THE RAFFLES BULLETIN OF ZOOLOGY 2007
THE RAFFLES BULLETIN OF ZOOLOGY 2007 Supplement No. 16: 227–231
Date of Publication: 31 Dec.2007© National University of Singapore

A NEW GENUS FOR CAVE-DWELLING CRABSPREVIOUSLY ASSIGNED TO SESARMOIDES(CRUSTACEA: DECAPODA: BRACHYURA: SESARMIDAE)Peter J. F. Davie
Queensland Museum, PO Box 3300, South Brisbane, Queensland, Australia
Email: Peter.Davie@qm.qld.gov.au

Peter K. L. Ng
Department of Biological Sciences, National University of Singapore, Kent Ridge,
Singapore 119260, Republic of Singapore
Email: peterng@nus.edu.sg

ABSTRACT. – The semi-terrestrial crab genus Sesarmoides is restricted to three species of Indo-West Pacificintertidal mangrove and estuarine crabs, S. kraussi, S. longipes and S. borneensis. Twelve other species
previously placed in Sesarmoides are here transferred to a new genus, Karstarma. The species of Sesarmoides
are distinguished from those of Karstama by the presence of a stridulatory structure consisting of a longitudinal
crest on the cheliped merus that is used to rub against a ridge of suborbital granules. This form of stridulation
is apparently unique within the Sesarmidae, although common in the Varunidae. Other features distinguishingspecies of Sesarmoides sensu stricto from those of Karstama are their relatively smaller adult carapace size,a relatively shallower median epigastric groove, the presence of a longitudinal row of small granules or ridgeon the outer surface of the cheliped pollex, and a relatively more slender male first gonopod.
KEY WORDS. – Taxonomy, Sesarmidae, new genus, caves, stridulation, intertidal, mangrove.

INTRODUCTION
Ng (2002) recently reappraised the sesarmid crab genus
Sesarmoides Serène & Soh, 1970, and recognised 14 species,
S. balicus Ng, 2002, S. boholano Ng, 2002, S. borneensis(Tweedie, 1950), S. cerberus (Holthuis, 1964), S. emdi Ng& Whitten, 1995, S. guamensis Ng, 2002, S. jacksoni (Balss,
1934), S. jacobsoni (Ihle, 1912), S. kraussi (De Man, 1887),
S. longipes (Krauss, 1843), S. loyalty Ng, 2002, S.
novabritannia Ng, 1988, S. sulu Ng, 2002 and S. ultrapes
Ng, Guinot & Iliffe, 1994 all from the Indo-West Pacific.
Naruse & Ng (2007) subsequently described another new
species from Sulawesi, S. microphthalmus.
However, Ng (2002: 420) commented that there were two
species groups in Sesarmoides. One group included three
species usually associated with mangroves, estuarine areasand sometimes coastal caves, viz., S. longipes, S. kraussi and
S. borneensis. These species are morphologicallydistinguished by their smaller adult carapace size, a relatively
shallower median epigastric groove, the presence of alongitudinal row of small granules or ridge on the outersurface of the pollex and the male first gonopod being
relatively more slender. The second group of species are

typically larger, have a swollen epigastric region separated
by a deeper median groove, often possess a row of granules
on the dorsal margin of the dactylus of the chelipeds and lacka longitudinal row of granules or ridge on the outer surfaceof the pollex (Ng, 2002). This group included the remaining
12 species listed above, and all are typically found inanchialine cave systems (Ng, 2002; Naruse & Ng, 2007).
In this paper, we argue that these two groups of Sesarmoides
must be recognised as distinct genera. In addition to thecharacters listed by Ng (2002), species of Sesarmoides sensu
stricto possess a unique stridulatory mechanism consistingof a ridge on the inner surface of the merus of the cheliped
and a milled suborbital ridge.
All the specimens listed in Ng (2002) were utilised for thepresent study and the reader should consult this reference forthis specimen data. As such, we only list additional material
which is deposited in the Queensland Museum (QM),
Brisbane and the Zoological Reference Collection (ZRC) of
the Raffles Museum of Biodiversity Research, National
University of Singapore. Measurements (in millimetres, mm)
are of the carapace width and length respectively. The
abbreviations G1 and G2 are used for the male first and second
gonopods respectively.

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Davie & Ng: New cave-dwelling crab genus
TAXONOMYFamily SESARMIDAE Dana, 1851
Sesarmoides Serène & Soh, 1970
Type species. – Sesarma krausii De Man, 1887, by original
designation; gender masculine.
Diagnosis. – Carapace distinctly trapezoidal in shape, posterolateral
margins strongly converging towards posterior carapace margin;epigastric region with relatively shallow median groove; dorsal
margin of cheliped dactylus smooth or almost so; outer surface ofpollex of males with longitudinal row of small granules, spinules,
or ridge; inner surface swollen, with patch of distinctly raised
granules; cheliped merus with inner dorsal angle forming small tooth
but not prominent expansion, inner surface with distinctly raised
ridge just behind inner margin (Fig. 1); suborbital margin distinctly
milled, without setae; G1 relatively stout, short.
Included species. – Sesarmoides borneensis (Tweedie, 1950), S.
kraussi (De Man, 1887) and S. longipes (Krauss, 1843).
Material examined. – In addition to the material of the Sesarmoides
species listed in Ng (1988, 2002), Ng et al. (1994) and Naruse &Ng (2007), we add the following: Sesarmoides kraussi: 1 male (ZRC
1964.9.8.7), Pasir Ris, Singapore, coll. M. W. F. Tweedie, 1950; 1
damaged specimen (ZRC 1987.560), Mandai, Singapore, coll. P.
K. L. Ng, Mar.1986; 1 female (ZRC 1968.11.22.1), Buloh Besar
River, Singapore, coll. C. L. Soh, 20 Mar.1966; 1 female (ZRC
1968.11.22.2), River Simpang Mak Wai, Singapore, coll. C. L. Soh,
18 Mar.1966; 2 females (ZRC 1965.8.3.49–50), Prai, Province
Wellesley, Peninsular Malaysia, coll. Dec.1938; 1 male (ZRC
1965.8.3.50), Port Swettenham, Peninsular Malaysia, coll. M. W.
F. Tweedie, Dec.1934; 1 male (ZRC 1965.8.3.51), Port Swettenham,
Peninsular Malaysia, coll. M. W. F. Tweedie, Dec.1934; 1 male
(ZRC 1965.8.3.52), Port Swettenham, Peninsular Malaysia, coll. M.
W. F. Tweedie, Dec.1934; 10 specimens (ZRC 1965.8.3.53–60),
Port Swettenham, Peninsular Malaysia, coll. M. W. F. Tweedie,
Dec.1934; 25 juveniles (ZRC 1965.8.3.53–60), Port Swettenham,
Peninsular Malaysia, coll. M. W. F. Tweedie, Dec.1931; 1 male
(ZRC 1965.8.3.61), Kuantan, Pahang, Peninsular Malaysia, coll.
Sep.1935; 1 male, 1 female (ZRC 1970.1.20.5–6), Port Swettenham,
Peninsular Malaysia, coll. Sase Kuman, 10 Jan.1970; 1 female (ZRC
1999.1136), Matang mangroves, Perak, Peninsular Malaysia,
Jun.1995.
Sesarmoides borneensis: 1 male (20.3 × 15.9 mm) (QM W5365),
Pulgul Ck., south of Urangan, Hervey Bay, southeast Queensland,
Australia, 25º19'S 152º54'E, coll. P. Shanco & E. Hegerl, 19
Jul.1975; 1 male (12.3 × 9.7 mm) (ZRC), Kampung Ladong, Pulau
Tekong, Singapore; 1 male (Museum of Florence), lateral branch,
St. Paul N. P. Cave, Palawan Island, Philippines, coll. Borri & C.
Volpi, 25 Feb.1991; 1 female (Museum of Florence), Palawan Island,
Philippines, coll. G. Messana, 25 Jan.1991; 1 male (ZRC
1970.1.20.4), Johor Straits, Singapore, coll. C. L. Soh, 18 Jan.1970;
1 male (ZRC 1968.11.23.1), Seletar River, Singapore, coll. C. L.
Soh, 22 Jun.1967; 2 males, 1 female (ZRC 1970.1.23.12–13), Johor
Straits, Singapore, coll. C. L. Soh, 21 Jan.1970; 1 male (ZRC
1999.0567), T. Pandan Besar, Bako, Sarawak, East Malaysia,
Borneo, coll. P. K. L. Ng, 28 Jun.1994; 2 males, 2 females (ZRC
2000.2056), Ajkwa River, Irian Jaya, Indonesia, coll. D. L. Rahayu,
11 Jan.2000; 2 males, 2 females (ZRC 2003.0496), station A33/II/
C1, mangrove, Ajkwa River, Irian Jaya, coll. D. L. Rahayu, 23
Oct.2002; 1 male, 1 ovigerous female (ZRC 2003.0494), Irian Jaya,
Indonesia, coll. D. L. Rahayu, 17 Oct.2001; 4 males (ZRC
2003.0495), Irian Jaya, Indonesia, coll. D. L. Rahayu, 19 Oct.2001.

Remarks. – The genus Sesarmoides Serène & Soh, 1970, wasoriginally established for species which have a relatively
flattened carapace with strongly divergent lateral margins, anarrow frontal margin which is shorter than the posterior
carapace margin, long and slender ambulatory legs (especiallythe third), a basal antennal segment which is swollen andglobular with a sub-longitudinal peduncle and an epistome
with a distinct transverse rim and deep median notch. As
discussed earlier, Ng (2002) had noted that there were twogroups of species in the genus but preferred not to separate
them at that time. The present discovery of a uniquestridulatory mechanism (Fig. 1) in one group (but not theother) now requires that the two groups be recognised as
separate genera. Ng’s (2002) first group must retain the name
Sesarmoides as it contains Sesarma krausii De Man, 1887,
the type species of Sesarmoides. Thus a new genus, Karstama,
is established here for the second group previously placed in
the genus and which typically live in caves in limestone or
karst systems.
Other than the characters given by Serène & Soh (1970) andNg (2002) (for his first group of Sesarmoides), we note thatthe three species of Sesarmoides sensu stricto recognised here,
share a stridulatory structure that is unique within the
Sesarmidae (to our knowledge). Stridulation has been longknown, or postulated, to occur in sesarmids although themechanisms have been little studied. Tweedie (1954: 123)
observed “The crab [Perisesarma eumolpe] rose on its feet,
twisted one cheliped downwards so that the upper margin of
the chela was vertical or even a little overturned, and rubbed
the upper margin of the dactylus of the other chela up anddown against that of the downturned one, at first rather slowly,
but with increasing speed, so that the active chela appeared
to be almost vibrating at the end of the performance.” Thisbehaviour was later verified as stridulatory by Mulstay (1980)
and Siebel & Salmon (1982) for the American species
Sesarma reticulatum and S. cinereum. The dactylar tubercles
are rasped over the pectinated crests on the upper surface ofthe palm. Other sesarmids that lack such pectinated crests,

Fig. 1. The stridulatory ridge and orbit of Sesarmoides borneensis,
a key character unique to Sesarmoides species (QM W5365, male,20.3 × 15.9 mm).

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THE RAFFLES BULLETIN OF ZOOLOGY 2007
appear to also stridulate by rubbing the inner face of the palm
against the pterygostome or suborbital crest (pers. obs.),
although the “pars stridens” (file) and the corresponding“plectrum” (see Guinot-Dumortier & Dumortier, 1960) areless obvious.
In Sesarmoides, the inner surface of the cheliped merus, justadjacent to the inner distal margin, has a short but distinct
smooth ridge. This ridge can be stridulated against the milled(finely, evenly, and smoothly granulated) suborbital margin(see Fig. 1). In Karstama species, there is no trace of any
such stridulatory ridge on the cheliped merus. These species
also have a very prominent sublamelliform distal process on
the merus of the cheliped, which in Sesarmoides sensu stricto
is shorter and dentiform. The suborbital margin of Karstama
species is not distinctly milled as in Sesarmoides sensu stricto,
but typical of other sesarmids in being granular under a dense
lining of short setae which almost completely obscure themargin. The unusual stridulatory structure of Sesarmoides
sensu stricto is a clear indication that they are genericallydifferent from the other species that have been included in
the genus.
In addition, Sesarmoides sensu stricto species have
trapezoidal carapaces (Fig. 2A), whereas those of Karstama
species are generally quadrate or rounded (Fig. 2B).
Sesarmoides species have a smaller adult carapace size, arelatively shallower median epigastric groove, the presenceof a longitudinal row of small granules/teeth or ridge on theouter surface of the pollex and a relatively more slender malefirst gonopod. Karstama species are typically larger, have aswollen epigastric region separated by a deeper mediangroove, often possess a row of granules on the dorsal marginof the dactylus of the chelipeds and lack a longitudinal row
of granules or ridge on the outer surface of the pollex (Ng,2002).
Ng (2002) also noted an interesting correlation in habitatpreferences. The three species of Sesarmoides sensu stricto
are typically coastal or mangrove species (see Tweedie, 1950;
Barnard, 1950; Tan & Ng, 1994; Ng & Sivasothi, 1999; Ng,
2002), although S. borneensis has been found in a cave nearmangroves in the Philippines (Ng, 2002: 428). In contrast,
all the members of Karstama have only been found in karst
areas or limestone caves, sometimes some distance from thesea (Holthuis, 1964; Ihle, 1912; Naruse et al., 2004; Naruse& Ng, 2007).

Key to the species of Sesarmoides sensu stricto
1. Carapace with one epibranchial tooth; length of third ambulatory
leg less than 3 times extraorbital width, propodus about 2–3
times as long as broad; anterior frontal margin nearly straight;
male abdomen with telson slightly shorter than broad at base;length of segment 6 slightly less than twice width [East Africa]............................................................................... S. longipes– Carapace with two epibranchial teeth; length of third ambulatoryleg more than 3 times extraorbital width, propodus about 4 times
as long as broad; anterior frontal margin weakly sinuous; maleabdomen with telson slightly shorter than broad at base; lengthof segment 6 not as above .................................................... 2
2. Outer surface of pollex with a longitudinal row of 8–13 small
conical tubercles; adult male abdomen with segment 6 a little
shorter than half its proximal breadth [Indian Ocean andSoutheast Asia as far as the Ryukyu Islands] ....... S. kraussi– Outer surface of pollex with a longitudinal row of 3–5 elongatedtubercles; adult male abdomen with segment 6 slightly longer
than half its proximal breadth [Southeast Asia and northern
Australia] ............................................................ S. borneensis

Karstarma, new genus
Type species. – Sesarmoides boholano Ng, 2002, by present
designation.
Diagnosis. – Carapace more or less rectangular, with rounded
anterolateral margins, posterolateral margins weakly converging
towards posterior carapace margin; epigastric region with a relatively
deep median groove; dorsal margin of cheliped dactylus usually witha row of granules; outer surface of pollex smooth; inner surface ofpalm smooth or with scattered very small granules, never swollen;cheliped merus with inner dorsal angle prominently expanded toform sub-lamelliform lobe, inner surface smooth, without ridge;
suborbital margin granulated, densely lined with setae which
obscures margin; G1 relatively slender, elongate.

Fig. 2. Dorsal views of representatives of Sesarmoides andKarstarma showing typical differences in overall carapace shape:
A, Sesarmoides borneensis, note the more trapezoidal carapace (QM
W5365, male, 20.3 × 15.9 mm); and B, Karstarma guamense, witha generally more quadrate and anterolaterally rounded carapace (QM
W27549, male, 15.2 × 12.3 mm).

Etymology. – The name is derived from the typical habitatof most of the constituent species, i.e. karst systems, inarbitrary combination with the genus name Sesarma. Genderneuter.
Remarks. – Karstama is distinguished from Sesarmoides by
a combination of carapace and cheliped characters (seeremarks for Sesarmoides sensu stricto).

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Davie & Ng: New cave-dwelling crab genus
Species included. – Sesarmoides balicus Ng, 2002, S.
boholano Ng, 2002, S. cerberus (Holthuis, 1964), S. emdi
Ng & Whitten, 1995, S. guamensis Ng, 2002, S. jacksoni(Balss, 1934), S. jacobsoni (Ihle, 1912), S. loyalty Ng, 2002,
S. microphthalmus Naruse & Ng, 2007, S. novabritannia Ng,
1988, S. sulu Ng, 2002, and S. ultrapes Ng, Guinot & Iliffe,
1994.
Material examined. – In addition to the material of the
Sesarmoides species listed in Ng (1988, 2002), Ng et al.
(1994) and Naruse & Ng (2007), we add the following:
Sesarmoides guamensis: QM W27549, male (15.2 × 12.3mm), Faifai Beach, Timon Bay, Guam, 13°48'N 144°45'E,
28 Jul.–1 Aug.2001, coll. P. K. L. Ng.

Key to the species of Karstarma(adapted from Ng, 2002)
1. Ocular peduncle short, cornea small, eyes occupying only partof orbit ................................................................................... 2– Ocular peduncle and cornea of normal size and occupying
entirely the orbital cavity ...................................................... 3
2. Lateral margin of carapace smoothly divergent, appears almost
straight; no mat of setae on inner margins of ambulatory propodi
and dactyli; chitinous part of G1 relatively long, directed
outwards at an angle of about 45º [Java: Indonesia] ..............
............................................................................. K. jacobsoni– Lateral margin of carapace distinctly convex; prominent mat ofsetae present on inner margins of ambulatory propodi anddactyli; chitinous part of G1 relatively shorter, directed outwards
at an angle of about 30º [Sulawesi: Indonesia] .......................
.................................................................. K. microphthalmus
3. Tufts of setae absent between coxae of first to third ambulatory
legs ........................................................................................ 4– Distinct tufts of setae present between coxae of first to thirdambulatory legs ..................................................................... 6
4. Ambulatory legs extremely long, third ambulatory leg (merus
to dactylus length) about 4.9 times carapace length [Solomon
Islands] .................................................................. K. ultrapes– Ambulatory legs long, but third ambulatory leg (merus todactylus) distinctly less than 4.5 times carapace length ...... 5
5. Tip of external orbital tooth reaching just beyond level of frontal
margin; male telson equal in length to segment 6 of maleabdomen [Ambon: Indonesia] ............................. K. cerberus– Tip of external orbital tooth not reaching level of frontal margin;
male telson shorter than segment 6 of male abdomen [NewBritain] ......................................................... K. novabritannia
6. Surface of carapace without obvious oblique striae on posterior
branchial region; meri of second and third ambulatory legs 4.7
and 5.1 times carapace length respectively [Christmas Island:
Indian Ocean] ......................................................... S. jacksoni– Surface of carapace with clear oblique striae on posterior
branchial region; meri of second and third ambulatory legs lessthan 4.5 and less than 5.0 times carapace length respectively............................................................................................... 7
7. Meri of first to third ambulatory legs with sharp distal dorsal
spine [Palawan: the Philippines] ................................. K. sulu
– Meri of first to third ambulatory legs without distal dorsal spine,
may be angular distally but never spiniform ....................... 8
8. External orbital tooth truncate, outer margin straight to gently
concave, separated from first anterolateral tooth by prominent,
deep V-shaped notch [Bohol: the Philippines; southern Ryukyu
Islands: Japan] ..................................................... K. boholano–

External orbital tooth not truncate, outer margin almost straightto gently convex, separated from first anterolateral tooth by
narrow cleft or relatively shallower notch ........................... 99. G1 with elongate, slender chitinous distal part ................. 10– G1 with relatively short, truncate chitinous distal part ..... 11
10. External orbital tooth separated from first anterolateral tooth byvery shallow notch; length to width ratios of meri of second
and third ambulatory legs 3.1 and 3.5 respectively ; inner marginof G1 with gentle submedian hump [Loyalty Islands] ............
................................................................................. K. loyalty– External orbital tooth separated from first anterolateral tooth byvery shallow notch; length to width ratios of meri of second
and third ambulatory legs 3.3 and 3.6 respectively; inner marginof G1 almost straight [Bali: Indonesia] ............... K. balicum
11. Length to width ratios of merus of second and third ambulatory
legs 4.3 and 4.0 respectively; length to width ratios of propodi
of second and third ambulatory legs 4.1 and 4.5 respectively;
G1 with chitinous distal part slightly bent downwards [Bali:
Indonesia] ................................................................... K. emdi
– Length to width ratios of meri of second and third ambulatory
legs 3.7 and 4.1 respectively; length to width ratios of propodi
of second and third ambulatory legs 3.4 and 4.2 respectively;
G1 with chitinous distal part bent at right angles [Guam] ......
............................................................................ K. guamense

ACKNOWLEDGEMENTS
The first author is grateful to the Raffles Museum of
Biodiversity Research, National University of Singapore(NUS), for travel funds that allowed this research to becompleted. Aspects of this study have also been supported
by grants to the second author from NUS.

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