Cavoportunus dubius (Laurie, 1906)

(Figs. 2D, 6A–G, 10I–L)

Neptunus (Achelous) dubia Laurie 1906: 416, fig. 9 (type locality: Gulf of Mannar, Sri Lanka).

Portunus (Achelous) dubius — Stephenson & Rees 1967: 4 (list) 20, fig 3a–b, pl. 2A.— Stephenson 1972a: 15, 38.— Stephenson 1972b: 135.— Stephenson 1976: 16.— Moosa 1981: 143 (list), 147.— Crosnier 1984: 401.— Crosnier 1985: 33, figs. 1a– e.— Zarenkov 1994: 115.— Moosa 1996: 521.— Poupin 1996: 31.— Neumann & Spiridonov 1999: 19.— Crosnier 2002: 404.— Ng et al. 2008: 151 (list).

Portunus (Cycloachelous) yoronensis Sakai, 1974: 90 .— Sakai 1976: 349, figs. 189a–d.— Ng et al. 2008: 151 (list).

Cavoportunus dubius — Nguyen & Ng 2010: 41, figs. 1–4.— Komatsu 2011: 257, fig. 19C.—Mantellato et al. 2018: 192, 194, fig. 1.

Material examined. 1 male (26.9 × 20.4 mm) (IO/SS/BRC/00191), FORVSS stn. 38811, west of Tillangchong Island, Nicobars, India, 8.44°N, 93.59°E, 69 m depth, chain bag dredge, coll. V.P. Padate, 13 August 2019 .

Description. Carapace hexagonal, 1.3 times as wide as long, dorsal surface glabrous; dorsal surface microscopically granular with distinct regions, bearing scattered granules; only metagastric ridge distinct, other ridges indistinct (Figs. 2D, 6A). Frontal margin with 4 teeth, projecting beyond tip of internal supraorbital angle; orbits relatively large, moderately dorsally inclined, supraorbital margin with single fissure, inner supraorbital margin rounded; infraorbital margin straight, granular, with forward-directed tooth; anterolateral margin with 9 teeth, first tooth largest, second to ninth teeth smaller; posterolateral junction of carapace rounded (Fig. 6A).

Antennule folding obliquely; basal antennal article with lobe-like process, flagellum lodged in orbital hiatus (Fig. 6B); epistome triangular, anterior and posterior margins elevated. Buccal cavity subquadrate; mxp3 merus subquadrate, shorter than ischium, anterolateral margin produced into rounded lobe (Fig. 6C).

Cheliped 1.8 times as long as CL, merus with 4 spines on anterior margin, 1 distal spine on posterior margin (Fig. 2D); carpus with long spine on inner angle; palm with 2 granular ridges on dorsal surface terminating in subdistal spines, short spine at carpal articulation, outer surface with 3 longitudinal granular costae on lower half; fingers slender, gently curved, as long as palm (Fig. 6D, E). Pereopods 2–4 dactyli longer than propodi (Fig. 2D); P5 merus posterior margin dentate, other segments unarmed (Fig. 6F).

Male pleon triangular, somite 3 posterior margin with median cleft, somite 4 lateral margins convex, those of somite 5 slightly convex; telson small, triangular, 0.8 times as long as wide (Fig. 6G). G1 stout, S-shaped; basal part relatively slender, subdistal portion distinctly dilated into a bluntly triangular lobe; distal tip with distinct folds (Fig. 10I–K); G2 slender, tapering, 0.6 times as long as G1 (Fig. 10L).

Colouration. Fresh specimen: Carapace white with densely-packed reddish reticulate markings on the dorsal surface, particularly the granules, granulated ridges and anterolateral spine tips (Fig. 2D). Posterolateral carapace junction, edges of first two pleonites, outer carpal spine, outer carina and dorsal spine of the cheliped palm and dorsal surface of cheliped dactylus with silvery iridescence (Fig. 2D).

Biology. The present specimens were collected along with gorgonians and sponges from coral reef habitat at 69 m depth. Known to inhabit coral reefs (Nguyen & Ng 2010), on fine coral sand (Stephenson and Rees 1967) at depths from 20–500 m (Nguyen & Ng 2010).

Remarks. Cavoportunus dubius was originally described by Laurie (1906) from two males collected from the Gulf of Mannar and off Negombo, Sri Lanka. Stephenson & Rees (1967) reported Portunus dubius from the Philippines. Subsequently, Sakai (1974) described Portunus yoronensis from a male collected from Yoron Island, Japan. Crosnier (1984) and Nguyen & Ng (2010) synonymized Portunus (Cycloachelous) yoronensis Sakai, 1974 with Neptunus (Achelous) dubia Laurie, 1906 . Further, Nguyen & Ng (2010) transferred this species to a new genus, Cavoportunus, owing to the presence of two distinct depressions lateral to the sternal medial groove on the male sternum to accommodate a distinct G1 (with a swollen and bilobed subdistal part). The present specimen conforms to the morphology of the male pleon, third maxillipeds, left cheliped and G1 depicted by Laurie (1906: fig. 9). However, the carapace of the Indian specimen possesses densely packed fine reddish orange reticulation on the carapace (Fig. 2D), as compared to reddish spots scattered on a white background in the Japanese specimen reported by Nguyen & Ng (2010: fig. 1).

Geographical distribution. Sri Lanka, Philippines, Japan, and French Polynesia (Nguyen & Ng 2010). The present observation is the first record for Indian waters.