Cymonomus dwi, Ahyong & Ng, 2023

Ahyong, Shane T. & Ng, Peter K. L., 2023, Cymonomus dwi, a new species of deep-water cymonomid crab from Indonesia (Crustacea: Brachyura), Raffles Bulletin of Zoology 71, pp. 116-119 : 116-118

Cymonomus dwi , new species

( Fig. 1 )

Type material. Holotype: MZB, male (cl 2.8 mm, pcl 2.4 mm, cw 2.7 mm), station CP07, Sunda Strait , between Tabuan Island and Sumatra, Java, Indonesia, Indian Ocean, 5° 44.678 –44.917 ′S, 104° 51.151 –52.061 ′E, 379–409 m, coarse sand, gravel, rubble and wood, SJADES cruise, coll. Baruna Jaya VIII , 25 March 2018.

Description of holotype. Carapace rounded-subquadrate, slightly wider than long, lateral margins subparallel on anterior half, posterior half with swollen branchial margins; regions weakly indicated, cervical groove weakly indicated; lower pterygostomian region swollen; anterior and lateral surfaces with few long, fine setae, other surfaces at most sparsely setose. Anterolateral spine small, directed anteriorly; similar spine on lateral margin behind anterolateral spine. Anterior carapace margin mesial to anterolateral spines with short, acute granules, margin approximately transverse, slightly sloping inwards at base of outer orbital processes. Dorsal and lateral surfaces covered with minute granules. Fronto-orbital margin (excluding rostrum and lateral projections) slightly advanced beyond anterolateral margins; 0.67 anterior carapace width; outer orbital processes sharply triangular, elongate, subparallel, directed anteriorly, situated below plane of rostrum, dorsally granular, laterally spinulate, apex acute, as long rostrum. Rostrum length exceeding half-length of eyestalks; 0.15 pcl; slender, tapering, sparsely granular; slightly inclined ventrally.

Eyestalks subparallel, slender, tapering, ventrally flattened, slightly movable, not fused to carapace, reaching anterior half of antennular peduncle article 1; dorsal surface and margins granulate, sparsely spinose; cornea apparently vestigial, not pigmented. Epistome smooth; with spine mesial to base of antenna.

Antennular peduncle 1.06 pcl; articles 1 and 2 minutely granulate; article 3 smooth. Antennal articles granulate or minutely spinular.

Maxilliped 3 ischiobasis subquadrate, surface and margins sparsely granulate; shallow longitudinal sublateral groove; ischium and basis demarcated by scarcely visible, faint shallow groove. Merus as long as ischiobasis; length 3.06× width (excluding spines); tapering distally to rounded apex; surface and margins granulate, spinulate. Palp articulating slightly proximal to merus midlength. Propodus and carpus spinulate. Dactylus unarmed. Exopod sparsely granulate, distally reaching apex of endopod merus.

Chelipeds (pereopod 1) equal in size and ornamentation, with long fine setae. Merus finely granulate, minutely spinose distomesially. Carpus granulate, with short mesial spine. Propodus palm surfaces sparsely granulate, minutely spinulate, dorsal and ventral margins spinose, extending onto pollex. Dactylus longer than dorsal palm length; dorsal margin spinose; outer surface with faint longitudinal carina, occlusal surfaces of dactylus and pollex smooth or irregularly crenulate, with slight gape when fingers closed.

Pereopods 2 and 3 sparsely setose, sparsely granulate; propodus and carpus extensor margins acutely granulate and with widely spaced spines, flexor margins unarmed or minutely spinose; merus extensor and flexor margins acutely granulate and with short spines; dactylus broadly curved, unarmed on P2, with small proximal spine on P3, without distinct longitudinal rib. Pereopod 3 longest, merus 1.10 pcl; dactylus slightly shorter than combined length of propodus and carpus.

Pereopods 4 and 5 finely granulate, sparsely spinose; longer than pereopod 3 merus; propodus distoextensor margin unarmed; dactylus markedly shorter than propodus, falcate, with corneous apex and 2 obliquely inclined, corneous spines on flexor margin. Pereopod 5 merus, when folded against carapace, reaching anterior one-third of carapace.

Thoracic sternite 3 pentagonal, width 0.76× length; lateral proximal margins subparallel; surface irregular, minutely granulate. Margins of sternites 4 and 5 smooth.

Pleon granulate, spinose, most prominent on somites 2 and 3, very sparsely ornamented on somites 4 and 5. Pleotelson without trace of demarcation between somite 6 and telson; triangular; margins slightly straight; apex blunt; width 1.36× length.

Gonopod 1 distal article cannulate, forming copulatory tube, with long distal setae. Gonopod 2 with articles fused; distomesial margin slightly hollowed, apex acute.

Etymology. Named for our friend and colleague, Dwi Listyo (“Yoyo”) Rahayu, one of the Chief Scientists on the expedition (SJADES 2018), for her numerous contributions to carcinology. The name is used as a noun in apposition.

Remarks. Cymonomus dwi , new species, belongs to the Cymonomus bathamae group, which is characterised by having the outer orbital processes inclined anterolaterally, straight rather than curved eyestalks, and the anterolateral margins curving inwards at the base of the outer orbital processes ( Ahyong, 2019). Of the species in the C. bathamae group, C. dwi is morphologically closest to those species having slightly movable eyestalks and a rostrum that is as long as the outer orbital processes, namely C. triplex Ahyong, 2019 (southern Australia, 367–539 m; Ahyong, 2019), C. umitakae Takeda, 1981 ( Japan, 219–500 m; Ahyong & Ng, 2017), and C. valdiviae Lankester, 1903 (East Africa, 518–638 m; Ahyong, 2014); all species occurring in outer shelf to upper slope depths. Cymonomus dwi , which is also from upper slope habitats, differs from each of these species in the proportionally more slender maxilliped 3 merus (length: width 3.1 versus 2.3–2.6), the longer maxilliped endopod (meral apex reaching level of apex of exopod versus falling short of the apex) and the shape of the lateral margins of the carapace, being straight in the anterior half and convexly swollen along the branchial margins, versus straight or evenly convex in the other species. The new species can be further distinguished from C. triplex by the finely granular surface of the carapace (spinular on the anterior half of the carapace in C. triplex ), and from C. umitakae and C. valdiviae by the much less prominent ocular and cheliped spines, as well as the arrangement of spines on pereopods 2 and 3 (compare Fig. 1A with Ahyong & Ng, 2017: fig. 6A, Ahyong, 2014: fig. 3A).

Six species of Cymonomus are now known from Indonesian waters: C. chani Ahyong & Ng, 2017 ; C. dianae Ahyong, 2018 ; C. hakuhoae Takeda & Moosa, 1990 ; C. indicus Ihle, 1916 ; C. java Ahyong, Mitra & Ng, 2020 ; and C. dwi . They can be distinguished in the key below.

Distribution. Currently known only from the Sunda Strait, Indonesia; 379– 409 m.

Figures

Fig. 1. Cymonomus dwi, new species, male holotype, cl 2.78 mm, pcl 2.41 mm, cw 2.69 mm, MZB: A, dorsal habitus; B, fronto-orbital region, dorsal view; C, right epistomial spine mesial to base of antenna, ventral view; D, right maxilliped 3, external view; E, thoracic sternite 3, ventral view; F, right G1, pleonal view; G, right G2, pleonal view; H, left P4 dactylus, dorsal view; I, left P5 dactylus, dorsal view. Scale: A = 1.0 mm; B–G = 0.5 mm; H, I = 0.25 mm.