RAFFLES BULLETIN OF ZOOLOGY 2015
RAFFLES BULLETIN OF ZOOLOGY Supplement No. 31: 193–198

Date of publication: 10 July 2015
http://zoobank.org/urn:lsid:zoobank.org:pub:CD1D559B-9122-49C0-ACA4-E20024D0C99A

Neorhynchoplax venusta, a new species of hymenosomatid crab
(Crustacea: Decapoda: Brachyura) from Singapore
Peter K. L. Ng
Abstract. A new species of hymensomatid crab is described from a reef in Chek Jawa in Pulau Ubin, Singapore.
Superficially resembling the mangrove-dwelling Neorhynchoplax mangalis (Ng, 1988) from southern Peninsular
Malaysia and Singapore, N. venusta n. sp. is easily distinguished by its unarmed ambulatory dactyli. It can beseparated from all congeners with unarmed ambulatory dactyli in the Indo-West Pacific by its combination ofrostral, carapace and ambulatory leg characters.
Key words. Hymenosomatidae, Neorhynchoplax, new species, taxonomy, Singapore

INTRODUCTIONTAXONOMY

Members of the Indo-West Pacific hymenosomatid crab
genus Neorhynchoplax Sakai, 1938, are characterised by theirrelatively narrow third maxillipeds which do not not cover
more than three‑quarters of the buccal field when closed,
the presence of a well developed rostrum, well developedeyes, position of the base of the antenna, a male pleotelson
which is triangular, and a male first gonopod which has thedistal part tapering and not armed with lobes and processes(cf. Ng & Chuang, 1996; Naruse & Ng, 2007a; Naruse etal., 2008b). The genus currently contains 31 species andincludes freshwater, estuarine and mangrove species (Chuang& Ng, 1994; Ng & Chuang, 1996; Ng et al., 2008; Naruseet al., 2008a, b; Ng et al., 2011).

Family Hymenosomatidae Macleay, 1838
Genus Neorhynchoplax Sakai, 1938
Neorhynchoplax venusta n. sp.
(Figs. 1A, 2)
Material examined. Holotype, female (2.6 × 2.7 mm) (ZRC2014.0258), station SW41, inside hole, sand-mud substratewith small stones, sea grass bed, Pulau Sekudu, off Chek
Jawa, Pulau Ubin, Singapore, 1°24’19.8”N 103°59’15.1”E,coll. A Anker, yabby-pump, 19 October 2012.
Comparative material. Neorhynchoplax mangalis: 1 male
(2.8 × 3.4 mm) (ZRC), Mandai mangroves, Singapore,coll. PKL Ng, 21 June 1990; 1 ovigerous female (2.7 × 3.3mm) (ZRC), station SW110, Lim Chu Kang, Singapore,
1°26.772’N 103°42.509’E, coll. CMBS team, 28 October
2012; 1 male (3.5 × 4.0 mm) (ZRC), station SW136, Sungei
Buloh, 1°26.753’N 103°43.836’E, coll. CMBS team, 30October 2012; 1 male (2.8 × 3.3 mm) (ZRC), station SW106,
Lim Chu Kang, Singapore, 1˚26.772’N 103˚42.509’E, coll.
CMBS team, 27 October 2010. See also Ng & Chuang(1996: 59) for other material.

A new species, Neorhynchoplax venusta, collected from acoral reef in Singapore, a specimen of which was obtained
during the Comprehensive Marine Biodiversity Survey(CMBS), a national project to provide a taxonomic baseline
for the island’s marine fauna. The abbreviations G1 and G2
are used for the male first and second gonopods, respectively.
Measurements provided (in millimetres, including spines) areof the carapace width and length, respectively. The specimen
is deposited in the Zoological Reference Collection (ZRC)of the Lee Kong Chian Natural History Museum (ex Raffles
Museum of Biodiversity Research), National University ofSingapore.

Diagnosis. Carapace subcircular, slightly longer than wide,surrounded by a distinct, complete rim; grooves on dorsal
surface distinct; dorsal surface gently depressed medially,smooth, flat (Fig. 2A). Rostrum distinctly tridentate,lateral teeth triangular with rounded tip; median tooth
spatuliform, lateral margins subparallel, tip rounded (Fig.
2A). Anterolateral tooth arcuate, anterior third with low but
visible lobe; junction of antero‑ and posterolateral margins
with well developed sharp, anteriorly curved tooth on subbranchial region, just outside carapace rim; margins linedwith numerous short setae (Fig. 2A). Infraorbital toothshort, sharp (Fig. 2A). Eye short, peduncle stout, well

Tropical Marine Science Institute and Lee Kong Chian Natural History Museum,
National University of Singapore, Kent Ridge, Singapore 119260, Republic ofSingapore; Email: peterng@nus.edu.sg© National University of Singapore
ISSN 2345-7600 (electronic) | ISSN 0217-2445 (print)

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Fig. 1. Colours in life. A, Neorhynchoplax venusta n. sp., holotype female (2.6 × 2.7 mm) (ZRC 2014.0258), Singapore; B, Neorhynchoplax
mangalis, male (2.8 × 3.3 mm) (ZRC), Singapore.

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fused, with only lateral suture just visible, suture between
somites 4 and 5 with submedian suture or hole; pleotelson
broad, lateral margins sinuous (Fig. 2C).

developed, visible dorsally; cornea large, pigmented (Fig.
2A). Third maxilliped longitudinally slender; covering ca.
75% of buccal field when closed; ischium shorter thansubovate merus; exopod long, gently curved, tip reaching to
distal margin of merus, flgellum long (Fig. 2B). Chelipeds
elongate, equal; outer surfaces with scattered pubescence;merus long, unarmed but proximal margins gently crenulate;
carpus subovate; fingers longer than palm, cutting edges withnumerous small and large teeth (Fig. 2A, D). Ambulatory
very long; articles slender; outer surfaces with scatteredpubescence; dorsal and ventral margins of ischium, merus,
carpus and propodus lined with numerous very short curved
setae, with scattered longer setae; distal margin of merus
with row of short setae; dactylus subcylindrical, distal part
sharply tapering to sharp tip, lateral margins (except fordistal part) lined with numerous very short curved setae
and scattered longer setae, otherwise unarmed (Fig. 2A, E).
Abdomen subovate, surface with scattered setae, denser along
margins; with 4 parts; somites 1 and 2 free, longitudinally
narrow, spanning width of thoracic sternum; somites 3–5

Etymology. The name is derived from the Latin “venustus”
for graceful; alluding to the long and delicate ambulatory
legs of the species.
Remarks. Neorhynchoplax venusta n. sp. is superficiallysimilar to N. mangalis (Ng, 1988) with regards to its carapace
shape, presence of a lateral carapace tooth and elongate
ambulatory legs. However, the structure of the ambulatory
dactylus easily separates the two species – the ventral marginof N. venusta is unarmed while that of N. mangalis is linedwith 2–5 teeth. The carapace shapes of the two species alsodiffer. That of N. mangalis is distinctly longer than broad,the carapace appearing longitudinally ovate (Fig. 3). In N.
venusta, these proportions are subequal and the carapace isalmost round (Fig. 2A).

Fig. 2. Neorhynchoplax venusta n. sp., holotype female (2.6 × 2.7 mm) (ZRC 2014.0258), Singapore. A, overall dorsal view (setae onsides of carapace not drawn); B, left third maxilliped; C, outer surface of right chela; D, right second ambulatory dactylus; E, female
abdomen. Setae not drawn for B–E. Scale bars = 1.0 mm [A]; 0.5 mm [B–E].

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N. attenuipes (Chopra & Das, 1930) (India), N. demeloi(Kemp, 1917) (India), N. inermis (Takeda & Miyake, 1971)(Palau), N. falcifera Naruse, Mendoza & Ng, 2008a (Bohol,Philippines), N. pageti Pretzmann, 1975 (Sri Lanka), N.
sinensis (Shen, 1932) (China), N. thorsborneorum (Lucas& Davie, 1982) (Queensland Australia), and N. tuberculata(Chopra & Das, 1930) (India) (cf. Lucas, 1980; Ng et al.,
1999; Naruse et al., 2008b).

The setation on the ambulatory legs is different. While
N. mangalis has numerous long setae which trap detritus,
giving the animal a “furry” appearance in life (Fig. 1B),
N. venusta has far fewer of these setae. Instead, N. venusta
has numerous short hooked setae lining the margins of thearticles which trap detritus much closer to the pereopods (Fig.
1A). As such, even before cleaning, the two species lookdifferent when alive. In life, the antennules and the distal
parts of the third maxillipeds are orange in N. venusta (Fig.
1A), but are light brown to white in N. mangalis (Fig. 1B).

While some species like N. aspinifera, N. attenuipes, N.
falcifera, N. inermis and N. sinensis (cf. Chopra & Das,
1930: text fig. 10; Shen, 1932: text fig. 33; Takeda & Miyake,
1971: fig. 1d; Naruse et al., 2008b: fig. 7a, d) have relatively
long ambulatory legs, these are proportionately not as longas N. venusta (Fig. 1A, 2A). Neorhynchoplax demeloi, N.

Most Neorhynchoplax species have at least one subterminal
spine on their ambulatory dactylus. In addition to N. venusta,
only nine other species have a completely unarmed ambulatory
dactylus: N. aspinifera (Lucas, 1980) (Queensland, Australia),Fig. 3. Neorhynchoplax mangalis, overall dorsal views. A, male (2.8 × 3.4 mm) (ZRC), Singapore; B, ovigerous female (2.7 × 3.3 mm)(ZRC), Singapore.

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pageti, N. thorsborneorum and N. tuberculata have evenshorter ambulatory legs. The rostrums of N. demeloi and N.
thorsborneorum (cf. Kemp, 1917: fig. 6; Lucas & Davie,
1982: fig. 1) are unusual in that they are very low andlobiform, not dentiform like N. venusta (Fig. 2A). While N.
venusta has a prominent lateral carapace tooth (Fig. 2A),
N. aspinifera, N. demeloi, N. inermis, N. sinensis and N.
thorsborneorum do not, with the lateral carapace marginunarmed (cf. Kemp, 1917: fig. 6; Shen, 1932: text fig. 33;
Takeda & Miyake, 1971: fig. 1a; Lucas, 1980: fig. 3L;
Lucas & Davie, 1982: fig. 1). Neorhynchoplax tuberculata,
N. attenuipes and N. pageti are unusual in having tubercles
on the gastric and cardiac regions (cf. Chopra & Das, 1930:
text figs. 6, 7, 10; Pretzmann, 1975: pl. 1 figs. 1, 2); in N.
venusta, the surface of the carapace is completely smooth(Fig. 2A). Neorhynchoplax inermis is also diagnostic in
having only a single rostral tooth (cf. Takeda & Miyake,
1971: fig. 1A) compared to three in N. venusta (Fig. 2A).

live specimen during the workshop.
LITERATURE CITED
Chopra B & Das KN (1930) Further Notes on Crustacea Decapoda
in the Indian Museum. I. On two new species of hymenosomatid
crabs, with notes on some other species. Records of the IndianMuseum, 32(4): 413–429.
Chuang CTN & Ng PKL (1991) Preliminary descriptions of one
new genus and three new species of hymenosomatid crabs
from Southeast Asia (Crustacea: Decapoda: Brachyura). Raffles
Bulletin of Zoology, 39(2): 363–368.
Chuang CTN & Ng PKL (1994) The ecology and biology ofSoutheast Asian false spider crabs (Crustacea: Decapoda:Brachyura: Hymenosomatidae). Hydrobiologia, 285: 85–92.
Kemp S (1917) Notes on Crustacea Decapoda in the Indian Museum.
X. Hymenosomatidae. Records of the Indian Museum, 12(5):243–279.
Lucas JS (1980) Spider crabs of the family Hymenosomatidae(Crustacea; Brachyura) with particular reference to Australianspecies: systematics and biology. Records of the AustralianMuseum, 33(4): 148–247, figs. 1–10.
Lucas JS & Davie PJF (1982) Hymenosomatid crabs of Queensland
estuaries and tidal mud flats, including descriptions of four newspecies of Elamenopsis A. Milne-Edwards and a new speciesof Amarinus Lucas. Memoirs of the Queensland Museum,20(3): 401–419.
MacLeay WS (1838) On the Brachyurous Decapod Crustacea.
Brought from the Cape by Dr. Smith. In: Smith A (ed.),Illustrations of the Zoology of South Africa; consisting chieflyof Figures and Descriptions of the Objects of Natural History
Collected during an Expedition into the Interior of South
Africa, in the Years 1834, 1835, and 1836; fitted out by “TheCape of Good Hope Association for Exploring Central Africa”.
Published under the Authority of the Lords Commissioners ofHer Majesty’s Treasury, Invertebratae, Smith, Elder, and Co.,
London. Pp. IV + 53–71, pls. 2, 3.
Naruse T & Ng PKL (2007a) On the taxonomy of the genus
Hymenicoides Kemp, 1917 (Crustacea: Decapoda: Brachyura:Hymenosomatidae), with resurrection of Limnopilos Chuang& Ng, 1991, and descriptions of two new species. Zootaxa,1621: 17–31.
Naruse T & Ng PKL (2007b) On a new species of Elamenopsis
from Singapore, with notes on Crustaenia palawanensis (Serène,1971) (Crustacea: Decapoda: Brachyura: Hymenosomatidae).
Raffles Bulletin of Zoology, 55(1): 121–125.
Naruse T, Mendoza JCE & Ng PKL (2008a) Descriptions offive new species of false spider crabs (Decapoda: Brachyura:Hymenosomatidae) from the Philippines. Marine Biology
Research, 4: 429–441.
Naruse T, Ng PKL & Guinot D (2008b) Two new genera andtwo new species of troglobitic false spider crabs (Crustacea:Decapoda: Brachyura: Hymenosomatidae) from Indonesia,
with notes on Cancrocaeca Ng, 1991. Zootaxa, 1739: 21–40.
Ng PKL (1988) Elamenopsis mangalis sp. nov., a new speciesof mangrove‑dwelling hymenosomatid crab from Singapore(Crustacea, Decapoda, Brachyura). Crustaceana, 55(3):274–278.
Ng PKL (1991) Cancrocaeca xenomorpha, new genus and species,a blind troglobitic freshwater hymenosomatid (Crustacea:Decapoda: Brachyura) from Sulawesi, Indonesia. Raffles
Bulletin of Zoology, 39(1): 59–63.
Ng PKL, Chen H-L & Fang S-H (1999) On some species ofHymenosomatidae (Crustacea: Decapoda: Brachyura) fromChina, with description of a new species of Elamena and a
key to the Chinese species. Journal of the Taiwan Museum,52(1): 81–93.

The female type specimen is not fully mature as its
abdomen is not fully rounded and not very swollen, as istypical for other Neorhynchoplax species. Members of thisgenus practice ovovivipary, possessing an internal broodcavity with relatively large eggs that hatch inside the body(Fig. 3B) (see Ng & Chuang, 1996; Ng et al., 2011). The
structure of the female abdomen, however, is typical for other
Neorhynchoplax species, with somites 3–5 fused, although
the lateral parts of the sutures are still visible (Fig. 2E).
Six other species of hymenosomatid crabs are known
from Singapore: Halicarcinus coralicola (Rathbun, 1909),
Crustaenia palawanensis (Serène, 1971), Neorhynchoplax
mangalis (Ng, 1988), Elamena globosa Chuang & Ng, 1991,
Elamena mendosa Chuang & Ng, 1991, and Elamenopsis
rotunda Naruse & Ng, 2007 (cf. Ng & Chuang, 1996; Naruse& Ng, 2007b).
Ecology. The single specimen of N. venusta was obtained
from a sandy-muddy burrow in a seagrass bed by a yabbypump, on a small island covered by reef at low tide. It appears
to be an intertidal reef species. Two other intertidal reef
species are known from Singapore, Crustaenia palawanensis
and Halicarcinus coralicola, although they have not beencollected together as yet. The closely related N. mangalis
is only known from intertidal mangroves.
ACKNOWLEDGEMENTS
The Johor Straits marine biodiversity workshop on Pulau
Ubin, Singapore was organised by the National Parks Board
and National University of Singapore and held from 15
October to 2 November 2012 at Outward Bound School. Theworkshop, as part of the Comprehensive Marine Biodiversity
Survey (CMBS) was supported by generous contributions
from Asia Pacific Breweries Singapore, Care-for-Nature
Trust Fund, Shell Companies in Singapore and The Air
Liquide Group. The management and staff of the Outward
Bound School are thanked for kindly accommodating ourspecial needs for a successful workshop. I am grateful toArthur Anker for collecting it and his photographs of the197

Ng: new hymenosomatid crab from Pulau Ubin, Singapore
Ng PKL & Chuang CTN (1996) The Hymenosomatidae (Crustacea:Decapoda: Brachyura) of Southeast Asia, with notes on otherspecies. Raffles Bulletin of Zoology, Supplement 3: 1–82.
Ng PKL, Guinot D & Davie PJF (2008) Systema Brachyurorum:
Part I. An annotated checklist of extant brachyuran crabs ofthe world. Raffles Bulletin of Zoology, Supplement 17: 1–286.
Ng PKL, Nesemann HF & Sharma G (2011) A new freshwater
species of Neorhynchoplax Sakai, 1938 (Crustacea: Decapoda:Hymenosomatidae) from Patna, Bihar, India. Zootaxa, 3063:
53–63.
Pretzmann G (1975) Brachyura aus Ceylon (Ausbeute einer
Expedition des Naturhistorischen Museums 1973/74). Annalen
des Naturhistorischen Museums in Wien, 79: 605–608, pl. 1.
Rathbun MJ (1909) New crabs from the Gulf of Siam. Proceedingsof the Biological Society of Washington, 22: 107–114.

Sakai T (1938) Studies on the Crabs of Japan. III. Brachygnatha,Oxyrhyncha. Yokendo, Tokyo. 193–364 pp., figs. 1–55, pls.
20–41, Table 1.
Serène R (1971) Observations préliminaires sur des Brachyoures
nouveaux ou mal connus du Sud‑Est Asiatique (CrustaceaDecapoda). Bulletin du Muséum national d’Histoire naturelle,(2)42(5): 903–918.
Shen C (1932) The Brachyuran Crustacea of North China. Zoologica
Sinica, Series A (Invertebrates of China), The Fan Institute ofBiology, Peking [= Beijing], China, 9(1): i–x + 1–321 pp.,pls. 1–9, 1 map.
Takeda M & Miyake S (1971) A new freshwater crab of thefamily Hymenosomatidae from the Palau Islands. Researches
on Crustacea, 4–5: 157–163.

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