, 7 May 1969.

MXP3 outer surface with scattered short setae; ischiomerus
completely fused, subrhomboidal, inner margin rounded atwidest point; carpus short; propodus about 3 times as long as
high, spatulate, distinctly longer than carpus, tip subtruncate;dactylus slender, inserted at midlength of propodus, tip notreaching propodal apex; exopod relatively slender, about
two-thirds length of ischiomerus, flagellum 2-segmented(Fig. 56K, L).
205

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 56. Arcotheres obesus (Dana, 1852), female (8.3 × 5.9 mm) (ZRC 2019.1876), Fiji. A, outer view of right chela; B–E, left P2–P5,
respectively; F–I, right P2–P5, respectively; J, right P5 dactylus (long setae omitted); K, inner view of left MXP3; L, outer view of left
MXP3. Scales = A–I, 1.0 mm; J–L, 0.5 mm.

206

RAFFLES BULLETIN OF ZOOLOGY 2022
Colour. Not known.

Chela not prominently elongate, dactylus about two-thirds
palm length; palm relatively slender, proximally narrower
than distally; outer surfaces of palm, fingers (except for distalpart) almost glabrous, with only scattered short setae; ventral
margin of palm gently concave; dactylus occlusal margin
with distinct subproximal tooth; pollex occlusal margin
with 1 low proximal tooth, 1 sharp submedian tooth, anddenticles; tips of fingers sharp, hooked (Figs. 55D, 56A).

Host. Known for certain from Gafrarium pectinatum(Linnaeus, 1758) and G. dispar (Holten, 1802) (Veneridae
Rafinesque, 1815).
Remarks. The identity of Pinnotheres obesus was long
uncertain until Komai et al. (2020) redescribed and transferred
it to Arcotheres based on a neotype from Fiji (type locality).
Arcotheres obesus appears to be an obligate commensal
of clams of the genus Gafrarium. The specimens from
Peninsular Malaysia, while soft and in poor condition, agree
very well with the material from Fiji. A male specimen willneed to be obtained to confirm the identity of these Malaysianspecimens, especially considering the geographical distanceseparating the two locations.

P2–P5 dorsally, ventrally unarmed; outer surface covered
with scattered, very short setae or glabrous; ventral margins
of propodus and dactylus more setose; merus long, slender,
relative lengths of meri P4>P3>P2>P5; right (sometimesleft) P4 distinctly longer; P2 and P3 dactyli relatively short,
subequal, tip slightly hooked, half or slightly longer than half
propodus length; P4 dactylus about half propodus length,
longer than P2 and P3 dactyli, shorter than P5 dactylus; P5
merus 4.9–5.0 times longer than wide; P5 dactylus longest,
margins lined with short and long setae, denser on ventral
margin, distoflexor margin without rows of graded spinules,
at most with scattered, very low, minute spinules or acutegranules of similar size (Fig. 56B–J).

Distribution. Western Pacific from Fiji and Peninsular
Malaysia.
Arcotheres placunicola Ng, 2018a(Figs. 57–62)
Pinnotheres similis — Tesch, 1918: 254 (list); Gordon, 1936: 172,
fig. 3 (Singapore); Silas & Alagarswami, 1967: 1210 (list) (notPinnotheres similis Bürger, 1895).
Arcotheres palaensis — Pohle & Marques, 1998: 231 (part) (notPinnotheres palaensis Bürger, 1895).
Arcotheres placunicola Ng, 2018a: 475, figs. 1–6 [type locality:
Changi, Singapore]; De Gier & Becker, 2020: tab. 1.

Pleon extending to buccal region, covering bases of P2–P5;
telson gently recessed into concave distal margin of somite
6 (Komai et al., 2020: fig. 3D).
Male: Carapace almost circular, as wide as long; dorsal
surface almost smooth, not prominently inflated, lateralsurfaces with setae; front projecting anteriorly, margin
gently sinuous (Komai et al., 2020: fig. 5A). Eyes distinctly
visible in dorsal view (Komai et al., 2020: fig. 5A). MXP3
as in female (Komai et al., 2020: fig. 5B). Anterior thoracic
sternum wide, sternites 1, 2 fused, partially sunken into
buccal cavity; suture between sternites 2 and 3 shallow;
sternites 3, 4 completely fused, separated only by shallow
grooves (Komai et al., 2020: fig. 10B). Chela relatively stout,
shorter than in female (Komai et al., 2020: fig. 6A, B). P2–
P5 dorsally, ventrally unarmed; outer surface covered withshort setae; P3 and P4 carpus and propodus without long
natatory setae; left and right meri equal, relative lengths ofmeri P4>P3>P2>P5; dactyli of P2–P4 progressively longer;
left P4 dactylus longer, more slender than right side; P4
and P5 dactylus longer to that of P3, covered with shortsetae (Komai et al., 2020: figs. 5A, 6E–N). Pleon slender,
triangular, widest at somite 3, lateral margins of somite 4gently concave; somite 6 trapezoidal or subquadrate; telson
semicircular, slightly wider than long (Komai et al., 2020:
fig. 5D). G1 relatively stout, arcuate, curved outwards, distal
part subtruncate, without elongate tip (Komai et al., 2020:
fig. 5E, F). G2 short, with spatuliform tip; exopod about halfendopod length (Komai et al., 2020: fig. 5G).

Type material. Holotype: ovigerous female (11.2 × 8.6mm) (ZRC 2017.1009), off Changi beach, next to Carpark7, near ferry terminal, 1.374223°N, 104.006827°E, coll. S.
K. Tan et al., 7 June 2016. Paratypes: 5 males (5.6 × 4.7mm, 4.7 × 4.1 mm, 4.0 × 3.7 mm, 3.9 × 3.6 mm, 3.6 × 3.4mm), 4 ovigerous females (10.4 × 8.3 mm, 8.7 × 7.5 mm,
7.5 × 6.1 mm, 7.0 × 5.5 mm), 1 female (8.8 × 6.9 mm), 3
females (with rhizocephalans) (8.6 × 6.3 mm, 8.6 × 7.1 mm,
9.0 × 6.9 mm), 2 juvenile females (3.6 × 2.9 mm, 6.2 × 5.1mm) (ZRC 2017.1010), same data as holotype; 14 males(largest 5.1 × 4.4 mm, smallest 2.4 × 2.3 mm), 2 males(with rhizocephalans) (5.7 × 4.7 mm, 5.5 × 5.0 mm), 16
ovigerous females (largest 10 × 8.7 mm, smallest 7.7 × 6.3mm), 1 ovigerous female (10.1 × 8.0 mm), 10 non-ovigerous
females (largest 10.3 × 7.8 mm, smallest 5.4 × 4.5 mm), 2
females (with rhizocephalan) (6.3 × 4.9 mm, 7.8 × 5.8 mm), 1
juvenile female (3.8 × 3.7 mm) (ZRC 2017.1012), off Changi
beach, next to Carpark 7, near ferry terminal, 1.374223°N,
104.006827°E, coll. S. K. Tan et al., 8 June 2016; 1 male
(4.2 × 3.8 mm), 2 ovigerous females (7.2 × 6.1 mm, 7.5 ×
6.2 mm) (ZRC 2012.432), off Changi beach, next to Carpark7, near ferry terminal, 1.374223°N, 104.006827°E, coll. P.
K. L. Ng et al., 5 July 2012; 4 ovigerous females (largest 9.1
× 7.4 mm, smallest 6.5 × 5.4 mm), 3 non-ovigerous females(largest 8.9 × 6.9 mm, smallest 8.1 × 6.7 mm), 2 females(with rhizocephalan) (8.2 × 7.3 mm, 11.2 × 9.1 mm) (ZRC
2017.1013), off Changi beach, next to Carpark 7, near ferry
terminal, 1.374223°N, 104.006827°E, coll. P. K. L. Ng &P. Y. C. Ng, June 2016; 2 males (2.2 × 2.2 mm, 2.4 × 2.4mm) (ZRC 2017.1014), Tanah Merah Ferry Terminal Beach,

Variation. This is the most poorly chitinised and calcified
Arcotheres treated here, with the carapace appearing
almost membranaceous, even in the relatively fresh
specimens recently collected from Fiji (Komai et al., 2020).
Nevertheless, the carapace shape is relatively invariate, being
always transversely ovate (see Komai et al., 2020).

207

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 57. Arcotheres placunicola Ng, 2018, colour in life, after Ng (2018a: figs. 5, 6). A, paratype female (size not known) (ZRC 2017.1010);
B, paratype ovigerous female (8.0 × 6.1 mm) (ZRC 2017.1015); C, paratype non-ovigerous female with rhizocephalan (9.0 × 7.4 mm) (ZRC
2017.1015); D, paratype ovigerous female (7.3 × 5.9 mm) (ZRC 2017.1015); E, paratype female (8.8 × 6.9 mm) (ZRC 2017.1010), Changi,
Singapore; F, paratype male (4.7 × 4.1 mm) (ZRC 2017.1010); G, paratype male with rhizocephalan (5.6 × 5.1 mm) (ZRC 2017.1015); H,
paratype ovigerous female (7.3 × 5.9 mm) (ZRC 2017.1015), in situ on Placuna (crab indicated by arrow). All specimens from Singapore.

208

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 58. Arcotheres placunicola Ng, 2018, holotype ovigerous female (11.2 × 8.6 mm) (ZRC 2017.1009), Changi, Singapore; after Ng(2018a: fig. 1). A, overall dorsal view; B, ventral view of cephalothorax; C, frontal view of cephalothorax; D, outer view of left chela.

209

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 59. Arcotheres placunicola Ng, 2018, after Ng (2018a: fig. 3). A, C, E–K, holotype ovigerous female (11.2 × 8.6 mm) (ZRC
2017.1009), Changi, Singapore; B, D, paratype ovigerous female (10.1 × 8.0 mm) (ZRC 2017.1012), Changi, Singapore. A, B, outerview of right MXP3; C, left chela and carpus; D–G, left P2–P5, respectively; G’, distal part of dactylus of left P5 showing stiff subdistal
spinules (dorsal view, soft setae omitted); H–K, right P2–P5, respectively; K’, distal part of dactylus of right P5 showing stiff subdistal
spinules (ventral view, soft setae omitted); K”, distal part of dactylus of left P5 showing stiff subdistal spinules (dorsal view, soft setae
omitted). Scales = A, B, G’, K’, K”, 0.5 mm; C–K, 1.0 mm.

near Changi Point, coll. S. K. Tan et al., 17 November 2016;
1 male (with rhizocephalan) (5.6 × 5.1 mm), 3 ovigerous
females (7.3 × 5.9 mm, 8.0 × 6.1 mm, 9.5 × 7.2 mm), 1
non-ovigerous female (with rhizocephalan) (9.0 × 7.4 mm)(ZRC 2017.1015), off Changi beach, next to Carpark 7, near
ferry terminal, 1.374223°N, 104.006827°E, coll. P. K. L. Ng& P. Y. C. Ng, 31 March 2017; 1 male (with Sacculina) (4.8
× 4.5 mm) (ZRC 2017.1016), off Changi beach, adjacentto National Service Recreational Country Club, coll. H. H.
Tan et al., 26 May 2017; 4 females, 1 juvenile female (ZRC
2017.1011), 1 male, 2 ovigerous females (AM P105907),
off Changi beach, next to Carpark 7, near ferry terminal,
1.374223°N, 104.006827°E, coll. S. K. Tan et al., 5 July
2017; 1 male (with rhizocephalan), 3 females (AM P105908),
intertidal area, off Changi beach, next to Carpark 7, near
ferry terminal, 1.374223°N, 104.006827°E, coll. S. K. Tan et210

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 60. Arcotheres placunicola Ng, 2018. A–C, paratype male (5.6 × 4.7 mm) (ZRC 2017.1010), Changi, Singapore; D–H, male (3.7 ×
3.3 mm) (ZRC 2021.802), Batu Pahat, Peninsular Malaysia. A, D, overall dorsal view; B, E, frontal view of cephalothorax; C, F, outerview of left chela; G, left G1; H, distal part of P5 (ventral view). B after Ng (2018a: fig. 2B).

211

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 61. Arcotheres placunicola Ng, 2018, after Ng (2018a: fig. 4). A, B, D–P, paratype male (5.6 × 4.7 mm) (ZRC 2017.1010); C,Q, paratype young male (2.4 × 2.4 mm) (ZRC 2017.1014). Both specimens from Changi, Singapore. A, right chela and carpus (setae
omitted); B, C, male pleon; D, right MXP3 (setae omitted); E–H, right P2–P5, respectively; I–L, left P2–P5, respectively (setae omitted);L’, distal part of P5 dactylus (dorsal view, soft setae omitted); M, Q, left G1 (ventral view, setae omitted); N, left G1 (dorsal view, setae
omitted); O, left G2 (setae omitted); P, right G2 (mesial view showing exopod, setae omitted). Scales = A, B, E–L, 1.0 mm; D, M–P,
0.5 mm; C, L’, Q, 0.25 mm.

212

RAFFLES BULLETIN OF ZOOLOGY 2022
al., 26 June 2017; 1 female (ZRC 2017.1265), Telok Paku,
beach, in “Placuna sella”, coll. 8 April 1965. All locationsin Singapore.

P5 dactylus; P5 merus 4.3–4.4 times longer than wide; P5
dactylus longest, longer than propodus, distal half covered
with short setae, ventral margin of distal part with 2 rowsof up to 11 spinules each, lower row usually with few
spinules, lower and upper rows with subequal number of
spinules (Fig. 50D–K”).

Other material examined. Singapore: 3 males, 6 females(ZRC 2019.1113), probably in Placuna ephippium, intertidalarea, off Changi beach, next to Carpark 7, near ferry terminal,
1.374223°N, 104.006827°E, Singapore, coll. P. K. L. Ng etal., 3 August 2012; 1 female (ZRC 2019.1021), intertidal area,
off Changi beach, next to Carpark 6, Singapore, coll. H. H.
Tan et al., 24 December 2018; 1 female (ZRC 2019.1037),
intertidal area, off Changi beach, next to Carpark 7, near
ferry terminal, 1.374223°N, 104.006827°E, Singapore, coll.
H. H. Tan et al., 4 July 2019; 1 female (11.6 × 8.6 mm), 1
juvenile female (ZRC 2021.80), inside Placuna ephippium,
Changi Bay, ex Red Cliff Shoal area, Singapore, coll. S. K.
Tan et al., 13 January 2021. Peninsular Malaysia: 1 male
(3.7 × 3.3 mm) (ZRC 2021.802, ex part ZRC 2000.2613),
Batu Pahat, Johor, supposedly from Tegillarca granosa,
mangroves, coll. H. K. Tan, 16 August 1983; 1 male, 3females, 1 juvenile female (ZRC 2009.905), supposedly
from Tegillarca, Pahang, coll. students, Universiti Teknologi
Malaysia, 2006.

Pleon extending to buccal region, covering bases of P2–P5;
telson recessed into distal margin of somite 6 (Fig. 58B).
Male: Carapace almost circular, slightly wider than long;
dorsal surface covered with scattered short setae or smooth,slightly convex to almost flat; front distinctly projecting
anteriorly, margin gently sinuous (Figs. 60A, D, 62A). Eyes
distinctly visible in dorsal view (Figs. 60A, D, 62A). MXP3
as in female (Figs. 61D, 62C). Anterior thoracic sternum
wide, sternites 1, 2 fused, partially sunken into buccal
cavity; suture between sternites 2 and 3 shallow; sternites
3, 4 completely fused, separated only by shallow grooves;sternopleonal cavity reaching to junction between sternites
3 and 4. Chela relatively stouter, shorter than in female, tip
of fingers longer (Figs. 60C, F, 61A, 62D). P2–P5 dorsally,
ventrally unarmed; outer surface covered with short setae;
carpus and propodus with long natatory setae; left and rightmeri equal, relative lengths of meri P4>P3>P2>P5; dactyli
of P2–P4 progressively longer; left (sometimes right) P4
dactylus slightly longer, more slender than right side; P4
and P5 dactyli subequal to that of P3, completely covered
with short setae (Figs. 61E–L, 62E–L’); P5 dactylus with 2
rows of up to 11 or 12 distoflexor spinules each (Figs. 60H,
61L). Pleon slender, triangular, widest at somite 3, lateral
margins of somite 4 distinctly concave, tapering distally totelson; telson subquadrate-linguiform with convex lateral
margins, wider than long (Figs. 61B, C, 62B). G1 relatively
stout, arcuate, curved outwards, short extended tip gently
curved upwards or straight (Figs. 60G, 61M, N, Q, 62M,
N). G2 short, with spatuliform tip; exopod almost as longas endopod (Figs. 61O, P, 62O).

Description. Carapace and pereopods well chitinised.
Female: Carapace subcircular to subhexagonal, distinctly
wider than long; dorsal and lateral surfaces smooth, glabrous;
front usually projecting anteriorly beyond orbits, entire,
margin slightly convex; anterior quarter of anterolateral
margin gently sloping posteriorly, forming angle with restof convex margin (Fig. 58A). Eyes small, not visible indorsal view; mobile, completely filling orbit (Fig. 58A–C).
Epistome with median part triangular, lateral margins gently
concave (Fig. 58C).
MXP3 outer surface with scattered short setae; ischiomerus
completely fused, subrhomboidal, inner margin angular atwidest point; carpus short; propodus about 3 times as longas high, subspatulate, distinctly longer than carpus; dactylus
slender, inserted at midlength of propodus, tip not reaching
propodal apex; exopod relatively slender, about one-third
length of ischiomerus, flagellum 2-segmented (Fig. 59A, B).

Variation. Several female specimens have the frontal margin
weakly projecting, being almost level with the anterolateral
margin, making the carapace appear more evenly ovate (Fig.
58A). Significantly, the male pleon changes shape (notably
somite 6 and telson) with size, with somite 6 becoming
proportionally narrower and the telson less tapering withincreasing body size (Figs. 61B, C, 62B). The G1 in A.
placunicola varies slightly, with the tapering tip generally
longer in larger males (Fig. 61M, N) and relatively shorterin some smaller males (Fig. 61Q), but also sometimes longin small specimens (Figs. 60G, 62M, N).

Chela slender, dactylus almost two-thirds palm length; palm
slender, proximally narrower than distally; outer surface ofpalm, fingers (except for distal part) almost glabrous, with
only scattered short setae; dactylus occlusal margin withlarge subproximal tooth; pollex occlusal margin with 1 low
proximal tooth, 1 submedian tooth; tips of fingers sharp,
hooked (Figs. 58D, 59C).

Colour. Rather variable: females pale yellow to orange or
dirty white with brown patterns; males more uniform brownto greenish brown (Fig. 57F).

P2–P5 dorsally, ventrally unarmed; outer surface covered
with scattered, very short setae or glabrous; ventral margins
of propodus and dactylus more setose; merus relatively
longer, more slender, relative lengths of meri P4>P3>P5>P2;
left (sometimes right) P4 distinctly the longer; P2 and P3
dactyli short, subequal, tip gently hooked, half propodus
length; P4 dactylus elongate, weakly falciform, three-fourths
propodus length, longer than P2 and P3 dactyli, shorter than

Host. Known with certainty only from Placuna ephippium(Philipsson, 1788) (Placunidae Rafinesque, 1815). One
lot (ZRC 2009.905) of A. placunicola collected in 2006
from Pahang, Malaysia, as part of aquaculture surveys by213

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 62. Arcotheres placunicola Ng, 2018, male (3.7 × 3.3 mm) (ZRC 2021.802), Batu Pahat, Peninsular Malaysia. A, dorsal view ofcarapace; B, pleon (setae omitted); C, outer view of left MXP3 (setae denuded); D, outer view of left chela; E–H, left P2–P5, respectively(setae omitted); I–L, right P2–P5, respectively; M, left G1 (ventral view); N, left G1 (dorsal view, setae omitted); O, left G2. Scales =A, B, D–L, 0.5 mm; C, M–O, 0.2 mm.

214

RAFFLES BULLETIN OF ZOOLOGY 2022
placunicola and A. palaensis are also superficially similar,
although that of A. placunicola is slightly more curved aswell as relatively shorter and stouter (Figs. 60G, 61M, N,
Q, 62M, N) (cf. Fig. 22K–M for A. palaensis).

students from the Universiti Teknologi Malaysia, werelabelled as collected from “Anadara”. We suspect the hostwas incorrectly recorded.
Remarks. Arcotheres placunicola most closely resembles
A. palaensis and A. similis, with which it has been confused(see Ng, 2018a). Arcotheres placunicola and A. similis aresuperficially similar, with similar MXP3 and P5 morphology,
so the two species can be easily mistaken if not examined
carefully. The female carapaces of the two species aresuperficially similar but can usually be distinguished,especially with fresh material. In A. placunicola (Figs.
57A–D, 58A), the carapace shape is more polygonal, withthe anterolateral corners of the carapace bluntly angular,
contrasting with the more rounded carapace of A. similis(Figs. 27A, C, 28A, C), in which the anterolateral corners
are evenly curved. As discussed earlier, carapace shape canchange to varying degrees when preserved. That being said,
there is some overlap in carapace shape between the two
species, so other characters must also be checked to confirm
identifications. The female P2 and P3 dactyli also differ inshape, being evenly curved in A. similis (Fig. 30A, B, E,
F), and tightly curved distally with a more falcate apex in
A. placunicola (Fig. 59D, E, H, I). While the P5 dactylus ofboth species possess two rows of distoflexor spinules, in A.
placunicola, the lower row is shorter and usually has only a
few more spinules than the upper row, the two rows oftenwith subequal numbers of more closely packed spinules (Fig.
59G’, K’, K”). In A. similis, on the other hand, the lower
row is almost always long, with distinctly more, widely
spaced spinules (Fig. 30D’, H’, L).

Arcotheres exiguus also superficially resembles A. placunicola,
especially with regards to the general carapace shape andrelatively longer P2–P5. Female A. placunicola, however,can be reliably distinguished by the P5 dactylus possessing adouble row of subdistal spinules on the ventral margin (Fig.
59G’, K’, K”) (versus absent in A. exiguus; Fig. 40G); andthe chela is often proportionately longer and more slenderproximally than distally (Figs. 58D, 59C) (versus short andstout in A. exiguus; Fig. 40C). In addition, the frontal marginof A. placunicola is relatively wider and curves gently to join
the anterolateral margin (Fig. 58A) (versus shorter frontal
margin which joins lateral margins at a sharper angle in A.
exiguus; Fig. 40A).
Distribution. South China Sea from Singapore and Peninsular
Malaysia.
Plenotheres, new genus
Type species. Pinnotheres coarctatus Bürger, 1895, bypresent designation.
Diagnosis. Carapace dorsal surface smooth, with distinct,
relatively deep, clearly incised gastric grooves. MXP3
dactylus slender, inserted near proximal one-third ofventral margin of propodus, exceeding tip of propodus;
propodus conical; inner margin of widest part of ischiomerus
prominently angular; proximal part of exopod very wide.
Female P4 asymmetrical, with left or right side distinctly
longer (merus and propodus); dactyli of P2–P4 relatively
short, subequal; P5 dactylus long, weakly falciform, different
in structure from those of P2–P4. Male pleon relatively
broadly triangular, telson semicircular. G1 very stout,
arcuate, with prominent subdistal projection, dorsal fold welldeveloped; G2 exopod half length of endopod.

Males of A. placunicola and A. similis are quite different.
Compared to A. similis, the carapace of A. placunicola isdistinctly more transversely ovate with the dorsal surface
gently convex to almost flat (Figs. 60A, B, E, 62A) (versus
carapace round or longitudinally ovate with the dorsal
surface inflated and distinctly convex in A. similis; Figs.
31A, B, D, 32A). The G1 of A. placunicola and A. similis
are superficially similar, but that of A. placunicola is more
evenly curved with the slender apical projection directed
oblique to the main axis (Figs. 60G, 61M, N, Q, 62M, N),
while in A. similis, the median and subdistal parts are almost
straight and the apical projection almost in line with the main
axis (Figs. 32L–N, 33E–H, J, K). Arcotheres placunicola
closely resembles A. palaensis in the structure of thecarapace, P2 and P3 dactyli, MXP3, and G1, but adults canbe distinguished by the relatively longer P2–P5 (P5 merus
4.3–4.4 times longer than wide, versus 3.6–4.0 times in A.
palaensis) and by the P5 dactylus possessing two rows of
distoflexor spinules (Fig. 59G’, K’, K”) (versus absent in A.
palaensis). The adult chela is also usually relatively longer
and more slender in A. placunicola, with the proximal partmore narrow than the distal section (Fig. 59C); it is often
proportionately and evenly stouter in A. palaensis (Figs.
5D, 6D), although the palm may also be elongated (Fig.
4A, B; Ahyong & Ng, 2007b: fig. 9B). The MXP3 of A.
placunicola usually has a proportionately shorter dactylus(Fig. 59A, B), but it varies too much in A. palaensis (Fig.
13) to reliably distinguish the two species. The G1 of A.

Etymology. From the Latin ‘plenus’ for plump and wellbuilt, alluding to the size and shape of the type species; inarbitrary combination with a typical ending ‘-theres’ for pea
crabs, from the genus name Pinnotheres. Gender masculine.
Comparative material. Nepinnotheres corbiculae (Sakai,1939), new combination: 1 male (2.1 × 1.8 mm) (KPMNH 106679), from Corbicula japonica (Prime, 1864)(Corbiculidae), Kumamoto Prefecture, Japan, coll. T. Sakai,
1963.
Remarks. Pinnotheres coarctatus Bürger, 1895, transferred
to Arcotheres by Ahyong & Ng (2007b), is unusual in thegenus in that the gastric grooves on the female carapace are
relatively deep and clearly incised (Figs. 64A, 65), resemblingsome species of Fabia Dana, 1851; the MXP3 propodus isconical and stout with the dactylus digitiform and extending
beyond the tip of the propodus (Figs. 66I, 68J–L); and the
MXP3 exopod is relatively broader basally (Fig. 66I). In
215

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia
addition, males of P. coarctatus, together with A. exiguus
and A. rayi, differ from other species of Arcotheres in
the G1 having a prominent, triangular subdistal lobe (Fig.
68N–P) (absent in other species of Arcotheres), and in thearticulation between the P5 ischium and merus being obliquerather than perpendicular to the segment axis (Figs. 67A,
68D, H). As such, a separate genus, Plenotheres, new genus,
is here established for A. coarctatus.

longish hairs but the carpus is not crossed by an oblique rowof hairs contrary to P. pholadis, which is also the nearest
kin of this species. I could not examine the male specimen,
but its seventh segment seems to be triangular according to
Urita, who referred this species erroneously to P. pholadis”(Sakai, 1939: 591).
The only overall habitus figure of P. corbiculae, that of Sakai(1976: pl. 200 fig. 4) (present Fig. 69A), is rather schematicand does not clearly show the structure of the ambulatory
dactyli, but the general appearance agrees well with Pl.
coarctatus. The only figure of the MXP3 by Sakai (1939:
fig. 77b) does not show the exopod (Fig. 69B).

The similarities between the G1 of A. exiguus, A. rayi, andPl. coarctatus, new combination, are, at present, difficult tointerpret; whether they are evidence of close phylogenetic
proximity or convergence is unclear. All three species are
currently subject of wider phylogenetic studies by us.

The male specimen of Pinnotheres corbiculae examined(Figs. 70, 71) is relatively well preserved, and the general
structure of the chela, MXP3, and P2–P5 are not very
different from that of the male Plenotheres coarctatus.
The MXP3 propodus is not as short or conical in shape,
but the inner angle of the MXP3 ischiomerus is angular.
The male pleon (Fig. 71K) is relatively more elongate thanthat in Pl. coarctatus (Fig. 68M). The G1 (Fig. 71L–O),
however, is very different, being long and slender with the
tip tapering to a sharp point, without any folds or projections(cf. Pl. coarctatus; Fig. 68N–P). The absence of a female
specimen makes a decision on its genus affiliation moredifficult, but based on Sakai’s (1976: pl. 200 fig. 4) figure(present Fig. 69A), the P2–P5 on both sides appear to besymmetrical, with the dactylus of all legs relatively shortwith none prominently elongated. Significantly, Sakai (1939:
591) commented that Pinnotheres corbiculae was close to
Pinnotheres tsingtaoensis Shen, 1932, and both species haverelatively short ambulatory dactyli. Their G1s are also similar(Fig. 71L–O; Shen, 1932: text-fig. 94c). On the basis of theavailable data, both Pinnotheres corbiculae and Pinnotheres
tsingtaoensis should now be referred to Nepinnotheres,as N. corbiculae, new combination, and N. tsingtaoensis,
new combination: the MXP3 dactylus is slender in both
species and inserts sub-basally (rather than basally), thefemale P2–P4 do not show any obvious asymmetry, and theambulatory dactyli are all relatively short and not strongly
dissimilar in length.

In the carapace shape, structure of the MXP3, and general
P2–P4 leg proportions, Plenotheres coarctatus superficiallyresembles the ecologically similar species from Japan,
Pinnotheres corbiculae Sakai, 1939 (Figs. 69–71). The hostsof Pinnotheres corbiculae are noteworthy, being freshwaterand estuarine clams of the family Corbiculidae Gray, 1847.
Plenotheres coarctatus is known from mangrove clams of
the families Cyrenidae Gray, 1840, and Glauconomidae
Gray, 1853, and all three bivalve families belong to the
Cyrenoidea Gray, 1840, with most of the species inhabiting
brackish or freshwater habitats.
Pinnotheres corbiculae was first referred to “Pinnotheres
pholadis” by Urita (1926: 18), but Sakai (1939: 591, textfig. 77a, b) described it as a new species from two female
specimens (one measured, 5.5 × 4.5 mm) supposedlycollected from Nagasaki from Corbicula japonica (Prime,1864) (Corbiculidae), figuring only part of the MXP3 andchela (Fig. 69B, C). Sakai (1976: 572, pl. 200 fig. 4) figured
the overall animal (Fig. 69A), corrected the type locality to“Sendai-gawa, Kagoshima”, and also recorded additional
specimens: five males and seven females from the same
host, from Kikuchi-gawa in Kumamoto (see also Silas &Alagarswami, 1967: 1197, 1224; Schmitt et al., 1973: 42;
Ng et al., 2008: 250). The whereabouts of the type material
is uncertain; Muraoka (1998: 48) records one specimen from
Kumamoto in the Kanagawa Museum, but this specimen,
collected in 1963, is not a type. This specimen is examined
and figured here. The whereabouts of the types is not known.

Plenotheres coarctatus (Bürger, 1895), newcombination(Figs. 63–68)

Sakai’s (1939, 1976) description of P. corbiculae is relatively
brief, but nevertheless useful to quote here as it is the only
diagnosis of the species: “The carapace is broad and roundish,the lateral borders are subparallel but the shoulders are weakly
angular and the posterior border slightly convex. The front ismoderately produced, and the eyes may be observed in dorsal
aspect. The external maxilliped has the dactylus slender andmuch exceeding the tip of propodus (in tsingtaoensis, thissegment is distally broadened). The immovable finger has
four or five denticles near the base, the movable finger has
a stout tooth near the base and its apex is strongly curved
inwards. The anterior three pairs of ambulatory legs are
subequal in length, the last pair conspicuously smaller. The
anterior and posterior borders of all pairs are fringed with

Pinnotheres coarctatus Bürger, 1895: 369, pl. 9 fig. 7, pl. 10 fig. 7[type locality: Tipoeka, Timika, Indonesian Papua, Indonesia,
by present neotype designation]; Tesch, 1918: 248 (list);
Estampador, 1937: 546 (list); Silas & Alagarswami, 1967: 1197
(list); Serène, 1968: 93; Schmitt et al., 1973: 42.
Pinnoteres coarctatus — Balss, 1957: 1419 (list).
Arcotheres coarctatus — Ahyong & Ng, 2007b: 195, fig. 3A; Ng
et al., 2008: 248; Ng et al., 2017: 1094; Trivedi et al., 2018a:
197; De Gier & Becker, 2020: tab. 1; Ahyong & Ng, 2021:
207–209, fig. 17.
Pinnotheres parvulus — Rathbun, 1910: 331, fig. 13, pl. 2 fig. 9;
Naiyanetr, 1980: 42; 1998: 104; 2007: 118 (not Pinnotheres
parvulus Stimpson, 1858).

216

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 63. Plenotheres coarctatus (Bürger, 1895). A, B, specimens in situ (ZRC 2018.765) in Polymesoda expansa (Mousson, 1849) (Cyrenidae
Gray, 1840), Indonesian Papua; C–G, female (11.7 × 11.2 mm) (ZRC 2017.1283), Peninsular Malaysia. C, in situ in Glauconome rugosa
Hanley, 1843 (Glauconomidae); D, overall habitus; E, F, frontal view; G, ventral view. Photographs: A, B, Sigit Dwiono; C–G, Paul Y.
C. Ng.

217

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 64. Plenotheres coarctatus (Bürger, 1895), female (6.8 × 6.5 mm) (ZRC 2018.246), Peninsular Malaysia. A, overall dorsal view; B,
ventral view of cephalothorax; C, frontal view of cephalothorax; D, outer view of right chela.

Type material. Neotype (here designated): female (13.1
× 11.4 mm) (ZRC 2020.13), Tipoeka, Timika, Indonesian
Papua, Indonesia, in Polymesoda expansa, coll. D. L. Rahayu
et al., 26 April 2000.

females (3.4 × 3.3 mm, 3.2 × 3.2 mm) (ZRC 2018.765), samedata as neotype; 1 female (7.9 × 7.2 mm) (ZRC 2019.1789),
in Glauconome rugosa, from Kandang Kerbau market,
Singapore, from Indonesia, coll. P. K. L. Ng, 29 December
2019; 2 females (11.3 × 10.7 mm, 10.2 × 9.5 mm) (ZRC
2017.1284), Ajkwa, Timika Province, Indonesian Papua,
no host data, coll. D. L. Rahayu et al., 20 January 2000; 1

Other material examined. Indonesia: 2 males (2.7 × 2.8mm, 3.3 × 3.4 mm), 1 female (11.9 × 10.8 mm), 2 juvenile
218

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 65. Plenotheres coarctatus (Bürger, 1895), overall dorsal view. A, neotype female (13.1 × 11.4 mm) (ZRC 2020.13), Indonesian
Papua; B, female (11.5 × 10.7 mm) (ZRC 2018.766), Indonesian Papua; C, female (13.9 × 12.4 mm) (ZRC 2003.681), Singapore; D,
female (13.3 × 11.9 mm) (ZRC 2012.1223), Singapore; E, female (8.0 × 7.6 mm) (ZRC 2018.254), Peninsular Malaysia; F, female (11.9
× 10.6 mm) (ZRC 2018.254), Peninsular Malaysia.

female (11.3 × 10.2 mm) (ZRC 2017.1285), Ajkwa, Timika
Province, Indonesian Papua, in Geloina coaxans, coll. D. L.
Rahayu et al., 11 January 2000; 1 female (13.2 × 11.6 mm)(ZRC 2017.1286), Tipoeka, Timika Province, Indonesian
Papua, in Polymesoda expansa, coll. D. L. Rahayu et al.,21 December 1999; 2 females (11.5 × 10.7 mm, 6.7 × 6.1mm) (ZRC 2018.766), in Batissa violacea, peat site, Timika,
Indonesian Papua, Indonesia, coll. D. L. Rahayu et al., 27
November 1999. Singapore: 1 female (14.1 × 13.1 mm)(ZRC 2001.2237), Loyang mangroves, off Loyang Avenue,
in Polymesoda expansa, coll. N. Sivasothi, 17–18 July 2001;
3 females (16.5 × 14.4 mm, 15.5 × 13.2 mm, 13.9 × 12.4mm) (ZRC 2003.681), in “Geloina zeylanica (Lamarck,1806)” (= present Geloina coaxans), coll. A. Ibrahim, June

2003; 2 females (14.4 × 13.2 mm, 13.3 × 11.9 mm) (ZRC
2012.1223), in Polymesoda expansa, mudflats, Pulau Ubin,coll. G. Jahvel, 5 November 2012. Peninsular Malaysia: 1
female (10.1 × 9.1 mm) (ZRC 1995.438), in Glauconome
rugosa, from market, coll. P. K. L. Ng, 21 December 1992; 5females (11.7 × 11.2 mm, 9.5 × 8.8 mm, 9.4 × 9.0 mm, 8.7
× 8.2 mm, 6.6 × 6.3 mm) (ZRC 2017.1283), in Glauconome
rugosa, from Chinatown market, Singapore, from Malaysia,
coll. P. K. L. Ng, 21 January 2018; 9 ovigerous females (6.8
× 6.5 mm, 8.0 × 7.1 mm, 8.1 × 6.7 mm, 8.1 × 7.6 mm, 8.4
× 8.6 mm, 9.0 × 8.0 mm, 9.1 × 8.0 mm, 9.9 × 8.8 mm, 10.5
× 9.2 mm) (ZRC 2018.246), 2 ovigerous females (9.1 × 8.4
mm, 7.2 × 6.1 mm) (AM P105903), in Glauconome rugosa,
Chinatown market, Singapore, from Malaysia, coll. P. K. L.
219

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 66. Plenotheres coarctatus (Bürger, 1895). A–I, neotype female (13.1 × 11.4 mm) (ZRC 2020.13), Indonesian Papua; J, K, female(14.1 × 13.1 mm) (ZRC 2001.2237), Singapore. A–D, left P2–P5, respectively; E–H, right P2–P5, respectively; I, left MXP3; J, distal
part of left P5 dactylus (ventral view, soft setae omitted); K, distal part of right P5 dactylus (ventral view, soft setae omitted). Scales =A–H, 1.0 mm; I, 0.5 mm; J, K, 0.25 mm.

Description. Carapace and pereopods poorly chitinised.
Female: Carapace subcircular, slightly wider than long;
appearing domed in frontal view; dorsal and lateralsurfaces smooth, glabrous; with pair of fine longitudinallyarcuate grooves almost always well defined, extending
from behind orbits to indistinct gastrocardiac groove; front
slightly projecting anteriorly beyond anterolateral margins,
margin straight to gently convex; anterolateral marginsmooth, convex, gently curving to join rounded angle withposterolateral margin (Figs. 63D, 64A, 65). Eyes small, justNg, 11 February 2018; 4 females (8.0 × 7.6 mm, 9.1 × 8.0mm, 10.2 × 9.5 mm, 11.9 × 10.6 mm) (ZRC 2018.254), in
Glauconome rugosa, Chinatown market, Singapore, from
Malaysia, coll. P. K. L. Ng, 11 March 2018; 3 females(photographed 8.4 × 7.9 mm) (ZRC 2020.10), in Glauconome
rugosa, from Kandang Kerbau market, Singapore, coll. P.
K. L. Ng, 29 March 2020. Thailand: 1 ovigerous female(8.8 × 7.6 mm) (NHMD Cr-284250), Lem Ngob, Gulf ofThailand, mangrove swamp, coll. Th. Mortensen, 23–27
December 1900.
220

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 67. Plenotheres coarctatus (Bürger, 1895), male (3.3 × 3.4 mm) (ZRC 2018.765), Indonesian Papua. A, overall dorsal view; B, frontal
view of cephalothorax; C, outer view of right chela.

visible in dorsal view in adults; mobile, filling orbit (Figs.
63D–F, 64A, C). Epistome with median part triangular,
lateral margins concave (Fig. 64C).

with long dense setae; merus relatively longer, more slender,
relative lengths of meri P4>P3>P2>P5; right (sometimesleft) P4 merus and propodus distinctly the longer; P2–P4
dactyli relatively short, subequal, tip hooked, shorter than half
propodus length; dactylus of longer P4 stout, weakly curved,
longer than P2 and P3 dactylus, shorter than P5 dactylus; P5
merus 4.3–4.4 times longer than wide; P5 dactylus longest,
gently falciform, ventral margin lined with short and longsetae, distoflexor margin with relatively short row of 6–12
small spinules (Fig. 66A–K).

MXP3 outer surface with scattered short setae; ischiomerus
completely fused, subrhomboidal, inner margin prominently
angular (almost 90°) at widest point; carpus short; propodus
conical, about 2.5 times as long as high, distinctly longer
than carpus, tip rounded; dactylus slender, inserted at
proximal one-third of propodus, tip slightly overreaching
propodal apex; exopod relatively stout, with proximal partvery wide, about two-thirds length of ischiomerus, flagellum
2-segmented (Figs. 64C, 66I).

Pleon extending to buccal region, covering bases of P2–P5;
telson slightly recessed into concave distal margin of somite
6 (Fig. 64B).

Chela not prominently elongate, stout, dactylus exceeding
two-thirds palm length; palm proximally narrower than
distally; outer surfaces of palm, fingers (except for distalpart) almost glabrous, with only scattered short setae; ventral
margin of palm gently convex to almost straight; dactylus
occlusal margin with blunt triangular tooth proximal to
midlength, margin straight in distal half, finely denticulate,sparsely setose; pollex occlusal margin with 2 blunt triangularteeth proximal to midlength, straight margin in distal half,
finely denticulate, sparsely setose, with fringe of short setae
on inner ventral margin, extending to inner surface of palm
(Fig. 64D).

Male: Carapace almost circular, slightly longer than wide;
dorsal surface smooth, inflated, lateral surfaces with setae;
front projecting anteriorly, margin almost straight (Fig. 67A,
B). Eyes distinctly visible in dorsal view (Fig. 67A, B).
MXP3 as in female but dactylus shorter than propodus (Fig.
68J–L). Anterior thoracic sternum wide, sternites 1, 2 fused,partially sunken into buccal cavity; suture between sternites
2 and 3 shallow; sternites 3, 4 completely fused, separated
only by shallow grooves. Chela relatively stout, shorter
than in female (Figs. 67C, 68I). P2–P5 dorsally, ventrally
unarmed; outer surface covered with short setae; P3 and P4
carpus and propodus with long natatory setae; left and rightmeri equal, relative lengths of meri P3=P4>P2>P5; dactyli
of P2–P4 progressively longer; left P4 merus slightly longer
than right side; P4 and P5 dactylus subequal to that of P3,

P2–P5 dorsally, ventrally unarmed; margins of P2 and P3
covered with short setae; outer surface of P3 (especiallymerus) with dense long setae; ventral margins of dactylus
221

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 68. Plenotheres coarctatus (Bürger, 1895), male (3.3 × 3.4 mm) (ZRC 2018.765), Indonesian Papua. A–D, left P2–P5, respectively;
E–H, right P2–P5, respectively; I, outer view of right chela; J, left MXP3; K, palp of left MXP3 (outer view); L, palp of left MXP3 (innerview); M, pleon; N, left G1 (ventral view); O, distal part of left G1 (ventral view, setae omitted); P, left G1 (dorsal view, setae omitted);Q, left G2. Scales = A–I, M, 0.5 mm; J–L, N–Q, 0.2 mm.

222

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 69. Nepinnotheres corbiculae (Sakai, 1939). A, female, overall dorsal view; B, left female MXP3; C, outer view of left female chela.
A, after Sakai (1976: pl. 200 fig. 4); B, C, after Sakai (1939: text-fig. 77).

exiguus and A. rayi in most respects including the gonopod
and P5 structure; they can be externally distinguished by theshape and elongation of the MXP3 propodus, which is shorterand conical in Pl. coarctatus, with the dactylus articulatingslightly proximal to the midlength (Fig. 68J–L) (versus
elongate and spatulate with the dactylus articulating slightly
distal to the midlength in the two species of Arcotheres; Figs.
44A, 45A, 52B). In view of the strong similarities between
males of A. exiguus, A. rayi, and what is here considered to
be Pl. coarctatus; and because this species is herein madethe type species of a new genus, a neotype selection isrequired to fix its identity. This will stabilise the taxonomy of
Pinnotheres coarctatus and the genus. The neotype selected
is a female (13.1 × 11.4 mm; ZRC 2020.13) collected fromPolymesoda expansa in Indonesian Papua. This location issoutheast of the original type locality in southern Philippines.

covered with short setae (Fig. 68A–H). Pleon triangular,
widest at somite 3, lateral margins of somite 4 gently concave;
somite 6 subrectangular; telson semicircular, slightly wider
than long (Fig. 68M). G1 relatively stout, widely C-shaped,
curved outwards, with elongate sharp subdistal dorsal flap atright angles to main structure (Fig. 68N–P). G2 short, with
spatuliform tip; exopod long, curved (Fig. 68Q).
Variation. General setation is often less dense in larger
specimens. The gastric grooves on the carapace are usually
distinct, but less obvious in a few specimens, probably as aresult of poor preservation. The single row of spinules on theventral margin of the distal part of the P5 dactylus varies in
number from 6 to 12, but the proximal ones are frequently
very low and barely visible (Fig. 66J, K).
Colour. In life, the carapace varies from brown and grey
to dirty white overall, with the ovaries appearing purplish
and the eyes dull red (Fig. 63).

Ahyong & Ng (2021) showed that Rathbun’s (1910) record of
Pinnotheres parvulus from the Gulf of Thailand is referable
to Pl. coarctatus.

Host. Confirmed from Geloina coaxans (Gmelin, 1791),
Polymesoda expansa (Mousson, 1849), and Batissa violacea(Lamarck, 1818) (Cyrenidae Gray, 1840); or Glauconome
rugosa Hanley, 1843 (Glauconomidae Gray, 1853). Bothfamilies are in the superfamily Cyrenoidea Gray, 1840. Bürger
(1895) recorded the host as “Cahebe” (= Polymesoda spp.)(Schmitt et al., 1973). Note that Bürger’s (1895) “Cahebe”
is probably a misspelling of the local name for a commonly
harvested brackish water clam (Geloina sp.), known as“kabibe” in Tagalog and other Philippine languages, or“cabebe” or “cabibe” under old Hispanised renderings (J.
C. E. Mendoza, pers. comm.; Ahyong & Ng, 2021).

Distribution. South China Sea from the Philippines,
Indonesia, Peninsular Malaysia, Singapore, and Thailand.
Magnotheres, new genus
Type species. Pinnotheres globosum Hombron & Jacquinot,
1846, by present designation.
Diagnosis. Carapace dorsal surface smooth, without obvious
gastric grooves. MXP3 dactylus slender, inserted near orslightly proximal to ventral midlength of propodus, variable
length; propodus spatuliform, about 3.0 times longer thanwide; inner margin of widest part of ischiomerus rounded
to angular; exopod relatively slender with proximal part of
exopod not especially wide. Female P4 slightly asymmetrical,
with merus on left or right side slightly longer; dactyli of P2
and P3 subequal, weakly curved, longer than half propodus
length, P4 dactylus similar in shape but slightly longer thanP2 and P3 dactyli; P5 dactylus elongate, weakly falciform,
with 2 rows of distoflexor spinules. Male pleon relatively

Remarks. Ahyong & Ng (2007b: 195) regarded Plenotheres
coarctatus, new combination, originally described from
Zamboanga, Philippines, and whose original types are
lost, to be sufficiently recognisable as to not require aneotype designation at that time. Females of Pl. coarctatus
as understood here are distinguishable from species ofArcotheres by the pair of anterior dorsal carapace grooves.
Males of Pl. coarctatus, however, closely resemble male A.
223

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 70. Nepinnotheres corbiculae (Sakai, 1939), male (2.1 × 1.8 mm) (KPM-NH 106679), Japan. A, overall dorsal view; B, frontal viewof cephalothorax; C, outer view of left chela.

narrowly triangular, telson broadly trapezoidal, widest
distally, distal margin sinuous. G1 very slender, proximal
two-thirds straight, broadly curved at distal one-third, withsharply tapering tip; G2 exopod as long as endopod.

locality: Singapore) have revealed that it is not only a seniorsynonym of A. borradailei, A. ridgewayi, and A. latus, butthat it warrants transfer to a new genus, Magnotheres, newgenus (see also Remarks for M. globosus).

Etymology. The name is derived from the Latin ‘magnus’ forbig, alluding to the relatively large size of the type species;in combination with a typical ending for pinnotherid crabs.
Gender masculine.

The symmetry or near symmetry of the female P4 is diagnosticof Magnotheres (see Fig. 83A–H) relative to Arcotheres s.
str. In all other species that have been referred to Arcotheres,the asymmetry in the female P4 is very distinct and can bediscerned even in males (e.g., see Ng, 2018a). Females of
Magnotheres also differ from most female Arcotheres by
the gently curved P2 and P3 dactyli (Fig. 83A, B, E, F), in
contast to the condition found in most species of Arcotheres,in which the apex of the P2 and P3 dactyli have a distinctly
turned, gently hooked, spiniform tip (e.g., Figs. 14A, B, E,
F, 54F, G, J, K). Males of M. globosus, new combination,
are notable because the pleon is unlike that of known
Arcotheres, with somites 5 and 6 narrow and the telson
broadly trapezoidal, being widest across the distal margin(Figs. 85C, 86J); and with the G1 very long and slender (Fig.
86K). The unusual male telson of M. globosus is similar tothat of Fabia concharum (Rathbun, 1893) (Campos, 1996:
fig. 4A) from the Americas. In all other species of Arcotheres,male pleonites 5 and 6 are proportionally wider and theRemarks. Ng et al. (2017: 1081, 1092) alluded to thepossible polyphyly of Arcotheres in treating the taxonomyof two nominal species from India, Pinnoteres borradailei
Nobili, 1906a (replacement name for Pinnotheres tenuipes
Borradaile, 1903), and Pinnoteres ridgewayi Southwell,1911. Although they referred both species to Arcotheres, theyrecognised that the asymmetry of the female left and rightP4 was slight at best, as in Arcotheres latus (Bürger, 1895).
As a result of the present review of all of these nominal
species, A. borradailei, A. ridgewayi, and A. latus havebeen found indistinguishable and are synoynymised.
Moreover, reassessment and revision of the poorly known
Pinnotheres globosum Hombron & Jacquinot, 1846 (type224

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 71. Nepinnotheres corbiculae (Sakai, 1939), male (2.1 × 1.8 mm) (KPM-NH 106679), Japan. A, right MXP3 (outer view); B, right
MXP3 (inner view); C, D, propodus and dactylus of right MXP3 (outer view); E, outer view of left chela; F–I, left P2–P5, respectively; J,right P3; K, pleon; L, left G1 (ventral view); M, left G1 (dorsal view); N, distal part of left G1 (ventral view, setae omitted); O, distal partof left G1 (dorsal view, setae omitted); P, right G2; Q, distal part of right G2. Scales = A, B, E–M, P, 0.2 mm; C, D, N, O, Q, 0.1 mm.

a range of telson forms expressed in Arcotheres, parallelingsimilar variations observed in Fabia, for instance (Campos,
1996). However, unlike Fabia in which the G1 follows acommon pattern despite variation in telson shape, the G1of M. globosus is also of a distinctly different form fromall known species of Arcotheres, being proportionately
more elongated, straighter, and more slender (Fig. 86K),
approaching that seen in Afropinnotheres Manning, 1993
(Manning, 1993b: figs. 4i, 5c; Ahyong, 2019: fig. 2K). In

telson is a more typical polygonal to semicircular shape,
being widest proximally (e.g., Fig. 62B). A few species ofArcotheres (e.g., A. ocularius Komai, Kawai & Ng, 2020),
have the male telson trapezoidal or almost pentagonal inshape, but the widest point is near the midlength (Komaiet al., 2020: figs. 10B, E, 13D, H, 15F), and somites 5 and6 are still proportionally wider. It could be argued that theinverted trapezoidal male telson in M. globosus is simply
an autapomorphy, being one mode of polymorphism within225

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia
all species of Arcotheres, however, where known, the G1
is much stouter and shorter than in M. globosus, even for
species like A. palaensis (see Figs. 22K–M, 32L, M, 33G,J, 44K, L, 45H, I; Komai et al., 2020: figs. 5E, F, 13I, J,
15G, H). A preliminary genetic study by Tsang Ling Ming
from the Chinese University of Hong Kong also recovers
Magnotheres globosus in a separate clade from otherspecies of Arcotheres from Southeast Asia, India, Japan,
Hong Kong, and Fiji (L. M. Tsang, pers. comm.). As such,with the presence of correlated differences compared to all
known male Arcotheres, we consider Magnotheres to begenerically distinct.

Pinnoteres ridgewayi Southwell, 1911: 226, pl. 3 figs. 1, 2, 2a [typelocality: Kondatchi Paar, Sri Lanka, by lectotype designation,
Ng et al., 2017].
Pinnotheres ridgewayi — Tesch, 1918: 250, 251 (list); Gravely,
1927: 146, pl. 23 fig. 37; Prasad & Tampi, 1957: 22, fig. 1(larvae); Sankarankutty, 1966: 349, 350, 358, 360, figs. 23, 28;
Silas & Alagarswami, 1967: 1177, 1183, 1208, 1219, 1223,
1225, 1227 (discussion); Serène, 1968: 94; Schmitt et al., 1973:
84; Ng et al., 2008: 251 (list); Ng et al., 2017: 1080.
Pinnotheres similis — Rathbun, 1924: 15; Davie, 2002: 434 (notPinnotheres similis Bürger, 1895).
Pinnotheres alcocki — Rathbun, 1909: 114 (part); Gordon, 1936:
176, fig. 5 [not Pinnotheres alcocki Rathbun, 1909].
Pinnotheres sp. — Ward, 1967: 62.
Pinnotheres globosa — Silas & Alagarswami, 1967: 1199, 1204;
Schmitt et al., 1973: 46; Ng et al., 2008: 250.
Pinnotheres globosus — Serène, 1968: 93; Schmitt et al., 1973:
46 (part); De Gier & Becker, 2020: tab. 1.
Arcotheres similis — Ahyong & Brown, 2003: 9; De Gier & Becker,
2020: tab. 1 (part) [not Pinnotheres similis Bürger, 1895].
Arcotheres latus — Ahyong & Ng, 2007b: 198, fig. 6; Ng et al.,
2008: 248; Ng et al., 2017: 1093; Trivedi et al., 2018a: 197;
Ahyong, 2020b: 880; De Gier & Becker, 2020: tab. 1; Ahyong& Ng, 2021: 195, 207.
Nepinnotheres ridgewayi — Ng & Kumar, 2015: 265 (discussion).
Arcotheres borradailei — Ng & Kumar, 2015: 265; Ng et al.,
2017: 1081, figs. 1–3, 8A; Trivedi et al., 2018a: 197; Trivedi
et al., 2018b: 61; Trivedi et al., 2018c: 50; De Gier & Becker,
2020: tab. 1.
Arcotheres ridgewayi — Ng et al., 2017: 1085, figs. 4–7, 8B;
Trivedi et al., 2018a: 197; Trivedi et al., 2018b: 61; Trivedi
et al., 2018c: 50; De Gier & Becker, 2020: tab. 1.

Ng et al. (2017: 1081) noted that in A. borradailei and
A. ridgewayi, the P5 basis-ischium appeared to be moreelongated and cylindrical (Fig. 83D, H) than in most otherpinnotherids, and that this may have diagnostic value among
species of Arcotheres in the future. Indeed, females of
Magnotheres and Arcotheres share the “bar-like” P5 merus
and elongated ischium in which the articulation between the
two articles is perpendicular to the segment axis, rather than
diagnonal as seen in other pinnotherids. The condition of theP5 ischium-merus articulation in males of Magnotheres andmost species of Arcotheres (where known) is similar to thatof females. In males of A. exiguus and A. rayi, however, theischium-merus articulation is diagonal, indicating possible
heterogeneity in Arcotheres.
Sankarankutty (1966: text-figs. 23, 28) figured the distal
half of the G1 and male pleon of a specimen from southern
India (as P. ridgewayi), and his figures, although somewhat
simple, show the same features as our male from Singapore.
The slender G1 and trapezoidal male telson are diagnostic.

Type material. Neotype (here designated): female (10.2 × 8.4mm) (ZRC 2016.164), off Changi Beach, Singapore, adjacentto National Service Recreational Country Club, from Pinna
atropurpurea, intertidal area, coll. S. K. Tan et al., 6 July
2016. Myanmar: 1 ovigerous female (ca. 8.3 × 6.7 mm,
carapace soft) (NHM 1886.52), Mergui Archipelago, coll.
J. Anderson (paralectotype of Pinnotheres alcocki Rathbun,1909). Philippines: 1 female (10.6 × 8.7 mm) (SMF-ZMG
956a) (lectotype of Pinnotheres latus Bürger, 1895), from
Pinna sp., Burias; 3 females (9.5 × 7.5–10.5 × 9.0 mm)(SMF-ZMG 956b) (paralectotypes of Pinnotheres latus
Bürger, 1895), same data as lectotype. India: 1 ovigerous
female (8.0 × 6.8 mm) (CUMZ I.63816) (holotype of
Pinnotheres borradailei Nobili, 1906a, replacement namefor Pinnotheres tenuipes Borradaile, 1903), Minikoi (present
day = Minicoy), Lakshadweep Islands (previously known
as the Laccadive, Minicoi, and Aminidivi Islands), India,
previously part of Maldive and Laccadive Islands, supposedly
from Mya, coll. J. S. Gardiner. Sri Lanka: 1 female (14.9
× 12.8 mm) (ZSI/WGRC/IRINV 8102/10) (lectotype of
Pinnotheres ridgewayi Southwell, 1911) [photographs
examined], from Pinna bullata Gmelin, 1791, pearl banks,
Kondatchi Paar, Ceylon (= Sri Lanka), coll. T. Southwell,1910; 2 females (14.4 × 11.9 mm, one with carapace in poor
condition) [photographs examined], same data as lectotype(paralectotypes of Pinnotheres ridgewayi Southwell, 1911).

Magnotheres globosus (Hombron & Jacquinot, 1846),
new combination(Figs. 72–86)
Pinnotheres globosum Hombron & Jacquinot, 1846: pl. 5 figs.
21–26 [type locality: Changi, Singapore, by present neotype
designation]; Jacquinot & Lucas, 1853: 58; Ng & Corbari,
2019: 245, fig. 6.10–6.13 (discussion).
Pinnotheres rouxi — ?Paulson, 1875: 70, pl. 9 fig. 2-2c (notPinnotheres rouxi H. Milne Edwards, 1853).
Pinnotheres latus Bürger, 1895: 374, 375, pl. 9 fig. 16, pl. 10
fig. 15 [type locality: Ubay, Philippines, by present lectotype
designation, from Pinna sp.]; Tesch, 1918: 249 (Sumba
and Amboina, Indonesia); Gordon, 1936: 171 (discussion);
Estampador, 1937: 547 (list); Miyake, 1939: 221 (list); Silas& Alagarswami, 1967: 1201 (list); Serène, 1968: 94; Schmitt
et al., 1973: 52.
Pinnotheres parvulus — De Man, 1888: 105; Alcock, 1900: 339[not Pinnotheres parvulus Stimpson, 1858].
Pinnoteres borradailei Nobili, 1906a: 410 (replacement name for
Pinnotheres tenuipes Borradaile, 1903) [type locality: Minicoy,
Lakshadweep Islands, India]; 1906b: 306 (part); Guinot, 1967:
279 (list).
Pinnotheres borradailei — ?Lenz, 1910: 558, 576; Laurie, 1915:
415 (list); Tesch, 1918: 248, 253, 287 (list and key); Silas &Alagarswami, 1967: 1197 (list); Schmitt et al., 1973: 40 (list);
Ng et al., 2008: 250 (list); Ng et al., 2017: 1080.

Other material examined. Singapore: 1 ovigerous female(13.2 × 9.9 mm), 1 non-ovigerous female (14.4 × 10.8 mm)

226

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 72. Magnotheres globosus (Hombron & Jacquinot, 1846), colour in life. A, ovigerous female (13.0 × 10.0 mm) (ZRC 2018.782),
Philippines; B, ovigerous female (9.0 × 6.8 mm) (ZRC 2018.781), Philippines; C, male (5.4 × 5.5 mm) (ZRC 2018.783), Philippines.
Photographs: A, Chan Tin-Yam.

227

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 73. Magnotheres globosus (Hombron & Jacquinot, 1846). A, B, colour in life and in situ in Pinna sp., ovigerous female (11.8 × 9.5mm) (ZRC 2017.1017), Panglao, Philippines; C, female in situ in Pinna atropurpurea, Singapore, coll. 12 January 2017 (not collected).
Photographs: A, Chan Tin-Yam; B, C, Tan Heok Hui.

228

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 74. Magnotheres globosus (Hombron & Jacquinot, 1846), colour in life. A, non-ovigerous female (14.4 × 10.8 mm) (ZRC 2017.1018),
Changi, Singapore; B–D, ovigerous female (13.2 × 9.9 mm) (ZRC 2017.1018), Changi, Singapore. A, B, overall dorsal view; C, D, frontal
view of cephalothorax. Photographs: Tan Heok Hui.

229

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 75. Magnotheres globosus (Hombron & Jacquinot, 1846), colour in life, non-ovigerous female (14.6 × 10.7 mm) (ZRC 2017.1020),
with bopyrid parasite (Rhopalione sp.), Changi, Singapore. A, overall habitus; B, frontal view of cephalothorax showing asymmetrical pleon;
C, female pleon with left side folded inwards; D, ventral surface of pleon showing folding of left side; E, ventral view of Rhopalione sp.;F, dorsal view of Rhopalione sp. Photographs: Lee Bee Yan.

(ZRC 2017.1018), from Pinna atropurpurea, intertidal area,
Tanah Merah Ferry Terminal Beach, near Changi Point, coll.
S. K. Tan et al., 13 December 2016; 3 ovigerous females(11.9 × 9.3 mm, 13.1 × 11.0 mm, 13.6 × 10.5 mm) (ZRC
2017.1019), from Pinna atropurpurea, intertidal area, Changi
East, east of Tanah Merah Ferry Terminal Beach, near Changi

Point, coll. S. K. Tan et al., 14–16 December 2016; 1 nonovigerous female (14.6 × 10.7 mm) (ZRC 2017.1020), with
Rhopalione sp. bopyrid parasite, from Pinna atropurpurea,
intertidal area, Changi East, east of Tanah Merah Ferry
Terminal Beach, near Changi Point, coll. S. K. Tan et al.,
14–16 December 2016; 2 ovigerous females (ZRC 2018.786),
230

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 76. Magnotheres globosus (Hombron & Jacquinot, 1846), colour in life. A, ovigerous female (12.1 × 9.4 mm) (ZRC 2016.166),
Vanuatu; B, ovigerous female (13.5 × 10.7 mm) (ZRC 2016.165), Vanuatu. Photographs: Chan Tin-Yam.

from Pinna atropurpurea, intertidal area, Tanah Merah Ferry
Terminal Beach, near Changi Point, coll. P. K. L. Ng & S. K.
Tan, 12–15 January 2017; 1 ovigerous female (13.6 × 10.4mm) (ZRC 2017.176), from Pinna atropurpurea, intertidalarea, Tanah Merah Ferry Terminal Beach, near Changi Point,coll. P. K. L. Ng & S. K. Tan, 26 May 2017; 1 ovigerous
female (13.2 × 10.1 mm) (AM P105903), from Pinna
atropurpurea, intertidal area, Tanah Merah Ferry Terminal
Beach, near Changi Point, coll. S. K. Tan et al., 27 May
2017; 3 females (ZRC 2018.786), from Pinna atropurpurea,
intertidal area, Changi East, east of Tanah Merah Ferry
Terminal Beach, near Changi Point, coll. S. K. Tan et al., 12
and 15 January 2017; 1 male (8.0 × 7.8 mm) (ZRC 2020.12),

station IT140 (collection number station SS-4132), intertidal,
from Pinna atropurpurea, Pulau Tekukor, Southern Islands,
01°13.899′N, 103°50.265′E, 0.4–0.8 m, coll. Comprehensive
Marine Biodiversity Survey, 31 May 2013. Philippines: 3
ovigerous females (9.8 × 7.6 mm, 11.0 × 8.9 mm, 11.8 ×
9.5 mm), 1 ovigerous female (carapace broken, carapace
length 11.9 mm), 1 non-ovigerous female (7.2 × 6.2 mm)(ZRC 2017.1017), from Pinna sp., Panglao Island, Bohol,
Visayas, coll. Panglao 2004 Expedition, 5 July 2004; 1
ovigerous female (9.0 × 6.8 mm) (ZRC 2018.781), station
M11, host not recorded, rocky intertidal, fringe mangrove,and seagrass habitat, Sungcolan Bay, Panglao Island, Bohol,
Visayas, 0–3 m, 9°38.3′N, 123°49.6′E, coll. Panglao 2004
231

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 77. Magnotheres globosus (Hombron & Jacquinot, 1846), male (8.0 × 7.8 mm) (ZRC 2020.12), Singapore. A, overall dorsal view;B, frontal view of cephalothorax; C, ventral view of cephalothorax. Photographs: Arthur Anker.

Expedition, 6 June 2004; 1 ovigerous female (13.0 × 10.0mm) (ZRC 2018.782), station M18, in Pinna muricata,
on sandy bottom and seagrass, Gak-Ang Islet, Panglao
Island, Bohol, Visayas, 0–1 m, 9°33.0′N, 123°43.5′E, coll.
Panglao 2004 Expedition, 10 June 2004; 1 male (5.4 × 5.5mm) (ZRC 2018.783), station M18, in Pinna muricata, on

sandy bottom and seagrass, Gak-Ang Islet, Panglao Island,
Bohol, Visayas, 0–1 m, 9°33.0′N, 123°43.5′E, coll. Panglao
2004 Expedition, 10 June 2004. Palau: 2 females (8.7 × 7.2mm, 7.3 × 6.3 mm) (SMF-ZMG 955), from Pinna nigrina,
coll. Semper. Vanuatu: 1 female (13.5 × 10.7 mm) (ZRC
2016.165), from unknown host, intertidal area, sand and232

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 78. Magnotheres globosus (Hombron & Jacquinot, 1846), type female (20.0 × 18.0 mm). A, overall dorsal view; B, D, left MXP3;
C, outer view of right chela. A–C, after Hombron & Jacquinot (1846: pl. 5 figs. 21–23); D, after Milne Edwards (1853: pl. 11 fig. 6).

muddy rock substrate, Belmoul Lagoon, Santo, 15°35.8′S,
167°06.1′E, coll. SANTO Expedition, 13 September 2006;
1 female (12.1 × 9.4 mm) (ZRC 2016.166), specimen data
BC 3888, from inside unidentified Pinnidae, Santo, coll. 9
October 2006. Australia: 2 males (5.1 × 5.1–6.8 × 6.8 mm),
7 females (4.0 × 4.3–9.0 × 11.7 mm) (AM P7067), Fields
Reef, Port Denison, Queensland, 20°01′S, 148°15′E, fromyoung Pinna sp. in shallow water, coll. E. Rainford, 1923;
1 female (8.9 × 7.1 mm) (AM P12173), Michelmas Cay,Great Barrier Reef, Queensland, 16°35′S, 146°02′E, from
Pinna sp. on coral reef, coll. G. P. Whitley & T. Iredale, June
1926; 1 female (12.2 × 9.0 mm) (AM P64685), Lindeman
Island, Queensland, dredged, coll. M. Ward, December
1928; 1 female (11.5 × 9.2 mm) (AM P20161), East Arm,
Darwin Harbour, Northern Territory, in mytilid Stavilla [sic]horrida (= Stavelia subdistorta (Reclúz, 1852)), coll. O.
Cameron, 19 December 1971. India: 1 female (17.7 × 13.6mm) (ZSI/WGRC/IR-INV 8235), from Pinna atropurpurea,
Kovalam, 8.3°N, 77.2°E, Thiruvananthapuram, Kerala,
southwestern India, 5–10 m depth, coll. A. B. Kumar &R. Ravinesh, 2014; 1 ovigerous female (14.7 × 11.2 mm)(ZRC 2016.184), same locality and host as preceding, coll.
R. Ravinesh, early 2016; 1 ovigerous female (16.6 × 12.3mm) (ZRC 2016.185), same locality and host as preceding,coll. R. Ravinesh, June 2016.

tip rounded to subtruncate; dactylus slender, inserted near orslightly proximal to ventral midlength of propodus, not orjust reaching propodal apex; exopod relatively slender, about
two-thirds length of ischiomerus, flagellum 2-segmented(Figs. 74D, 83I–K, 84B–G).
Chela not prominently elongate, dactylus about half to threequarters palm length; palm relatively slender, proximallynarrower than distally; outer surface of palm, fingers (except
for distal part) almost glabrous, with only scattered shortsetae; ventral margin of palm gently concave to sinuous;
dactylus occlusal margin with distinct subproximal tooth;pollex occlusal margin with 1 low proximal tooth, 1
submedian tooth, and denticles; tips of fingers sharp, hooked(Figs. 79D, 80D).
P2–P5 dorsally, ventrally unarmed; outer surface covered with
scattered, very short setae or glabrous; ventral margins ofpropodus and dactylus slightly more setose; merus relatively
slender, relative lengths of meri P4>P3>P2>P5; left or
right P4 merus slightly longer than other side, propodus
and dactylus not clearly bilaterally asymmetrical; P2–P4
dactyli relatively long, subequal, weakly curved, longer
than half propodus length, P4 dactylus similar in shape but
slightly longer than P2 and P3 dactyli; P5 merus 4.5–4.6
times longer than wide; P5 dactylus longest, margins linedwith short and long setae, denser on ventral margin, with
2 rows of distoflexor spinules, upper row usually with 5–8
spinules, lower row with 12–18 small spinules, increasing
in size distad (Figs. 81F, 83A–H).

Description. Carapace and pereopods well chitinised.
Female: Carapace subcircular to transversely subhexagonal,
wider than long; dorsal and lateral surfaces smooth, glabrous;
appearing domed in frontal view; front occasionallyprojecting anteriorly beyond orbits, margin straight to gently
convex; anterolateral margin gently convex (Figs. 72A, 73A,
74A–C, 75A, B, 76, 79A, C, 80A, C, 81A–E, 82). Eyes
small, usually not clearly visible in dorsal view in adults,
filling orbit (Figs. 72A, 73A, 74A, B, 75A, 76, 79A, 80A,
81A–D, 82, 84A). Epistome with median part triangular,
lateral margins gently concave (Figs. 79C, 80C, 81E).

Pleon extending to buccal region, covering bases of P2–P5;
telson gently recessed into concave distal margin of somite
6 (Figs. 72B, 79B, 80B).
Male: Carapace almost circular, slightly wider than long;
dorsal surface almost smooth, not prominently inflated,
lateral surfaces with setae; front projecting anteriorly,
margin almost straight (Figs. 72C, 77A, B, 85A, B, 86A).
Eyes distinctly visible in dorsal view (Figs. 72C, 77A, 85A,
B, 86A). MXP3 as in female (Fig. 86D). Anterior thoracic
sternum wide, sternites 1, 2 fused, partially sunken into

MXP3 outer surface with scattered short setae; ischiomerus
completely fused, subrhomboidal, inner margin rounded toangular at widest point; carpus short; propodus about 3 times
as long as high, spatuliform, distinctly longer than carpus,
233

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 79. Magnotheres globosus (Hombron & Jacquinot, 1846), neotype female (10.2 × 8.4 mm) (ZRC 2016.164), Changi, Singapore. A,
overall dorsal view; B, ventral view of cephalothorax; C, frontal view of cephalothorax; D, outer view of left chela.

234

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 80. Magnotheres globosus (Hombron & Jacquinot, 1846), ovigerous female (11.8 × 9.5 mm) (ZRC 2017.1017), Panglao, Philippines.
A, overall dorsal view; B, ventral view of cephalothorax; C, frontal view of cephalothorax; D, outer view of left chela.

buccal cavity; suture between sternites 2 and 3 shallow;
sternites 3, 4 completely fused, separated only by shallow
grooves (Figs. 77C, 85C, 86C). Chela relatively stout, shorter
than in female (Figs. 85D, 86E). P2–P5 dorsally, ventrally
unarmed; outer surface covered with short setae; P3 and P4
carpus and propodus with long natatory setae; left and rightmeri equal, relative lengths of meri P4>P3>P2>P5; dactyli
of P2–P4 progressively longer; P5 dactylus falciform, long,covered with short setae (Figs. 85A, 86F–I). Male pleon
relatively narrowly triangular, widest at somite 3, lateral
margins of somite 4 gently concave; somite 6 trapezoidal;
telson broadly trapezoidal, distal margin sinuous, much wider
than proximal one (Figs. 77C, 85C, 86J). G1 very slender,
proximal two-thirds straight, broadly curved at distal onethird, with sharply tapering tip (Fig. 86K). G2 short, with
spatuliform tip; exopod as long as endopod (Fig. 86L).
235

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 81. Magnotheres globosus (Hombron & Jacquinot, 1846). A, ovigerous female (12.1 × 9.4 mm) (ZRC 2016.166), Vanuatu; B, E,
F, ovigerous female (13.5 × 10.7 mm) (ZRC 2016.165), Vanuatu; C, ovigerous female (11.9 × 9.3 mm) (ZRC 2017.1019), Changi,
Singapore; D, ovigerous female (13.6 × 10.5 mm) (ZRC 2017.1019), Changi, Singapore. A–D, overall dorsal view; E, frontal view of
cephalothorax; F, left P2–P5.

Variation. The variation in carapace shape in this species isquite substantial, from transversely ovate with a low front(e.g., Fig. 82F) to more subhexagonal with a rounded, moreproduced front (e.g., Fig. 82D). The holotype of Pinnotheres
latus has a relatively low truncate front (cf. Ahyong & Ng,
2007b: fig. 6A) as does another specimen from the Philippines(ZRC 2017.1017) (Fig. 84A), but in most specimens, thefront is broadly convex (e.g., Fig. 82A, B). The shape ofthe MXP3 ischiomerus varies to some degree, with the innerangle at the widest point usually angular and relatively welldefined (e.g., Figs. 83J, K, 84A, B), but in several specimens,
it is more rounded (e.g., Fig. 84D, E) with one appearing
almost broadly curved (Fig. 84F, G). The slender MXP3
dactylus varies slightly in length, usually not underreaching(Figs. 83I, 84B, E), but occasionally reaching the apex (Figs.
83J, K, 84C, D). The number of spinules on the two rows
on the ventral distal part of P5 dactylus is quite variable.
In the neotype female, there are only six spinules on theouter row (Fig. 83D’) and 14 spinules on the inner row on
the left side (Fig. 83D”), but on the right side, there are six
236

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 82. Magnotheres globosus (Hombron & Jacquinot, 1846), dorsal view of female carapace. A, neotype ovigerous female (10.2 × 8.4mm) (ZRC 2016.164), Singapore; B, ovigerous female (13.2 × 9.9 mm) (ZRC 2017.1018), Changi, Singapore; C, ovigerous female (13.6 ×
10.5 mm) (ZRC 2017.1019), Changi, Singapore; D, ovigerous female (11.9 × 9.3 mm) (ZRC 2017.1019), Changi, Singapore; E, ovigerous
female (13.5 × 10.7 mm) (ZRC 2016.165), Vanuatu; F, ovigerous female (12.1 × 9.4 mm) (ZRC 2016.166), Vanuatu.

spinules on the outer row (Fig. 83H’) and only seven very
short, barely visible spinules on the inner row (Fig. 83H”).
The spinules on the inner row of the right side seem abraded,very short, and barely visible (Fig. 83H”).

purple (Figs. 72A, B, 73, 74, 75A, B, 76), with the eyes
dark brown to red and white (Figs. 74C, D, 75B). In life,
males are light brown with pale ventral surfaces (Fig. 77).
Host. Known with certainty from Pinna (Subitopinna)atropurpurea Sowerby, 1825, or Pinna (Quantulopinna)
muricata Linnaeus, 1758 (Pinnidae Leach, 1819; Pinnoidea

Colour. The colour of live females varies from pale brown,with numerous small, almost black flecks, to pale or bright237

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 83. Magnotheres globosus (Hombron & Jacquinot, 1846), neotype ovigerous female (10.2 × 8.4 mm) (ZRC 2016.164), Singapore.
A–D, left P2–P5, respectively; D’, distal part of dactylus of left P5 showing stiff subdistal spinules (dorsal view, soft setae omitted); D”,
distal part of dactylus of left P5 showing stiff subdistal spinules (ventral view, soft setae omitted); E–H, right P2–P5, respectively; H’,distal part of dactylus of right P5 showing stiff subdistal spinules (dorsal view, soft setae omitted); H”, distal part of dactylus of right P5
showing stiff subdistal spinules (ventral view, soft setae omitted); I, outer view of right MXP3 (basal part with breakage line, exopod,
and setae omitted); J, left MXP3 (setae omitted); K, left MXP3 (slightly tilted, exopod and setae omitted). Scales = A–H, 2.0 mm; D’,
D”, H’, H”, 0.2 mm; I–K, 1.0 mm.

238

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 84. Magnotheres globosus (Hombron & Jacquinot, 1846). A, ovigerous female (carapace width 11.9 mm, carapace broken) (ZRC
2017.1017), Panglao, Philippines; B, ovigerous female (13.6 × 10.5 mm) (ZRC 2017.1019), Changi, Singapore; C, ovigerous female (11.8
× 9.5 mm) (ZRC 2017.1017), Panglao, Philippines; D, ovigerous female (11.0 × 8.9 mm) (ZRC 2017.1017), Panglao, Philippines; E,ovigerous female (16.6 × 12.3 mm) (ZRC 2016.185), Kerala, India; F, G, ovigerous female (14.7 × 11.2 mm) (ZRC 2016.184), Kerala,
India. A, frontal margin of carapace; B, C, E–G, left MXP3 (setae denuded); D, right MXP3 (setae denuded). F, G, drawn at different
angles. Scales = A, 1.0 mm; B–G, 0.5 mm.

239

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia

Fig. 85. Magnotheres globosus (Hombron & Jacquinot, 1846), male (5.4 × 5.5 mm) (ZRC 2018.783), Philippines. A, overall dorsal view;B, frontal view of cephalothorax; C, ventral view of sternum and pleon; D, outer view of right chela.

Leach, 1819). We have not found more than one specimen
in any single Pinna collected. In Europe, another species,
Nepinnotheres pinnotheres (Linnaeus, 1758), lives in Pinna(see Becker & Türkay, 2010; Trigos & Vicente, 2018; Acarli
et al., 2019).

reaching 17.7 × 13.6 mm (from India). The carapace of
this species is rather variable, with some almost circular inoutline (Fig. 82).
Hombron & Jacquinot’s (1846: pl. 5 fig. 21) figure of thespecies (present Fig. 78A–C) is very schematic, showing around carapace with all the ambulatory legs symmetricaland the dactyli short and equal in length. Similarly, his
figure of the MXP3 is inaccurate, with the presence of afaint suture between the ischium and merus, and the exopod
long, exceeding the length of the ischiomerus (Hombron &Jacquinot, 1846: pl. 5 fig. 22). No species of pinnotherid fromSingapore and Peninsular Malaysia has this kind of carapace,ambulatory legs, and MXP3. In fact, the MXP3 structure(shape as well as separation of the ischium and merus, andlength of the exopod) is not found in any pinnotherid. Henri
Milne Edwards (1853: pl. 11 fig. 6) depicted the MXP3
of a specimen from Vavao in the Solomon Islands moreaccurately, showing a long ischiomerus and relatively short
dactylus which does not reach the tip of the propodus (Fig.
78D), features that agree very well with what is currently
identified as A. latus (cf. Figs. 83I–K, 84B–G). Hombron& Jacquinot’s (1846) figures are inaccurate and cannot be
relied on.

Remarks. The authorship and year of publication of
Pinnotheres globosum was discussed by Clark & Crosnier(2000) and Holthuis (2002); we follow their recommendations
to recognise the taxon as being validly published from theearly published plate, i.e., the taxon should be cited as
Pinnotheres globosum Hombron & Jacquinot, 1846. The
text by Jacquinot & Lucas (1853) that actually described
the species appeared later. Jacquinot & Lucas (1853: 59)
apparently only had one 20 × 18-mm female, commentingthat “Nous ne connaissons pas le mâle de ce Pinnotheres,qui a été pris dans une grosse Modiole à Singapour [Wedo not know the male of this Pinnotheres, who was caught
in a big mussel in Singapore]” (Jacquinot & Lucas, 1853:
60). The French name for the host, “Modiole”, is not a
specific reference to Modiolus, but a vernacular reference
to a mussel of some kind, which, at that time, would haveincluded pinnids as well as mytilids. On the basis of thesize and description of a “big mussel”, it is almost certainthat they obtained their specimen from a large species ofPinna (even today often called horse mussels), and that the
crab species is what is currently known as Arcotheres latus(Bürger, 1895) (see Ng & Corbari, 2019). This is one of
the two largest pinnotherids known from Singapore, withthe largest known recent local specimen measuring 14.6 ×
10.7 mm (ZRC 2017.1020), and the largest known specimen

The types of Pinnotheres globosus Hombron & Jacquinot,
1846, cannot be located in the MNHN and are almost
certainly lost. In view of the complex and often confusingtaxonomy of this species, uncertain identity, and the revised
synonymy recognised here, a neotype should be selected. We
here designate a recently collected female specimen (10.2
240

RAFFLES BULLETIN OF ZOOLOGY 2022

Fig. 86. Magnotheres globosus (Hombron & Jacquinot, 1846). A–L, male (8.0 × 7.8 mm) (ZRC 2020.12), Singapore; M–R, female (13.0
× 10.0 mm) (ZRC 2018.782), Philippines. A, M, overall dorsal view; B, O, frontal view of cephalothorax; C, P, anterior thoracic sternum;
D, R, right MXP3; E, Q, right chela; F–I, right P2–P5; J, abdomen; K, left G1, abdominal view; L, left G2, abdominal view; N, carapace
front, oblique dorsal view. Scales = A, 4.0 mm; B–J, 2.0 mm; K, L, 1.0 mm; M, N, 5.0 mm; O–Q, 2.5 mm; R, 1.25 mm.

241

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia
× 8.4 mm; ZRC 2016.164) from Singapore as the neotypeof Pinnotheres globosus Hombron & Jacquinot, 1846. This
neotype is a recently collected, fully intact specimen froma known host and ethanol-preserved.

poor condition, appearing to have been fixed in formalingiven the “stiffness” of the appendages and cuticle texture.
The carapace is collapsed and the frontal margins sunken
in, but the walking legs are intact with the right P4 only
slightly longer than the left. De Man (1887) reported thehost as Pinna atropurpurea Sowerby, 1825, and Pinna
vexilllum Born, 1778 (now Atrina), consistent with what
we know of M. globosus. The present record constitutes thefirst confirmation of M. globosus from the Andaman Sea.

Comparisons of a large series of specimens of M. globosus,
new combination, from many parts of the Indo-West Pacific,
show that it is in fact a senior synonym of Pinnotheres
latus Bürger, 1895 (type locality: Philippines), Pinnoteres
ridgewayi Southwell, 1911 (type locality: Kondatchi Paar,
Sri Lanka), and P. borradailei Nobili, 1906a (type locality:
Lakshadweep, India). The lectotype of P. latus is relatively
well preserved, but now relatively soft. Bürger (1895: pl.
9 fig. 16) figured the front as somewhat triangular, while
Ahyong & Ng (2007b: fig. 6A) showed it to be truncate(see Variation). Ng et al. (2017: 1093) distinguished A.
ridgewayi from A. latus by the proportionately longer MXP3
ischiomerus and differing proportions of the ambulatory
dactyli, but the large series of specimens on hand now showthat the differences are well within the range of variation for
one species. Sankarankutty’s (1966: text-figs. 23, 28) figuresof the distal half of the G1 and male pleon of a specimen
from southern India (as P. ridgewayi), although somewhat
simple, show the characteristic slender G1 and trapezoidal
male telson of M. globosus, new combination.

Females of several species of Arcotheres superficiallyresemble M. globosus in sharing similar MXP3 form and asimilarly elongated P5 dactylus with two distoflexor rowsof spinules on the dactylus: A. alcocki, A. similis, and A.
placunicola. Although the differences in P4 symmetry andcarapace shape are distinctive in intact, well-preserved
specimens, incomplete, poorly preserved specimens, or
specimens with regenerating pereopods can be more difficult
to separate generically. In such a situation, the distally hooked
P2 and P3 dactyli will separate A. alcocki and A. placunicola
from M. globosus, which has broadly curved dactyli. BothA. similis and M. globosus share similarly broadly curved P2
and P3 dactyli, and are difficult to separate without reference
to the P4 and carapace. In female M. globosus, however, thebuccal margin of the epistome is transverse with a narrowly
triangular median point (Figs. 79C, 81E), rather than verybroadly angular in A. similis (Figs. 28D, 30K).

The taxonomy of Arcotheres borradailei (Nobili, 1906a) (areplacement name for Pinnotheres tenuipes Borradaile, 1903)
from the Maldives was revised by Ng et al. (2017: 1093),
who separated it from A. ridgewayi in possessing denser
setation on the occlusal surfaces of the fingers of the chela,more rounded inner angle of the MXP3 ischiomerus, andproportionately shorter and stouter P3 and P4. Following
our assessment of M. globosus based on the extensive series
examined here, it is evident that the supposed distinguishingfeatures of A. borradailei and A. ridgewayi are well within
the range of variation of M. globosus, so the two species are
herein synonymised. The original host record by Borradaile(1903: 432) is uncertain, said to be a “? Mya” (Myidae
Lamarck, 1809), but this is probably a misidentification as
the family is primarily a temperate one.

The Australian specimens of “A. similis” reported by Ahyong& Brown (2003) and Ward (1967), mostly from Pinna, were
restudied and referred to A. globosus by Ahyong (2020b).
Among these is a large female (AM P20161) with the hostrecorded as the mytilid Stavelia subdistorta (Récluz, 1852);
the host data for this specimen may be incorrect given thatM. globosus is otherwise not known from the Mytilidae.
Distribution. Western Pacific to central Indian Ocean,
including Palau, Vanuatu, Australia, Philippines, Singapore,
Myanmar, Sri Lanka, and India.
ACKNOWLEDGEMENTS

Rathbun (1909) proposed the name, Pinnotheres alcocki,
for specimens reported as P. parvulus Stimpson, 1858,
by De Man (1887, 1888) from Indonesia and the MerguiArchipelago, Bürger (1895) from the Philippines, andAlcock (1900) from an unspecified locality (presumably in
Indian waters), rendering the “P. parvulus” material of all ofthese authors as syntypes. To fix the identity of the species,
Ahyong & Ng (2007b) designated one of Bürger’s (1895)
Philippine specimens as the neotype of Pinnotheres alcocki
and transferred it to Arcotheres. Gordon (1936: fig. 5) reported
and partially figured one of the Mergui specimens of De Man(1887) as Pinnotheres alcocki, but after reexamination of
the specimen (NHM 1886.52, a paralectotype of P. alcocki),
we refer the specimen instead to M. globosus. Indeed, De
Man (1887) had already noted the similarity of his material
to M. globosus (as Pinnotheres) based on the limited typedescription and figures despite ultimately referring it to P.
parvulus. The Mergui specimen, an ovigerous female, is in

Many of the specimens from this study came throughmalacologist Tan Siong Kiat (ZRC), who found the crabs
as part of his work on Singapore bivalves, and we also
thank him for advice on their taxonomy. We are grateful to
Dwi Listyo Rahayu and Sigit Dwiono (LIPI, Indonesia) for
passing us important material from Lombok and Indonesian
Papua. We thank Meng Kai (CAS), Ng Ngan Kee, and Lee
Bee Yan (ZRC) for helping search for pinnotherid material
in China, as well as checking and taking photographs ofthe extant material. Lee Bee Yan also helped search for
Arcotheres species during her visit to Palau. We are gratefulto Laure Corbari and Danièle Guinot (MNHN) and Ng Ngan
Kee for searching for types in Paris. Thanks are also due toArthur Anker for checking and photographing the holotype
of A. guinotae, and to Sébastien Soubzmaigne (MNHN)
for additional photographs of the species. For material
from SMF, we are grateful to Angelika Brandt, Andreas
242

RAFFLES BULLETIN OF ZOOLOGY 2022
Allspach, and the late Michael Türkay for access and loans.
Takehiro Sato kindly facilitated loans of specimens from
KPM while Tomoyuki Komai (CBM) kindly shared relevantmaterial from Fiji with him. From India, we are grateful
to R. Ravinesh and A. Biju Kumar (University of Kerala)
and Jignesh Trivedi (Gujarat University) for sharing theirmaterial with us. Tsang Ling Ming (Chinese Universiy ofHong Kong) kindly assisted with molecular comparisons ofsome key taxa to help us clarify species relationships, forwhich we are most grateful. Thanks are due to Paul Y. C.
Ng, Tan Heok Hui, Chan Tin-Yam, Lee Bee Yan, Arthur
Anker, Sigit Dwiono, and R. Ravinesh for permitting use of
their colour photographs. We are most grateful to Ernesto
Campos, Darryl Felder, and an anonymous reviewer for
their many, very detailed comments on the manuscript andimportant suggestions; they helped to greatly improve thepaper. We also thank Jose Mendoza for handling and editing
the long manuscript and picking up various important points.

Ahyong ST & Ng PKL (2008) Alain raymondi, a new species of
deepwater pinnotherid crab (Crustacea: Decapoda: Brachyura)from the Philippines, commensal with holothurians. Zootaxa,
1919: 61–68.
Ahyong ST & Ng PKL (2009) Aphanodactylidae, a new familyof thoracotreme crabs (Crustacea: Brachyura) symbiotic withpolychaete worms. Zootaxa, 2289: 33–47.
Ahyong ST & Ng PKL (2020) New species of pinnotherid crabs
from Southeast Asia and Papua New Guinea (Crustacea:
Decapoda: Brachyura). Zootaxa, 4816(3): 333–349.
Ahyong ST & Ng PKL (2021) The pinnotherid crabs from the Gulfof Siam described by Rathbun (1909) (Decapoda: Brachyura):
Revisited and revised. Raffles Bulletin of Zoology, 69: 188–211.
Alcock A (1900) Materials for a carcinological fauna of India. No.
6. The Brachyura Catometopa, or Grapsoidea. Journal on the
Asiatic Society of Bengal, 59(2): 279–456.
Anderson GL (1975) The effects of intertidal height and the parasitic
crustacean Fabia subquadrata. Veliger, 17: 299–306.
Antony A & Kutyamma VJ (1971) A new species of the pea-crab
Pinnotheres Latreille (Crustacea: Brachyura: Pinnotheridae)
from the clam Meretrix casta (Chemnitz). Bulletin of the
Department of Marine Biology and Oceanography, Universityof Cochin, 5: 59–68.
Balss H (1957) Decapoda. VI. Ökologie. In: H. G. Bronn(ed.) Klassen und Ordnungen des Tierreichs. Volume 5,
Arthropoda. Abt. 1, Crustacea. Buch 7, Decapoda. Akademische
Verlagsgesellschaft, Leipzig, pp. 1369–1476.
Becker C & Türkay M (2010) Taxonomy and morphology ofEuropean pea crabs (Crustacea: Brachyura: Pinnotheridae).
Journal of Natural History, 44: 25–26.
Bierbaum RM & Shumway SE (1988) Filtration and oxygen
consumption in mussels, Mytilus edulis, with and without pea
crabs, Pinnotheres maculatus. Estuaries, 11: 264–271.
Borradaile LA (1903) Marine crustaceans. V. The crabs of thecatometope families. In: Gardiner JS (ed.) The Fauna andGeography of the Maldive and Laccadive Archipelagoes: Being
the Account of the Work Carried on and of the Collections
Made by an Expedition During the Years 1899 and 1900.
Volume 1, Part 4. University Press, Cambridge, pp. 429–433.
Bürger O (1895) Ein Beitrag zur Kenntnis der Pinnotherinen.
Zoologische Jahrbucher, Abtheilung fur Systematik, Geographie
und Biologie der Thiere, 8: 361–390, pls. 9, 10.
Campos E (1989) Comments on taxonomy of the genus Orthotheres
Sakai, 1969 (Crustacea, Brachyura, Pinnotheridae). Bulletin ofMarine Science, 44(3): 1123–1128.
Campos E (1996) Partial revision of the genus Fabia Dana, 1851
(Crustacea: Brachyura: Pinnotheridae). Journal of Natural
History, 30: 1157–1178.
Campos E (2001) A new crab species of the genus Arcotheres
Manning, 1993, from Thailand (Crustacea, Brachyura,
Pinnotheridae). Zoosystema, 23(3): 493–497.
Campos E (2009) A new species and two new genera of pinnotheridcrabs from the northeastern Pacific Ocean, with a reappraisalof the subfamily Pinnotherinae de Haan, 1833 (Crustacea:
Brachyura: Pinnotheridae). Zootaxa, 2022: 29–44.
Campos E (2013) Remarks on the sexual dimorphism and taxonomy
of Fabia Dana, 1851 (Crustacea, Brachyura, Pinnotheridae).
Zootaxa, 3616(2): 190–200.
Campos E (2016) The Pinnotheridae of the northeastern Pacific(Alaska to Mexico): Zoogeographical remarks and new bivalve
hosts (Crustacea, Brachyura, Pinnotheridae). Zootaxa, 4170(2):311–329.
Campos E (2017) Taxonomy of Pinnotheres bipunctatus Nicolet,
1849 with a distributional checklist of the Pinnotheridae ofChile and Peru, and a list of the Crustacea described by Hercule
Nicolet in the Atlas of the Physical and Political History of Chile.
Latin American Journal of Aquatic Research, 45(2): 379–390.

LITERATURE CITED
Acarli S, Vural P & Öktener A (2019) Association between thecrab, Nepinnotheres pinnotheres (Linnaeus, 1758), and theendangered species fan mussel, Pinna nobilis (Linnaeus, 1758),
from the Aegean Sea, Turkey. Alinteri Journal of Agriculture
Sciences, 34(2): 169–174.
Ahyong ST (2018) Revision of Ostracotheres H. Milne Edwards,
1853 (Crustacea: Brachyura: Pinnotheridae). Raffles Bulletinof Zoology, 66: 538–571.
Ahyong ST (2019) First Indonesian Viridotheres Manning, 1996, andredescription of male Afropinnotheres dofleini Manning, 1993,
from South Africa (Decapoda, Pinnotheridae). Crustaceana,
92: 107–118.
Ahyong ST (2020a) Discovery of Viridotheres Manning, 1996 in thesouthwestern Pacific and first record of Discorsotheres camposi
Ahyong, 2018 from New Caledonia (Crustacea: Brachyura:
Pinnotheridae). Zootaxa, 4763(3): 429–434.
Ahyong ST (2020b) Resolution of the identity of Pinnotheres
latipes Hombron & Jacquinot, 1846 and description of a new
species of Viridotheres Manning, 1996 (Decapoda: Brachyura:
Pinnotheridae): Two symbionts of bivalve molluscs. Journalof Crustacean Biology, 40: 879–886.
Ahyong ST & Brown DE (2003) Description of Durckheimia lochin. sp., with an annotated check-list of Australian Pinnotheridae(Crustacea: Decapoda: Brachyura). Zootaxa, 254: 1–20, figs.
1, 2.
Ahyong ST, Komai T & Watanabe T (2012) First Viridotheres
Manning, 1996, from Japan, with a key to the species(Decapoda, Brachyura, Pinnotheridae). In: Komatsu H, Okuno
J & Fukuoka K (eds.) Studies on Eumalacostraca: A Homage
to Takeda Masatsune. Crustaceana Monographs. Volume 17.
Brill, Leiden, pp. 35–47.
Ahyong ST & Ng PKL (2005) Review of Durckheimia and
Xanthasia, with descriptions of two new genera (Decapoda:
Brachyura: Pinnotheridae). Journal of Crustacean Biology,
25(1): 116–129.
Ahyong ST & Ng PKL (2007a) Visayeres acron, a new genus andspecies of pinnotherid crab (Crustacea: Decapoda: Brachyura)from the Philippines. Raffles Bulletin of Zoology, Supplement
16: 187–189.
Ahyong ST & Ng PKL (2007b) The pinnotherid type material of
Semper (1880), Nauck (1880) and Bürger (1895) (Crustacea:
Decapoda: Brachyura). Raffles Bulletin of Zoology, Supplement
16: 191–226.

243

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia
Campos E (2018) On the pea crabs found in the chiton Tonicia
chilensis (Frembly, 1827) (Mollusca, Polyplacophora:
Chitonidae) identified as “Orthotheres sp.” by Melzer &Schwabe (2008), and its reassignment to Calyptraeotheres
Campos, 1990 (Crustacea: Pinnotheridae). Zootaxa, 4434(2):385–390.
Campos E & Hernández-Aguilera JL (2019) First record and range
extension of the jewel box clam crab Gemmotheres chamae(Roberts, 1975) to the Gulf of Mexico, with comments on thesystematics of the pinnotherines with a 2-segmented palp on
the third maxilliped (Crustacea: Brachyura: Pinnotheridae).
Nauplius, 28: e2020005.
Campos E & Hernández-Ávila I (2010) Phylogeny of Calyptraeotheres
Campos, 1990 (Crustacea, Decapoda, Brachyura, Pinnotheridae)
with the description of C. pepeluisi new species from the tropical
Mexican Pacific. Zootaxa, 2691: 41–52.
Campos E & Manning RB (2001) Authorship and diagnosis of
the genus Arcotheres Manning, 1993 (Crustacea: Brachyura:
Pinnotheridae). Raffles Bulletin of Zoology, 49: 167–170.
Castro P (2015) Symbiotic Brachyura. In: Castro P, Davie PJF,
Guinot D, Schram FR & Von Vaupel Klein JC (eds.) Treatise
on Zoology — Anatomy, Taxonomy, Biology. The Crustacea,
Volume 9 Part C. Decapoda, Brachyura (Part 1). Brill, Leiden,
pp. 543–581.
Chhapgar BF (1957) On the marine crabs (Decapoda: Brachyura) ofBombay State, Part 2. Journal of the Bombay Natural History
Society, 54(3): 503–549.
Chhapgar BF (1958) On two new species and a variety of crabs(Decapoda: Brachyura) from Bombay state. Records of the
Indian Museum, 1955, 53: 251–257.
Christensen AM & McDermott JJ (1958) Life-history and biologyof the oyster crab, Pinnotheres ostreum Say. Biological Bulletin,
114: 146–179.
Chuang SH (1961) On Malayan Shores. Muwu Shosa, Singapore,
225 pp.
Clark PF & Crosnier A (2000) The zoology of the Voyage auPôle Sud et dans l’Océanie sur les corvettes l’Astrolabe et laZélée exécuté par ordre du Roi pendant les années 1837–1838–
1839–1840 sous le commandement de M. Dumont-d’Urville(1842–1854): Titles, volumes, plates, text, contents, proposed
dates and anecdotal history of the publication. Archives Natural
History, London, 27(3): 407–435.
Clark PF, Harrison K & Goodman S (1990) Decapoda andStomatopoda by De Man: 1887 or 1888? Archives of Natural
History, 17(1): 79–80.
Clark PF & Schram FR (2009) Raymond B. Manning: An
appreciation. Journal of Crustacean Biology, 29: 431–457.
Combosch DJ, Collins TM, Glover EA, Graf DL, Harper EM,
Healy JM, Kawauchi GY, Lemer S, McIntyre E, Strong EE,
Taylor JD, Zardus JD, Mikkelsen PM, Giribet G & Bieler
R (2017) A family-level tree of life for bivalves based on a
Sanger-sequencing approach. Molecular Phylogenetics andEvolution, 107: 191–208.
Cruz-Kaled AC, Boehs G & Absher TM (2004) Incidence of Fabia
insularis Melo, 1971 (Decapoda: Pinnotheridae) in Macoma
constricta (Bruguière, 1792) (Vivalvia: Tellinidae) in a tidal flatat the Paranagua Bay (SE Brazil). Journal of Coastal Research,
39: 1186–1189.
Cuesta JA, García Raso JE, Abelló P, Marco-Herrero E, Silva L &Drake P (2019) A new species of pea crab from southwestern
Europe (Crustacea, Decapoda, Brachyura): Species description,
geographic distribution and population structure with an
identification key to European Pinnotheridae. Journal of the
Marine Biological Association of the United Kingdom, 99:1141–1152.

Dai AY, Feng ZQ, Song YZ & Chen GX (1980) New species andnew records of family Pinnotheridae from Hainan Island. Acta
Zootaxonomica Sinica, 5(2): 129–143, figs. 1–11, tables 1–4.
Dai A-Y & Song Y-Z (1986) Intertidal crabs from Beibu Gulf ofGuangxi. Transactions of the Chinese Crustacean Society, 1:
54–62, pl. 1. [In Chinese]
Dai A-Y & Yang S-L (1991) Crabs of the China Seas. China
Ocean Press, Beijing, and Springer-Verlag, Berlin, 21 + 608
pp., figs. 1–295, pls. 1–74.
Dai A-Y, Yang S-L, Song Y-Z & Chen G-X (1986) Crabs of the
China Seas. China Ocean Press, Beijing, 11 + 642 pp. [In
Chinese]Dana JD (1851) On the classification of the Crustacea Grapsoidea.
American Journal of Science and Arts, 2: 283–291.
Dana JD (1852) Crustacea. Part I. United States Exploring
Expedition During the Years 1838, 1839, 1840, 1841, 1842,
Under the Command of Charles Wilkes, U.S.N. Volume 13.
C. Sherman, Philadelphia, 685 pp.
Dana JD (1855) Atlas. Crustacea. United States Exploring
Expedition During the Years 1838, 1839, 1840, 1841, 1842,
Under the Command of Charles Wilkes, U.S.N. Volume 14.
C. Sherman, Philadelphia, 27 pp., 96 pls.
Davie PJF (2002) Crustacea: Malacostraca. Eucarida (Part 2).
Decapoda – Anomura, Brachyura. Zoological Catalogue ofAustralia. Volume 19.3B. CSIRO Publications, Collingwood,
Melbourne, 641 pp.
Davie PJF, Guinot D & Ng PKL (2015) Anatomy and functional
morphology of Brachyura. In: Castro P, Davie PJF, Guinot D,
Schram FR & Von Vaupel Klein JC (eds.) Treatise on Zoology— Anatomy, Taxonomy, Biology. The Crustacea, Volume 9Part C. Decapoda, Brachyura (Part 1). Brill, Leiden, pp. 11–163.
De Gier W & Becker C (2020) A review of the ecomorphologyof pinnotherine pea crabs (Brachyura: Pinnotheridae), with an
updated list of symbiont-host associations. Diversity, 12: 431.
De Haan HM (1833–1850) Crustacea. In: von Siebold PF. Fauna
Japonica, sive Descriptio animalium, quae in itinere per
Japoniam, jussu et auspiciis superiorum, qui summum in India
Batavia imperium tenent, suscepto, annis 1823–1830. Apud
Auctorem, Lugduni Batavorum, xxx + i–xxxi + vii–xvii + 243+ viii–xvi pp., pls. I–LII, 2, A–Q.
De Man JG (1887) Report on the podophthalmous Crustacea ofthe Mergui Archipelago, collected for the Trustees of the
Indian Museum, Calcutta, by Dr. John Anderson, F.R.S.,
Superintendent of the Museum — Part II. Journal of the Linnean
Society, London, Zoology, 22: 65–128.
De Man JG (1887–1888) Bericht über die von Herrn Dr. J.
Brock im indischen Archipel gesammelten Decapoden und
Stomatopoden. Archiv für Naturgeschichte, 53(I, 2): 215–288,
pls. 7–10 [issued 1887]; 53(I, 3): 216–600, pls. 11–22a [issued
1888] [fide Clark et al., 1990].
De Man JG (1921) Sur quelques anomalies observées chez deux
espèces du genre Pinnotheres Latr. de l’Archipel Indien.
Bulletin biologique de la France et de la Belgique, Paris, 55:
260–265, pl. 8.
De Man JG (1929) On a collection of decapod and stomatopod
Crustacea from Pulau Berhala, an islet situated in the Straitsof Malacca. Bijdragen tot de Dierkunde, 26: 1–26, pls. 1–3.
Dev Roy K (2013) Diversity and distribution of marine brachyuran
crab communities inhabiting West Coast of India. In:
Venkataraman K, Sivaperuman C & Raghunathan C (eds.)
Ecology and Conservation of Tropical Marine Faunal
Communities. Part 1. Springer, Berlin & Heidelberg, pp.
147–169.
Dev Roy MK & Rath S (2017) An inventory of crustacean
fauna from Odisha Coast, India. Journal of Environment andSociobiology, 14(1): 49–112.

244

RAFFLES BULLETIN OF ZOOLOGY 2022
Devi KN & Shyamasundari K (1989) A new species of Pinnotheres
Latreille (Decapoda: Brachyura) from Visakhapatnam coast of
Bay of Bengal, Andhra Pradesh. Journal of the Bombay Natural
History Society, 86: 217–221.
Doflein F (1904) Brachyura. In: Wissenschaftliche Ergebnisse derDeutschen Tiefsee-Expedition auf dem Dampfer “Valdivia”
1898–1899. Volume 6. Verlag von Gustav Fischer, Jena, 6 +xiv + 314 pp., figs. 1–58, Atlas pls. 1–58.
Estampador EP (1937) A check list of Philippine crustacean
decapods. The Philippine Journal of Science, 62(4): 465–559.
Felder DL & Palacios Theil E (2020a) A new pinnotherid crab of
the genus Pinnixulala Palacios Theil, Cuesta & Felder, 2016
from uncertain infaunal hosts in the northern Gulf of Mexico,with a rediagnosis and updated synonymy for the polychaete
symbiont Pinnixulala retinens (Rathbun, 1918) (Decapoda:
Brachyura: Pinnotheridae). Journal of Crustacean Biology,
40(6): 887–898.
Felder DL & Palacios Theil E (2020b) Three new symbiotic crabs of
the genus Glassella Campos & Wicksten, 1997 from Atlantic andGulf of Mexico coasts of Florida, USA (Decapoda: Brachyura:
Pinnotheridae). Journal of Crustacean Biology, 40(6): 899–917.
George MJ & Noble A (1970) Occurrence of pea crabs Pinnotheres
gracilis Burger and Pinnotheres modiolicolus Burger in the
Indian coast. Journal of the Marine Biological Association ofIndia, 10(2): 392–394.
Gordon I (1936) On a few Indo-Pacific species of Pinnotheres, withspecial reference to asymmetry of the walking legs. Journal of
the Linnean Society of London, Zoology, 40: 163–180.
Gravely FH (1927) Decapoda and Stomatopoda. In: The littoral
fauna of Krusadai Island in the Gulf of Manaar. Bulletin of the
Madras Government Museum, New Series (Natural History),
1: 141–155, pls. 20–26.
Griffin DJG & Campbell BM (1969) The sub-littoral Goneplacidae
and Pinnotheridae (Crustacea: Brachyura) of Moreton Bay.
Memoirs of the Queensland Museum, 15: 141–164.
Guérin-Méneville FE (1829–1844) Iconographie du règne animalde G. Cuvier, ou représentation d’après nature de l’une desespèces les plus remarquables, et souvent non figurèes, de chaquegenre d’animaux, avec un text descriptif mis au courant de lascience: Ouvrage pouvant servir d’atlas à tous les traités dezoologie. Crustacea. Chez J.B. Baillière, Paris, 48 pp., 36 pls.
Guinot D (1967) La faune carcinologique (Crustacea Brachyura)de l’Océan Indien occidental et de la mer Rouge. Catalogue,
remarques biogéographiques et bibliographie. In: Réunion de
Spécialistes C. S. A. sur les Crustacés, Zanzibar 1964. Mémoires
de l’Institute fundamental d’Afrique Noire, 77: 235–352.
Hamel J-F, Ng PKL, McCurry S & Mercier A (2019) Note on
the pea crab Holotheres semperi (Bürger, 1895) parasitising
the sea cucumber Holothuria scabra in Rempang, Indonesia.
Beche de Mer Information Bulletin, 39: 36–38.
Hamel JF, Ng PKL & Mercier A (1999) Life cycle of the pea crab
Pinnotheres halingi sp. nov., an obligate symbiont of the sea
cucumber Holothuria scabra Jaeger. Ophelia, 50(3): 149–175.
Holthuis LB (2002) A few notes on the authors and dates of thenames of Crustacea collected by the “Voyage au Pôle Sud
et dans l’Océanie sur les Corvettes l’Astrolabe et la Zélée”.
Crustaceana, 75(3/4): 413–422.
Hombron JB & Jacquinot H (1842–1854) Crustacés. Atlas d’Histoire
Naturelle. Zoologie. Voyage au Pôle Sud et dans l’Océanie
sur les corvettes l’Astrolabe et la Zélée pendant les années
1837–1838–1839–1840. Gide et J. Baudry, Paris, pls. 1–9.
Huber M (2010) Compendium of Bivalves. A Full-Color Guide to
3,300 of the World’s Marine Bivalves. A Status on Bivalvia
After 250 Years of Research. Conchbooks, Hackenheim, 901 pp.
International Commission on Zoological Nomenclature (ICZN)(1999) International Code of Zoological Nomenclature.
Fourth Edition. Adopted by the XXI General Assembly of theInternational Union of Biological Sciences. International Trustfor Zoological Nomenclature, in association with the British
Museum (Natural History), London, 338 pp.
Jacquinot H & Lucas H (1853) Crustacés. Voyage au Pôle Sud etdans l’Océanie sur les corvettes l’Astrolabe et la Zélée pendantles années 1837–1838–1839–1840. Zoologie. Volume 3. Gideet J. Baudry, Paris, pp. 1–107.
Jahangir S, Siddiqui G, Ayub Z & Boyko CB (2015) New record of
the pea crab Nepinnotheres villosulus (Guérin-Meneville, 1831)(Brachyura: Pinnotheridae) from clams and cockles of Pakistan(northern Arabian Sea). Marine Biodiversity Records, 8: e135.
Jiang W & Liu R (2011) New species and new records of pinnotheridcrabs (Crustacea: Decapoda: Brachyura) from the Yellow Sea.
Zoologischer Anzeiger, 250: 488–496.
Johnson DS (1963) Commensalism and semi-parasitism amongst
decapod Crustacea in Singapore waters. In: UNESCO.
Proceedings of the First Regional Symposium on Scientific
Knowledge of Tropical Parasites, University of Singapore,
5 to 9 November, 1962. University of Singapore, Singapore,pp. 282–288.
Jones D (1990) Annotated checklist of marine decapod Crustacea
from Shark Bay, Western Australia. In: Berry PF, Bradshaw
SD & Wilson BR (eds.) Research in Shark Bay. Report of theFrance–Australe Bicentenary Expedition Committee. Western
Australian Museum, Perth, 325 pp.
Kannappan T, Shanmugavelu M, Sudhamathi C, Elumalai V &Karthikeyan MM (2012) New record of pea crabs (Pinnotheres
sinensis Shen, 1932) along the Manakudy Estuary, southwest
coast of India. European Journal of Biological Sciences, 4(2):45–48.
Kazmi QB, Sultana R & Ghory FS (2018) Redescription of
Arcotheres placunae and three new records, A. aff. alcocki,
A. casta and Pinnotheres quadratus from Pakistan with a noteon previously recorded Pakistani pinnotherid crabs. Pakistan
Journal of Marine Sciences, 25(1&2): 131–143.
Komai T, Kawai K & Ng PKL (2020) On the identity of thepoorly known pea crab, Pinnothera obesa Dana, 1852, anddescription of a new species of Arcotheres Manning, 1993 fromthe southwest Pacific (Decapoda: Brachyura: Pinnotheridae).
Zootaxa, 4822(2): 221–247.
Komatsu H & Ohtsuka S (2009) A new species of the genus
Abyssotheres (Crustacea, Decapoda, Brachyura, Pinnotheridae)
from the Ryukyu Islands, southwestern Japan, with taxonomic
notes on the genus. Bulletin of the National Museum of Natureand Science, Tokyo, Series A (Zoology), 35(1): 73–81.
Komatsu H & Takeda M (2009) A new species of the genus
Pinnixa (Decapoda: Brachyura: Pinnotheridae) from Otsuchi
Bay, northeastern Japan. Bulletin of the National Museum of
Nature and Science, Tokyo, Series A (Zoology), Supplement
3: 199–204, figs. 1, 2.
Kruczynski WL (1972) The effect of the pea crab, Pinnotheres
maculatus Say, on growth of the bay scallop, Argopecten
irradians concentricus (Say). Chesapeake Science, 13: 218–220.
Kumari LK & Rao VK (1974) Life cycle of pea crab, Pinnotheresvicaji Chhapgar, infesting the clam, Paphia malabarica. Indian
Journal of Marine Sciences, 3: 165–172.
Lalitha Devi L (1981) Occurrence of pea crabs Pinnotheres gracilis
Burger and P. alcocki Rathbun at Kakinada. Journal of the
Marine Biological Association of India, 23: 214–218.
Lanchester WF (1900) On a collection of crustaceans madeat Singapore and Malacca. Part I. Crustacea Brachyura.
Proceedings of the Zoological Society of London, 1900(3):719–770, pls. 44–47.
Laurie RD (1915) Reports on the marine biology of the Sudanese
Red Sea.—XXI. On the Brachyura. Zoological Journal of theLinnean Society, 31(209): 407–475, pls. 42–45.

245

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia
Lenz H (1910) Crustaceen von Madagaskar, Ostafrika und Ceylon.
In: Voeltzkow A. Reise in Ostafrika in den Jahren 1903–1905.
Volume 2, Part 4. E. Schweizerbart’sche Verlagsbuchhandlung,
Stuttgart, pp. 539–576.
Manning RB (1993a) Three genera removed from the synonymy
of Pinnotheres Bosc, 1802 (Brachyura: Pinnotheridae).
Proceedings of the Biological Society of Washington, 106:523–531.
Manning RB (1993b) West African pinnotherid crabs, subfamily
Pinnotherinae (Crustacea, Decapoda, Brachyura). Bulletin duMuséum national d’Histoire naturelle, Paris, 4e Série, Section
A, 15(1–4): 125–177.
Manning RB (1996) Viridotheres marionae, a new genus and speciesof pinnotherid crab from West Africa (Crustacea: Decapoda:
Brachyura). Zoologische Mededelingen, 70: 271–273, fig. 1.
Mena S, Salasmoya C & Wehrtmann IS (2014) Living with a crab:
Effect of Austinotheres angelicus (Brachyura, Pinnotheridae)
infestation on the condition of Saccostrea palmula (Ostreoida,
Ostreidae). Nauplius, 22(2): 151–158.
Miers EJ (1880) On a collection of Crustacea from the Malaysian
Region. Part II. Telphusidea, Catometopa, and Oxystomata.
Annals and Magazine of Natural History, Series 5, 5(28):
304–317, pl. 14.
Milne Edwards H (1853) Mémoire sur la famille des Ocypodides.
Suite (1). Deuxiéme Tribu Principale. Annales des Sciences
Naturelles, Comprenant la Zoologie, la Botanique, l’Anatomie
et la Physiologie comparées des deux Régnes et l’Histoire desCorps Organisés Fossiles, Librairie de Victor Masson, Paris,
Series 3, Zoology, 20: 163–228, pls. 6–11.
Miyake S (1939) Notes on Crustacea Brachyura collected by Prof.
Teiso Esaki’s Micronesia Expeditions 1937–1938 together withcheck list of Micronesian Brachyura. Records of Oceanographic
Works in Japan, 10(2): 168–247, figs. 1–13, pls. 12(1)–17(6).
Muraoka K (1998) Catalogue of the Brachyuran and Anomuran
Crabs Donated by Prof. Dr. Tune Sakai to the Kanagawa
Prefectural Museum. Catalogue of the Collection in the
Kanagawa Prefectural Museum of Natural History, Odawara,
No. 11. Kanagawa Prefectural Museum of Natural History,
Odawara, 67 pp., 16 pls.
Naiyanetr P (1980) Crustacean Fauna of Thailand (Decapoda andStomatopoda). Department of Biology, Faculty of Science,
Chulalongkorn University, Bangkok, 73 pp.
Naiyanetr P (1998) Checklist of Crustacean Fauna in Thailand(Decapoda, Stomatopoda, Anostraca, Myodocopa and Isopoda).
Office of Environmental Policy and Planning, Bangkok, 161 pp.
Naiyanetr P (2007) Checklist of Crustacean Fauna in Thailand(Decapoda, Stomatopoda, Anostraca, Myodocopa and Isopoda).
Office of Natural Resources and Environmental Policy andPlanning, Bangkok, 196 pp.
Narvarte MA & Saiz MN (2004) Effects of the pinnotherid crab
Tumidotheres maculatus on the Tehuelche scallop Aequipecten
tehuelchus in the San Matías Gulf, Argentina. Fisheries
Research, 67: 207–214.
Nauck E (1880) Das Kaugerust der Brachyuren (mit beschreibung
neur gattungen und arten, Z. T. von C. Semper). Zeitscrift
wissenschaften Zoologische, Leipzig, 34: 1–69, pl. 1.
Ng PKL (2018a) Arcotheres placunicola, a new species of pea
crab (Crustacea: Brachyura: Pinnotheridae) from the window
pane shell, Placuna ephippium Philipsson, 1788 (Placunidae)
in Singapore. Raffles Bulletin of Zoology, 66: 474–485.
Ng PKL (2018b) On the identities of Pinnotheres villosissimus
Doflein, 1904, P. dofleini Lenz, in Lenz & Strunck, 1914, and P.
pilumnoides Nobili, 1906 (Decapoda, Brachyura, Pinnotheridae)
from the western Indian Ocean. Crustaceana, 91(5): 611–633.
Ng PKL (2019) On the taxonomy of two rare freshwater
crabs (Decapoda, Brachyura, Chasmocarcinidae andHymenosomatidae) from Palau, western Pacific. Crustaceana,
92(8): 979–996.
Ng PKL, Ahyong ST & Campos E (2019b) Two new generaof pinnotherid crabs (Crustacea: Brachyura: Pinnotheroidea)from the Americas and Western Pacific. Raffles Bulletin of
Zoology, 67: 337–351.
Ng PKL, Ahyong ST & Clark PF (2019a) The identity of Pinnotheressocius Lanchester, 1902 (Decapoda, Brachyura, Pinnotheridae).
Crustaceana, 92(6): 685–692.
Ng PKL, Clark PF, Mitra S & Kumar AB (2017) Arcotheres
borradailei (Nobili, 1906) and Pinnotheres ridgewayi Southwell,1911: A reassessment of characters and generic assignment ofspecies to Arcotheres Manning, 1993 (Decapoda, Brachyura,
Pinnotheridae). Crustaceana, 90(7–10): 1079–1097.
Ng PKL & Corbari L (2019) Crustacea: Crab legacies. Confusion
Épique: 180 years and one pesky crab. In: Low MEY,
Pockington K & Jusoh WFA (eds.) Voyageurs, Explorateurset Scientifiques. The French and Natural History in Singapore.
Lee Kong Chian Natural History Museum, National Universityof Singapore, Singapore, and Muséum national d’Histoire
Naturelle, Paris, pp. 245–253.
Ng PKL, Guinot D & Davie PJF (2008) Systema Brachyurorum:
Part I. An annotated checklist of extant brachyuran crabs of
the world. Raffles Bulletin of Zoology, Supplement 17: 1–286.
Ng PKL & Ho H-P (2016a) A new genus for Fabia obtusidentata
Dai, Feng, Song and Chen, 1980, a pea crab (Decapoda:
Brachyura: Pinnotheridae) symbiotic with the moon scallop
Amusium pleuronectes (Linnaeus, 1758) (Mollusca: Pectinidae).
Journal of Crustacean Biology, 36(5): 740–751.
Ng PKL & Ho H-P (2016b) Orthotheres baoyu, a new speciesof pea crab (Crustacea: Brachyura: Pinnotheridae) associatedwith abalones from Tungsha Island, Taiwan; with notes on thegenus. Raffles Bulletin of Zoology, 64: 229–241.
Ng PKL & Kumar AB (2015) A new species of Afropinnotheres
Manning, 1993 (Crustacea, Brachyura, Pinnotheridae) fromsouthwestern India, the first record of the genus from theIndian Ocean, with a review of the Pinnotheridae of India andadjacent seas. Zootaxa, 3947(2): 264–274.
Ng PKL & Manning RB (2003) On two new genera of pea crabs
parasitic in holothurians (Crustacea: Decapoda: Brachyura:
Pinnotheridae) from the IndoWest Pacific, with notes on allied
genera. Proceedings of the Biological Society of Washington,
116(4): 901–919.
Ng PKL & Ngo VT (2010) Solenotheres prolixus, a new genus
and new species of pinnotherid crab (Crustacea: Decapoda:
Brachyura) associated with the razor clam, Solen corneus
Lamarck, 1818 (Solenidae) in Vietnam. Zootaxa, 2570: 61–68.
Nobili G (1906a) Diagnoses préliminaires de 34 espèces et variétés
nouvelles, et de 2 genres nouveaux de Décapodes de la Mer
Rouge. Bulletin du Muséum d’Histoire naturelle, 11: 393–411.
Nobili G (1906b) Faune carcinologique de la Mer Rouge. Décapodeset Stomatopodes. Annales de Sciences Naturelles, Zoologie,
9e Série, Volume 4. Masson et Cie, Paris, 347 pp., pls. 1–11.
Ocampo EH, Robles R, Terossi M, Nuñez JD, Cledón M &Mantelatto FL (2013) Phylogeny, phylogeography, andsystematics of the American pea crab genus Calyptraeotheres
Campos, 1990, inferred from molecular markers. Zoological
Journal of the Linnean Society, 169: 27–42.
Palacios-Theil E, Cuesta JA, Campos E & Felder DL (2009)
Molecular genetic re-examination of subfamilies and polyphyly
in the family Pinnotheridae (Crustacea: Decapoda). In: Martin
JW, Crandall KA & Felder DL (eds.) Crustacean Issues 18:
Decapod Crustacean Phylogenetics. CRC Press, England, pp.
457–474.
Palacios Theil E, Cuesta JA & Felder DL (2016) Molecular evidence
for non-monophyly of the pinnotheroid crabs (Crustacea:

246

RAFFLES BULLETIN OF ZOOLOGY 2022
Brachyura: Pinnotheroidea), warranting taxonomic reappraisal.
Invertebrate Systematics, 30: 1–27.
Palacios Theil E & Felder DL (2019) Molecular phylogeography of
Tumidotheres maculatus (Say, 1818) and Zaops ostreus (Say,
1817) (Crustacea: Decapoda: Pinnotheridae) in the western
Atlantic, with description of a new species and synonymy
of Epulotheres Manning, 1993. Marine Biology Research,
15(10): 548–567.
Palacios Theil E & Felder DL (2020a) Phylogeny of the genus
Pinnixa White, 1846 (Crustacea, Brachyura, Pinnotheridae)
and allies inferred from mitochondrial and nuclear molecularmarkers, with generic reassignment of twenty-one species.
Zoosystema, 42(6): 85–103.
Palacios Theil E & Felder DL (2020b) Phylogeny of the genus
Austinixa Heard & Manning, 1997, inferred from mitochondrialand nuclear molecular markers, with descriptions of three new
species and redescription of Austinixa felipensis (Glassell,1935) (Decapoda: Brachyura: Pinnotheridae). Zootaxa, 4778(1):101–134.
Pati P, Sahu BK & Panigrahy RC (2015) Pea crab Arcotheres
alcocki (Rathbun, 1909) as endoparasites of the backwater clam
Meretrix casta (Chemnitz, 1782). Indian Journal of Geo-Marine
Sciences, 44(7): 1067–1070.
Pati SK, Sahu KC, Swain D, Baliarsingh SK & Sharma RM(2012) Marine crabs (Decapoda: Anomura and Brachyura).
In: Director, Zoological Survey of India, Kolkata (ed.) Fauna
of Maharashtra. Part 2. Invertebrates. State Fauna Series, 20.
Zoological Survey of India, Kolkata, pp. 381–385.
Paulson O (1875) Studies on Crustacea of the Red Sea withNotes Regarding Other Seas. Part 1. Podophthalmata and
Edriophthalmata (Cumacea). S. V. Kul’zhenko, Kiev, 144pp., 21 pls., 21 tables. English translation by Por FD (1961)
Published for The National Science Foundation, Washington
D.C., and Smithsonian Institution, USA, by The Israel Programfor Scientific Translations, 164 pp.
Perez C (1921) Sur un Céponien nouveau, Onychocepon harpax (n.
g., n. sp.), parasite branchial d’un Pinnothère. Comptes Rendus
Hebdomadaires des Séances de l’Académie des Sciences,173(1): 59–61.
Pohle G & Marques F (1998) Phylogeny of the Pinnotheridae: Larval
and adult evidence, with emphasis on the evolution of gills.
Invertebrate Reproduction & Development, 33(2-3): 229–239.
Prasad RR & Tampi PRS (1957) Notes on some decapod larvae.
Journal of the Zoological Society of India, 9: 22–39.
Rathbun MJ (1893) Scientific results of explorations by the U.S.
Fish Commission steamer Albatross, XXIV. Descriptions of
new genera and species of crabs from the west coast of North
America and the Sandwich Islands. Proceedings of the UnitedStates National Museum, 16: 223–260.
Rathbun MJ (1909) New crabs from the Gulf of Siam. Proceedingsof the Biological Society of Washington, 22: 107–114.
Rathbun MJ (1910) The Danish Expedition to Siam 1899–1900.
V. Brachyura. Konelige Danske Videnskabernes Selskat,
Naturvidenskabelige Matematiske Afhandlinger, 5: 301–367,
pls. 1, 2, map.
Rathbun MJ (1924) Results of Dr. E. Mjöberg’s Swedish Scientific
Expeditions to Australia 1910–1913. 37. Brachyura, Albuneidae
and Porcellanidae. Arkiv för Zoologi, 16: 1–33.
Ravichandran S & Kannupandi T (2007) Biodiversity of crabs in
Pichavaram mangrove environment. In: Director, Zoological
Survey of India, Kolkata (ed.) National Symposium on
Conservation and Validation of Marine Biodiversity. Zoological
Survey of India, Kolkata, pp. 331–340.
Saeedi H & Ardalan AA (2010) Incidence and biology of Arcotheres
tivelae (Crustacea: Decapoda) in Amiantis umbonella (Bivalvia:
Veneridae) on the northern coast of the Persian Gulf, Iran.

Journal of the Marine Biological Association of the United
Kingdom, 90: 655–661.
Sakai T (1939) Studies on the Crabs of Japan. IV. Brachygnatha,
Brachyrhyncha. Yokendo Co. Ltd., Tokyo, 365–741 + 1–23[= Index] pp., figs. 1–129, pls. 42–111, tables 1–6.
Sakai T (1969) Two new genera and twenty-two new species ofcrabs from Japan. Proceedings of the Biological Society ofWashington, 82: 243–280, figs. 1–19, pls 1, 2.
Sakai T (1976) Crabs of Japan and the Adjacent Seas. Kodansha
Ltd., Tokyo, in three volumes; English: xxix + 773 pp.; Japanese:
461 pp.; Plates: 16 pp., pls. 1–251.
Salgado-Barragán J (2015) A new species of Pinnixa (Crustacea:
Brachyura: Pinnotheridae) from Mazatlan, Sinaloa, Mexico.
Revista Mexicana de Biodiversidad, 86: 629–633.
Salgado-Barragán J & Barragán-Zepeda AK (2021) Description ofa new species of Glassella Campos & Wicksten, 1997 from acoastal lagoon, SE Gulf of California (Crustacea, Decapoda,
Pinnotheridae). Zootaxa, 4952(2): 381–390.
Salgado-Barragán J, Raymundo-Huizar AR & Ayón-Parente M(2021) A new species of Austinixa Heard and Manning 1997
(Decapoda: Pinnotheridae) and new records of A. felipensis(Glassell, 1935) from the Mexican Pacific. Nauplius, 29:e2021022.
Sankarankutty C (1966) On Decapoda Brachyura from the Gulfof Mannar and Palk Bay. In: Proceedings of the Symposiumon Crustacea, held at Ernakulam from January 12 to 15, 1965.
Symposium series (Marine Biological Association of India),
2. Marine Biological Association of India, Mandapam Camp,
India, pp. 347–362.
Say T (1817) An account of the Crustacea of the United States.
Journal of the Academy of Natural Sciences of Philadelphia,
1(1): 57–63, 65–80, 97–101, 155–160, 161–169, pl. 4.
Schmitt WL, McCain JC & Davidson E (1973) Part 3. Decapoda
I. Brachyura I. Fam. Pinnotheridae. In: Gruner H-E & Holthuis
LB (eds.) Crustaceorum Catalogus. Dr. W. Junk, Den Haag,
1–160 pp.
Seed R (1969) The incidence of the pea crab, Pinnotheres pisumin two types of Mytilus (Mollusca: Bivalvia) from Padstow,
south-west England. Journal of Zoology, 158: 413–420.
Serène R (1968) The Brachyura of the Indo Pacific Region. In:
Prodromus for a Check List of the Non-planctonic Marine
Fauna of South East Asia. Special Publication of the Singapore
National Academy of Science, 1. Singapore National Academyof Science, Singapore, pp. 33–120.
Sethuramalingam S & Khan AS (1991) Brachyuran Crabs of
Parangipettai Coast. Centre of Advance Study in Marine
Biology, Annamalai University, Tamil Nadu, 93 pp.
Shen C (1932) The brachyuran Crustacea of North China. Zoologica
Sinica, Series A (Invertebrates of China), The Fan Institute of
Biology, Peking [= Beijing], China, 9(1): i–x + 1–321, figs.
1–171, pls. 1–10, 1 map.
Silas EG & Alagarswami K (1967) On an instance of parasitisationby the pea-crab (Pinnotheres sp.) on the backwater clam
[Meretrix casta (Chemnitz)] from India, with a review of thework on systematics, ecology, biology and etiology of pea crabsof the genus Pinnotheres, Latreille, 1802. In: Proceedings of the
Symposium on Crustacea, held at Ernakulam from January 12
to 15, 1965. Symposium series (Marine Biological Associationof India), 3. Marine Biological Association of India, Mandapam
Camp, India, pp. 1161–1227.
Southwell T (1911) Description of a new species of Pinnoteres[sic] and of female of Pinnoteres [sic] margaritiferae, Laurie.
[Article 19]. Reports from the Ceylon Marine Biological
Laboratory, 1(5): 226–227, pl. 3.
Stephenson W, Williams WT & Lance GN (1970) The macrobenthosof Moreton Bay. Ecological Monographs, 40: 459–494.

247

Ng & Ahyong: Revision of Arcotheres pea crabs from Singapore and Malaysia
Stevens PM (1990) Specificity of host recognition of individuals
from different host races of symbiotic pea crabs (Decapoda:
Pinnotheridae). Journal of Experimental Marine Biology andEcology, 143: 193–207.
Stimpson W (1858) Prodromus descriptionis animalium
evertebratorum quoe in Expeditione ad Oceanum Pacificum
eptentrionalem a Republica Federata Missa, Cadwaladaro
Ringgold et Johann Rodgers Ducibus, observavit et descripsit— Part V, Crustacea Ocypodoidea. Proceedings of the Academyof Natural Science, Philadelphia, 9: 93–110.
Suvatti C (1938) A Check-List of Aquatic Fauna in Siam (Excluding
Fishes). Bureau of Fisheries, Bangkok, 116 pp.
Suvatti C (1950) Fauna of Thailand. Department of Fisheries,
Bangkok, ii + 1100 pp., 1 pl.
Tablado A & López Gappa J (1995) Host-parasite relationships
between the mussel, Mytilus edulis L., and the pea crab,
Tumidotheres maculatus (Say), in the southwestern Atlantic.
Journal of Shellfish Research, 14: 417–423.
Takeda M & Konishi K (1988) Redescription of Pinnotheres
alcocki Rathbun, 1909, a commensal pea crab new to Japan.
Bulletin of the National Science Museum Series A (Zoology),
14(3): 137–144.
Tesch JJ (1918) The Decapoda Brachyura of the Siboga Expedition.
II, Goneplacidae and Pinnotheridae. Siboga Expeditie
Monographie, 39c1: 149–295, pls. 7–18.
Trigos S & Vicente N (2018) Occurrence of association of Pinna
nobilis with two decapod species in Mediterranean Frenchwaters. Thalassas: An International Journal of Marine Sciences,35(1): 151–154.
Trivedi JN, Campos E & Vachhrajani KD (2018a) A new pea crab
species of the genus Arcotheres Manning, 1993 (Crustacea:
Decapoda: Brachyura: Pinnotheridae) from India. Zootaxa,
4433(1): 195–200.

Trivedi JN, Gosavi S, Vachhrajani KD, Mitra S, Ravinesh R &Ng PKL (2020) On the identities of Nepinnotheres vicajii(Chhapgar, 1957) and Arcotheres casta (Antony & Kuttyamma,1971) from western India: Conspecificity and taxonomy(Decapoda, Brachyura, Pinnotheridae). Zootaxa, 4809(3):496–508.
Trivedi JN, Gosavi S, Vachhrajani KD & Ng PKL (2019)
Redescription of Arcotheres pernicola (Bürger, 1895)(Crustacea: Decapoda: Brachyura: Pinnotheridae) from theoyster Magallana gryphoides (Schlotheim, 1820) in India.
Zootaxa, 4706(4): 587–593.
Trivedi JN, Trivedi DJ, Vachhrajani KD & Ng PKL (2018b) An
annotated checklist of the marine brachyuran crabs (Crustacea:
Decapoda: Brachyura) of India. Zootaxa, 4502(1): 1–83.
Trivedi JN, Vachhrajani KD & Ng PKL (2018c) Redescription of
Arcotheres placunae (Hornell & Southwell, 1909) (Crustacea:
Decapoda: Brachyura: Pinnotheridae) from India and Pakistan.
Zootaxa, 4433(1): 50–58.
Tsang LM, Ahyong ST, Shih H-T & Ng PKL (2018) Further
polyphyly of Pinnotheroidea: The molecular phylogenetic
position of the polychaete-associated Aphanodactylidae.
Invertebrate Systematics, 32: 92–99.
Urita T (1926) A Check List of Brachyura Found in Kagoshima
Prefecture, Japan. The Tsingtao Times, Tsingtao, 40 pp.
Ward M (1967) Life on a sand beach. Proceedings of the Royal
Society of New South Wales, 1967: 59–65.
Watanabe T (2013) Host specificity and life history of pinnotheridcrabs. Cancer, 22: 45–50. [In Japanese]
Watanabe T & Henmi Y (2009) Morphological development of thecommensal pea crab (Arcotheres sp.) in the laboratory reared
specimens. Journal of Crustacean Biology, 29(2): 217–223.
Woodward H (1886) Animals commensal or parasitic in Meleagrina
margaritifera Lin. Proceedings of the Zoological Society of
London, 1886(2): 176–177.

248