Cymonomus java, Ahyong & Mitra & Ng, 2020

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 : 65-68

Cymonomus java , new species

( Figs. 3B , 4 )

Material examined. Holotype: MZB Cru 5024, male (cl 8.2 mm, pcl 7.5 mm, cw 7.6 mm), eastern Indian Ocean, south

of Java, Indonesia, 07°47.677′S, 107°41.904′E, 603–686 m, SJADES 2018 Expedition, stn CP35, 29 March 2018 .

Description of holotype. Carapace quadrate, lateral margins subparallel; regions weakly indicated; lower pterygostomian region swollen; surfaces sparsely setose. Anterolateral spine short, conical, minute. Dorsal and lateral surfaces entirely covered with minute granules, granules becoming larger and more elongate anterolaterally. Fronto-orbital margin (excluding rostrum and outer orbital processes) advanced slightly beyond anterolateral margins; 0.60× 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; 0.09× pcl; triangular, apex acute, margins slightly concave, coarsely granulate laterally and dorsally. Eyestalks distinctly divergent, ~42° to median axis, flattened, stout, width at midlength exceeding one-third length, fused to carapace below rostral base but demarcated from frontal margin; not reaching anteriorly 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 (male); 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 distal to merus mid-length. Exopod surface sparsely granulate; apex reaching to or slightly beyond carpo-meral articulation but not reaching beyond end of endopod merus.

Chelipeds (pereopods 1) equal in size and ornamentation, sparsely setose. Merus finely granulate, with scattered small spines. Carpus finely granulate, dorsal margin with few short spines. Propodus palm surfaces with fine granules and few scattered acute granules, dorsal and ventral margins irregularly spinulate. Dactylus longer than dorsal palm length; proximal dorsal three-fourths 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; carpus and merus with serrated granules and scattered spinules on extensor margins. Pereopod 3 longest; merus 1.02× pcl (male). Dactylus broadly curved, few granules or small spines proximally, with weak longitudinal rib, indicated on distal half; 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 merus; dactylus markedly shorter than propodus, length 1.5× height, falcate, apex corneous apex, flexor margin with 4 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 obtuse angle; surface granulate. Margins of sternites 4 and 5 granulate. Abdomen with margins and surface finely granulate or minutely spinulate. Somite 6 and telson fully demarcated, articulating; telson broadly rounded, width 2.4× length. Gonopod 1 with 3 articles; distal article cannulate, forming copulatory tube, with long distal setae. Gonopod 2 with articles fused, distomesial margin slightly hollowed, apex acute.

Etymology. The name refers to the known geographical distribution of the species; used as a noun in apposition.

Remarks. Cymonomus java , new species, is the sixth member of the C. delli group and morphologically closest to C. cognatus ( Japan, Taiwan, South China Sea) and C. andamanicus (Andaman Sea). Females of C. java and C. andamanicus are presently unknown but males of the three species agree in almost all respects, sharing similar antennular and walking leg proportions, carapace shape, frontal ornamentation, and telson shape, but differ in the stouter pereopod 4 and 5 dactyli (length about 1.5× height in C. java and C. andamanicus versus about twice height in C. cognatus ). Cymonomus java , however, is readily distinguished from C. andamanicus and C. cognatus by the articulation of the telson and abdominal somite 6. In C. cognatus and C. andamanicus , the male telson and abdominal somite 6 are immovably fused (although with an indistinct demarcation), but fully demarcated and articulated in C. java . In addition, the COI sequence of C. java is divergent from that of C. cognatus from the South China Sea by 4.3–4.4% (uncorrected p-distance) (Ahyong, in prep.). Significantly, the free telson and abdominal somite 6 of C. java is unique in Cymonomus . All other species of Cymonomus have a fused abdominal somite 6 and telson, even if the demarcation is indicated by a transverse groove as in other species of the C. delli group, or only partially indicated by lateral incisions as in C. karenae Ahyong, 2019 . Although C. java and C. andamanicus are known only from their respective holotypes, thus preventing evaluation of variation in the telson-abdominal somite 6 demarcation, the pleotelson condition in C. delli , for which a good of both sexes is available, is constant ( Ahyong, 2019).

In addition to C. java , two other members of the C. delli group are recorded from the Indian Ocean: C. andamanicus from the Andaman Sea, and C. delli from southern and southwestern Australia. Apart from the articulated (versus fused) abdominal somite 6 and telson, differences between males of C. java , C. delli , and C. andamanicus are minor. The new species is further distinguished from C. delli in the proportionally shorter and stouter pereopod 4 and 5 dactyli (length about 1.5× height in C. java versus about twice height in C. delli ). Cymonomus java is distinguished from C. andamanicus by the subparallel versus posteriorly divergent lateral margins of the carapace, narrower frontoorbital width (0.56× anterior carapace width versus 0.66), and position of the articulation of the maxilliped 3 palp, being distal to, instead of at the midlength of the merus (in C. andamanicus ).

Despite the telson being articulated, the demarcation in male C. java superficially resembles the well-defined groove present in the pleotelson in C. cubensis , C. delli , and C. diogenes , and could be misinterpreted without close examination or direct manipulation of the structure. The new species further differs from C. cubensis in having distinctly more robust, rather than slender, pereopods 4 and 5 (see Fig. 1A, B ; Chace, 1940: fig. 6A) and proportionally shorter male telson (width about 2.4× length versus 1.7 in C. cubensis ). From C. diogenes , C. java is also separated by the proportionally shorter walking legs in males (pereopod 2 merus 1.1 versus 1.4× pcl), the subparallel rather than divergent lateral margins of the carapace, and position of the articulation of the maxilliped 3 palp distal to, instead of at the midlength of the merus.

Distribution. Presently known only from the type locality.

Figures

Fig. 3. Colour in life. A, Cymonomus chani Ahyong & Ng, 2017, ovigerous female (cl 9.9 mm, pcl 9.1 mm, cw 10.0 mm), Java, stn CP44, ZRC 2019.1093. B, Cymonomus java, new species, male holotype (cl 8.2 mm, pcl 7.5 mm, cw 7.6 mm), Java, CP35, MZB Cru 5024. Photographs: T.-Y. Chan.

Fig. 4. Cymonomus java, new species, male holotype (cl 8.2 mm, pcl 7.5 mm, cw 7.6 mm), Java, CP35, MZB Cru 5024. A, dorsal habitus; B, left pereopod 3, dorsal view; C, fronto-orbital region; D, right outer-orbital process, lateral view; E, anterior epistome, ventral view; F, right maxilliped 3; G, thoracic sternite 3; H, abdominal somite 6 and telson; I, right G1, abdominal view; J, right G2, abdominal view; K, right epistomial spine mesial to base of antenna; L, pereopod 4 dactylus; M, pereopod 5 dactylus. Scale: A, B = 2.0 mm; D–J = 1.0 mm; K–M = 0.5 mm.

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).