Elamenopsis rotunda, Naruse & Ng, 2007

Naruse, Tohru & Ng, Peter K. L., 2007, 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), pp. 121-125 : 121-122

Elamenopsis rotunda , new species

( Fig. 1 )

Material examined. – Holotype: female, CL 3.7 mm, CW 4.8 mm, ZRC 2006.0147 , off Tuas, Singapore, coll. Z. Jaafar, 29 Aug.2000.

Paratype: female, CL 2.8 mm, CW 3.8 mm, ZRC 2006.0148 , data same as holotype .

Comparative material. Elamenopsis lineata (A. Milne Edwards, 1873) : 1 male, CL 2.1 mm, CW 3.1 mm, 1 female, CL 2.4 mm, CW 3.4 mm, Queensland Museum W2341, S. Dunwich, South East Queensland, Australia, coll. F. C. Vohra, 15 Jul. 1962. Elamenopsis ariakensis ( Sakai, 1969) : 1 male, CL 2.7 mm, CW 3.8 mm, ZRC 1999.0019, Mei-Zhou Bay, Fujian Province, China, coll. S.-H. Fang, 4 Sep. 1998. Elamenopsis comosa Ng & Chuang, 1996: 1 male, CL 2.8, CW 4.4 mm, ZRC 1994.4244 (holotype), Negeri Lama, Ambon, Indonesia, coll. M. Takeda, 24 Jan. 1993; 1 female, CL 2.6, CW 4.1 mm, ZRC 1994.4245 (paratype), data same as ZRC 1994.4244.

Description. – Carapace ( Fig 1a ) not swollen, ovoid, CW 1.30–1.33 times CL, width of dorsal rim 0.81–0.82 times CW, lateral surfaces around bases of second, third legs expanded laterally; complete dorsal rim subcircular, almost flat, regions well defined; cervical groove branching anteriorly near anterolateral angle, connected posteriorly with gastro-cardiac and thoracic grooves, thoracic groove merging with posterior rim, postbranchial region longitudinally divided by transverse groove. Rostrum ( Fig. 1b ) triangular, strongly deflexed downwards, concave medially, lateral margin not confluent with dorsal rim; dorsal rim fringed with short, curled setae; anterolateral angle very low, rounded, being formed by thickened rim with anterior inner notch; posterolatral border convergent posteriorly, without posterior lateral tooth. Posterior margin of epistome ( Fig. 1b ) with medial rounded convexity. Eye ( Fig. 1b ) with short peduncle, cornea small, rounded, visible in dorsal view. Third maxilliped narrow, leaving wide gape when closed, lined with long setae along inner margins of ischium and merus; ischium small, inner margin distinctly shorter than that of merus; exopod thin, distal end not reaching distal outer angle of merus, with distinct flagellum.

Chelipeds ( Fig. 1a, c ) almost symmetrical, with setose outer surfaces; merus with flat inner surface and gently convex outer surface; carpus rounded, distal part of dorsal margin toothlike, without inner angle; manus with inflated inner and outer surfaces; fingers as long as manus, with acute tips, with median gape when closed; immovable finger with lower margin slightly convex subproximally, cutting edge sparsely lined with small, triangular, acute teeth, proximal one largest; movable finger with upper margin slightly curved downwards distally, crossing posterior of immovable finger when closed, cutting edge lined with small triangular teeth on distal twothirds, long rectangular tooth on proximal third; chelae paler in coloration on distal part of manus (around base of immovable finger) and on both fingers, except for proximal dorsal part of movable finger.

Ambulatory legs ( Fig. 1d ) relatively slender, long, flattened laterally, margins densely fringed with long setae, setae progressively longer and denser from posterior (meri) to inner (carpi to dactyli) margins, second leg longest; merus laterally flattened, dorsal margin with sparse short setae, proximal part of posterior margin somewhat excavated; dactylus not prominently incurving, inner margin straight.

Abdomen ( Fig. 1e ) subquadrate, proximal part of first segment expanded laterally, backwards, third to fifth segment fused; abdominal cavity distally not reaching imaginary line joining proximal half of first ambulatory coxae.

Etymology. – From the Latin rotunda meaning “circular”, alluding to circular dorsal rim of the carapace. The name is used as an adjective.

Habitat. – Elamenopsis rotunda , new species, was collected from a sublittoral mud flat, off Tuas, Singapore, by using an Eckman grab (Zeehan Jaafar, personal communication).

Distribution. – Known only by type specimens from Singapore.

Remarks. – Elamenopsis currently contains three species, viz. E. lineata (A. Milne Edwards, 1873) , E. ariakensis ( Sakai, 1969) , and E. comosa Ng & Chuang, 1996 . Elamenopsis rotunda differs from all congeners by its proportionately narrower carapace and thinner ambulatory legs. Elamenopsis rotunda is most closely allied to E. ariakensis by its subcircular dorsal rim of the carapace (length to width ratios 1.19 in E. rotunda and 1.20 in E. ariakensis ). However, E. rotunda can easily be differentiated from E. ariakensis by the shape of the lateral lobe of the rostrum (very low vs. distinct, triangular, directed perpendicularly upwards), smaller anterior lateral angle which is formed by thickened rim with anterior notch (vs. distinct, triangular, upturned as high as the lateral lobe of the rostrum in E. ariakensis ), and the absence of a second anterolateral angle (vs. present in E. ariakensis ) ( Sakai, 1969: 250, text-fig. 2; Ng et al., 1999: 85, Fig. 3; Kosuge et al., 2002: 104, Fig. 2 ; present study).

Elamenopsis rotunda , new species, is also superficially similar to species of Neorhynchoplax in its relatively narrower dorsal rim of the carapace and more slender ambulatory legs. Elamenopsis rotunda , however, possesses a deflexed rostrum and acicular ambulatory dactyli with no dentition, and we have no doubt that it is a member of Elamenopsis .

Figures

Fig. 1. Elamenopsis rotunda, new species, female paratype (ZRC 2006.0148): a, dorsal view; b, frontal view; c, right chela; d, left second ambulatory leg; e, abdomen and telson (abdomen and telson are soft and most probably damaged). Scale bars = 1.0 mm.

Fig. 2. Posterolateral tooth of Crustaenia Ng & Chuang, 1996, and Neorhynchoplax Sakai, 1938: a, Crustaenia palawanensis (Serène, 1971) (paratype, ZRC 1969.12.11.2); b, Neorhynchoplax mangalis (Ng, 1988) (holotype, ZRC 1993.6556); c, Neorhynchoplax prima Ng & Chuang, 1996 (holotype, ZRC 1994.4233). Scale bars = 1.0 mm.