Thusaenys irami (Laurie, 1906)

(Fig. 2E–I)

Halimus irami Laurie, 1906: 379, pl. 1 figs. 4, 4a (type locality: Gulf of Mannar, Muttuvartu Par, Sri Lanka).— Rathbun 1924: 5.

Thusaenys irami . — Griffin & Tranter 1986: 193–196, fig. 66c–d.— Ng et al. 2008: 106 [list].

Material examined. Male (3.0 × 4.3 mm) (NIO /BOD/AB/BRY/00002), RVSS 2, west of Vijaydurg, Arabian Sea, 16.44°N, 72.07°E, 37 m depth, Van Veen grab, coll. R. Periasamy, 6 January 2014 .

Diagnosis. Carapace pyriform, longer than wide, smooth, covered with short, stiff setae, intestinal region with low tubercle (Fig. 2E, F). Rostrum length nearly 0.5 × post-rostral CL; rostral spines slender, horizontal, divergent from the base, lacking accessory spine; dorsal carapace smooth; Orbits reduced, supraorbital eave produced into indistinct preorbital spine, cupped postorbital process with an extension on posterior margin of upper orbital hiatus (Fig. 2F, G). Supraorbital eave margin weakly concave. Basal antennal article broad, anterolateral angle produced into strong spine separated from flagellar base by notch (indicated by arrowhead in Fig. 2G); lateral margin convex (Fig. 2G). Maxilliped 3 merus quadrilateral, as long as wide, length 0.5 × ischial length (Fig. 2G). Pterygostomian region with 2 tubercles (Fig. 2G). Chelipeds slender (Fig. 2H). Male pleon terminally tapering, telson sub-triangular, not inserted deeply into somite 6 (Fig. 2H). G1 expanded at distal tip, groove present near lateral margin near pleonal surface (Fig. 2I).

Colouration. Fresh specimen: reddish, chelae greyish (Fig. 2E). Preserved specimen: light brown (Fig. 2F).

Biology. Known to occur underneath sub-tidal rocks, inside pearl oysters, over muddy substrates, vicinity of corals and Lithothamnion; 36–37 m (Griffin & Tranter 1986; present study). Laurie (1906) reported Sacculina infestation from Sri Lanka. The present specimen was collected from rocky substrate, at 37 m depth, in the vicinity of Favites corals.

Remarks. Laurie (1906) described Halimus irami from an ovigerous female collected from Muttuvartu Par, in the Gulf of Mannar (Sri Lanka). Griffin & Tranter (1986) included Halimus irami in a new genus, Thusaenys, owing to the strongly produced basal antennal article with anterior notch and the broad, weakly lobed supraorbital eave. Thusaenys irami differs from its closest congener T. calvarius (Alcock, 1895) in having basal antennal article with weakly convex lateral margin (vs. strongly produced lateral margin in the latter species (cf. Takeda 2023: fig. 1B)), and low tubercle on the intestinal region (compared to short, strong median spine in the latter (cf. Takeda 2023: fig. 1A)). The Indian specimen conforms to the holotype description (Laurie 1906) in the nearly smooth dorsal surface of the carapace, the presence of a low tubercle on the intestinal region, a relatively long rostrum nearly half as long as the remaining carapace, supraorbital eave anteriorly projecting into triangular tooth, and a gently convex lateral margin of the basal antennal article which is produced into strong spine separated from flagellar base by a notch. On the other hand, the present specimen differs from the holotype in the absence of an epibranchial tubercle.

Geographical distribution. Australia, New Guinea, Indonesia, and Gulf of Mannar off Sri Lanka (Griffin & Tranter 1986); eastern Arabian Sea off India (present study). The present observation is the first record from Indian waters indicating westward extension of the known geographical range.