Cymonomus andamanicus Alcock, 1905

Ahyong, Shane T., Mitra, Santanu & Ng, Peter K. L., 2020, Cymonomid crabs from southwestern Indonesia and redescription of Cymonomus andamanicus Alcock, 1905, Raffles Bulletin of Zoology 68, pp. 62-69 : 64-65

Cymonomus andamanicus Alcock, 1905

( Figs. 1 , 2 )

Cymonomus andamanicus Alcock, 1905: 568–569 , pl. 18 figs. 1, 1a, 1b. — Alcock et al., 1907: pl. 79, 2, 2a, b. — Dell, 1971: 61, 62. — Griffin & Brown, 1976: 252. — Ahyong & Brown, 2003: 1372, 1373. — Ng et al., 2008: 32. — Ahyong & Ng, 2009: 237. — Dev Roy & Nandi, 2012: tab. 1. — Dev Roy, 2015: 78. — Ahyong & Ng, 2017: 1, 4, 6, 7. — Trivedi et al., 2018: 33. — Ahyong, 2019: 34, 50.

Cymonomus andamanica — Balss, 1922: 117.

Cymonomus granulatus andamanica — Balss, 1922: 118.

Material examined. Holotype: ZSI 4963/10, male (cl 8.7 mm, pcl 7.6 mm, cw 8.8 mm), Andaman basin, 11°26′30″N, 92°53′45″E, 378 fm (692 m), bottom green mud with Foraminifera, RIMSS INVESTIGATOR, stn 322, 20 November 1903.

Description of holotype. Carapace quadrate, lateral margins gently divergent posteriorly; regions weakly indicated; lower pterygostomian region swollen; surfaces sparsely setose. Anterolateral spine short, conical; with 1 or 2 small anteriorly directed spines on lateral margin behind anterolateral spine. Dorsal and lateral surfaces entirely covered with minute granules, granules becoming larger and more elongate anterolaterally. Fronto-orbital margin (excluding rostrum and outerorbital processes) advanced slightly beyond anterolateral margins; 0.66× anterior carapace width; outer orbital processes, pointed, directed anteriorly, triangular in lateral view, situated below plane of rostrum, laterally with acute granules, small spines, reaching midlength of rostrum. Rostrum small, about half-length of eyestalks; 0.15× pcl; triangular, apex acute, margins straight, coarsely granulate laterally and dorsally. Eyestalks distinctly divergent, ~30° to median axis, flattened, stout, width at midlength exceeding one-third length, fused to carapace below rostral base but demarcated from frontal margin; reaching anteriorly slightly beyond midlength of antennular peduncle article 1; dorsal surface minutely granulate, lateral and mesial margins weakly spinulate; cornea apparently vestigial, not pigmented.

Epistome surface granulate, small tubercle at base of rostrum and tubercle mesial to base of antennule; curved spine mesial to base of antenna. Antennular peduncle 0.88× pcl; articles 1 and 2 minutely granulate, article 3 smooth. Antennal articles 1–4 irregularly granulate or minutely spinular; article 5 minutely granulate. Maxilliped 3 ischiobasis subquadrate, granulate and minutely spinular distally; longitudinal sublateral groove; ischium and basis demarcated by faint groove. Merus shorter than ischium; length twice width, tapering distally to rounded apex; surface and margins spinulate. Dactylus, propodus and carpus sparsely spinulate. Carpo-meral articulation at merus mid-length. Exopod surface sparsely granulate; apex reaching to carpo-meral articulation.

Chelipeds (pereopods 1) equal in size and ornamentation, sparsely setose. Merus finely granulate, scattered small spines. Carpus finely granulate, dorsal margin with few short spines. Propodus palm surfaces coarsely granulate, dorsal and ventral margins irregularly spinulate. Dactylus longer than dorsal palm length; proximal dorsal two-thirds with spines and granules; outer surface with faint longitudinal carina; occlusal surfaces of dactylus and pollex crenulate, without gape when fingers closed. Pereopods 2 and 3 sparsely setose; all articles finely granulate; propodus, carpus and merus with serrated granules and scattered spinules on extensor margins. Pereopod 3 longest; merus 1.03× pcl. Dactylus broadly curved, few granules or small spines proximally, with longitudinal rib, though less distinct proximally; setose. Pereopod 3 dactylus about as long as combined length of propodus and carpus. Pereopods 4 and 5 minutely granulate, sparsely setose; longer than pereopod 3; dactylus markedly shorter than propodus, length 1.5× height, falcate, apex corneous apex, flexor margin with 3 obliquely inclined corneous spines. Pereopod 5 merus, when folded against carapace, reaching anterior one-third of carapace.

Thoracic sternite 3 pentagonal, width 1.5× length; margins divergent posteriorly, anteriorly divergent at acute angle; surface granulate. Margins of sternites 4 and 5 granulate.

Abdomen with margins and surface finely granulate or minutely spinulate. Pleotelson with somite 6 and telson immovably fused, demarcation indicated by lateral notch in margin and as faint, partial groove on lateral surface;

telson broadly rounded, width 2.4× length. Gonopods not preserved in holotype.

Remarks. The male holotype of Cymonomus andamanicus Alcock, 1905 , collected from off the Andaman Islands remains the only known specimen of the species to date. Apart from references to original account and figures of the holotype ( Alcock, 1905; Alcock et al., 1907), all subsequent published records of C. andamanicus ( Sakai, 1965, 1976; Nagai, 1994; Ho et al., 2004; Poore et al., 2008; Ahyong et al., 2009) are based on other species ( Ahyong & Ng, 2017; Ahyong, 2019). Although well-documented by the standards of its day, the holotype of C. andamanicus has not been re-studied since it was first described, and information on the form and segmentation of the abdomen is completely lacking in the original account and figures. Therefore, taxonomic decisions supporting other species in the C. delli group have relied entirely on the incomplete original description and figures of the holotype of C. andamanicus . Unfortunately, the holotype of C. andamanicus is now in poor condition, being significantly disarticulated, with damage to the carapace and posterior pereopods, and loss of the maxillipeds 3 and gonopods ( Fig. 1A , C–E). Nevertheless, what remains of the holotype of C. andamanicus permits clarification of ambiguities or errors in the original accounts ( Alcock, 1905: pl. 18; Alcock et al., 1907: pl. 79, 2, 2a, b). Our redescription is based on restudy of the holotype, apart from details of maxilliped 3, which are based on Alcock’s figures. Most significantly, the telson and abdominal somite 6 of C. andamanicus are confirmed to be demarcated, albeit weakly ( Figs. 1E , 2C ), enabling a more complete taxonomic characterisation of the species and of the C. delli group overall. A distinct notch on either side of the pleotelson marks the lateral marginal separation between the telson and abdominal somite 6 but the demarcation on the pleotelson surface is less obvious. Whereas the telson and abdominal somite 6 are clearly demarcated by a distinct, complete groove in C. cubensis (both sexes), C. delli (both sexes), C. diogenes (males only), and C. java , new species (male only known), the demarcation in male C. andamanicus is obscure, being indicated only by a partial, shallow, indistinct groove present laterally, similar to that of male C. cognatus (see Ahyong & Ng, 2017: fig. 2I, J). The anterolateral spines on the carapace, described and figured as absent by Alcock (1905) and Alcock et al. (1907) proved to be present in C. andamanicus ( Fig. 1D ), as also in C. delli and C. cognatus , invalidating the value of the feature for distinguishing these species ( Griffin & Brown, 1976; Ahyong & Ng, 2017). Identification of the presence and position of the anterolateral spines also allowed accurate determination of the frontoorbital width as 0.66× anterior carapace width.

Cymonomus andamanicus is morphologically nearest to C. cognatus , sharing similar walking leg and antennular morphometrics. The two species, however, are distinguished by subtle differences in carapace shape (lateral margins distinctly divergent posteriorly in C. andamanicus versus subparallel in C. cognatus ), rostral form (margins straight in C. andamanicus versus concave in C. cognatus ), greater fronto-orbital width (0.67× anterior carapace width in C. andamanicus versus 0.62 or less in C. cognatus ), and the position of the maxilliped 3 palp articulation on the merus (at midlength in C. andamanicus versus distal to midlength in C. cognatus ). Cymonomus java , new species, from Indonesia, and C. delli from Australia, can be separated from C. andamanicus by the same features as indicated for C. cognatus (see the account of C. java , below, for further distinctions). Cymonomus delli is further distinguished from C. andamanicus by the well-defined (versus indistinct) demarcation between the telson and abdominal somite 6 and the divergence of the anterior margins of thoracic sternite 3 (acute in C. andamanicus versus obtuse in C. delli ). Similarly, C. cubensis and C. delli also differ from C. andamanicus in the subparallel rather than posteriorly divergent lateral margins of the carapace and the strongly convex male telson. Cymonomus diogenes from the South China Sea resembles C. andamanicus in the divergent carapace margins and maxilliped 3 palp position but is readily separated by the proportionally longer walking legs in males (pereopod 3 merus length about 1.4× pcl versus 1.1 or less in C. andamanicus ), and clearly demarcated telson and abdominal somite 6.

Distribution. Presently known only from the type locality.

Figures

Fig. 1. Cymonomus andamanicus Alcock, 1905, male holotype (cl 8.7 mm, pcl 7.6 mm, cw 8.8 mm), Andaman Sea, ZSI 4963/10: A, B, dorsal habitus; C, right cheliped and pereopods 2 and 3, anteroventral view; D, carapace, left oblique view; E, abdomen; F, anterior cephalothorax, ventral view; G, cephalothorax, anterior view. (B, F, G from Alcock et al., 1907: pl. 79, 2, 2a, b).

Fig. 2. Cymonomus andamanicus Alcock, 1905, male holotype (cl 8.7 mm, pcl 7.6 mm, cw 8.8 mm), Andaman Sea, ZSI 4963/10: A, anterior epistome, ventral view; B, right epistomial spine mesial to base of antenna; C, pleotelson; D, right pereopod 5 dactylus. Scale: A, C = 1.0; B, D = 0.5 mm.