Cyrtocarcinus truncatus (Rathbun, 1906)

(Figs. 2E, 7A–F, 10M–O)

Harrovia truncata Rathbun, 1906: 832 (list), 886, fig. 40, pl. 14 fig. 8 (type locality: Kauai, Hawaii, Albatross stn. 3982, 233–

240 fathoms (= 427–439 m) depth).— Edmondson 1951: 217, figs. 21, 22. Harrovia truncata — Flipse 1930: 80, 90.— Serène et al. 1958: 196, 199 (key), fig. 7A.— Serène 1968: 63 (list). Glyptocarcinus truncatus — Takeda 1979: 68.— Števčic et al. 1988: 1311 (list). Cyrtocarcinus truncatus — Ng & Chia 1994: 725, figs. 11–13.— Eldredge & Miller 1997: 13 (list).— Ng et al. 2008: 196 (list).—

Castro 2011: 88 (list).— Lai et al. 2011: 440, Table 1.— Naruse et al. 2020: 1, figs. 1–5.

Material examined. 1 male (8.6 × 6.1 mm) (IO/SS/BRC/00193), 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 dorsal surface uneven, granular, regions 2M, 3M, 5L moderately elevated, 1P faintly bilobed, overhanging onto intestinal region, intestinal region low, flat (Figs. 2E, 7A). Frontal margin gently sinuous, slightly deflexed ventrally, bilobed by weak median fissure; supraorbital margin with 2 sutures, external orbital angle small; anterolateral margin with 3 triangular teeth, separated by sutures, second tooth largest; posterolateral margin nearly straight, with a V-shaped notch at level of cardiac protrusion; posterior margin concave (Fig. 7A).

Antennules folding obliquely; basal antennal article rectangular, flagellum lodged in orbital hiatus (Fig. 7B); epistome well-developed, posterior margin carinate, with median projection and sublateral concavities (Fig. 7B); buccal cavity subquadrate; mxp3 bearing papilliform setae, merus subquadrate, 0.6 times as long as ischium, with an oblique elevation separating two depressions (Fig. 7C).

Chelipeds subequal, 2.5 times as long as CL; merus with strong tooth distally on lower surface, anterior margin carinate with subdistal rounded lobe, posterior margin with large subdistal tooth; palm with 4 longitudinal ridges on outer surface; dactylus slightly shorter than palm (Fig. 7D). Pereopods 2–5 slightly shorter than cheliped, meri with granular carinate margins; carpi with 2 triangular lobes on outer margin, subdistal lobe larger than subproximal lobe, propodi with triangular lobe submedially on outer margin, dactyli as long as propodi, terminating in corneous spine, inner margins of propodi and dactyli densely setose (Fig. 7E).

Male pleon subtriangular, with 6 free somites and telson, somite 3 widest, telson subtriangular, half as long as wide (Fig. 7F). G1 slender, gently curved, distal portion tapering, margins armed with short, strong spines (Fig. 10M–N); G2 0.4 times G 1 in length, distal segment triangular (Fig. 10O).

Colouration. Fresh specimen: Carapace dorsal surface with pinkish margins enclosing bright red colouration, cardiac regions with small median and a pair of submedian elongate whitish patches (Fig. 2E). Chelipeds dark red, distal half of fingers brown (Fig. 2E). Pereopods 2–3 with bright red colouration from meri to proximal half of propodi, P4 bright red on proximal half of merus, entire carpus and proximal half of propodus, and P5 bright red only on proximal half of merus; other parts whitish (Fig. 2E).

Biology. The present specimens were collected along with gorgonians and sponges from 69 m depth. Known to occur at depths from 3–439 m (Rathbun 1906; Castro 2011) on rocky substrates (Ng & Chia 1994).

Remarks. Harrovia truncata was described by Rathbun (1906) from an immature male collected from deepwaters off Kauai, Hawaiian Islands. The Indian specimen resembles the description of the holotype (Rathbun 1906) in the morphology of the carapace, chelipeds and pereopods. Ng & Chia (1994) transferred this species to a new genus, Cyrtocarcinus, which differed from Antrocarcinus Ng & Chia, 1994 and Glyptocarcinus Takeda, 1973 in the presence of a distinct ridge and a depression on the fourth thoracic sternite formed by an abrupt downward sloping of its posterior part. The Indian specimen possesses a larger third tooth on the lateral carapace margin (Fig. 7A), as compared to the holotype male (6.5 × 5.5 mm; Ng & Chia 1994: figs. 11A, 12A), which according to Naruse et al. (2020), could be attributed to the difference in their carapace size. The Indian specimen closely resembles the morphology and colouration of the Japanese female specimen reported by Naruse et al. (2020: fig. 1C). On the other hand, the Indian specimen lacks a subproximal tooth on the outer surface of cheliped merus (Fig. 7D) (versus cheliped merus with a subproximal tooth on outer surface in the New Caledonian specimen reported by Naruse et al. (2020: fig. 2A)).

Geographical distribution. Hawaiian Islands (Rathbun 1906; Edmondson 1951; Takeda 1979; Castro 2011), Loyalty Islands, New Caledonia (Lai et al. 2011) and Ishigaki Island, Japan (Naruse et al. 2020). The present observation is the first record of the genus and species from Indian Ocean indicating westward extension of the known geographical range.