Thusaenys calvarius (Alcock, 1895)

Shaga, I Bevin, Priya, K, Ramar, Selvakumar, Srinidhi, S, Mohan, G Chandra, Ranganathan, Sukanya, Moulvi, S M M & Mani, Bhuvaneswari, 2023, A Small Collection of Subtidal Crabs (Crustacea: Decapoda: Brachyura) from the Palau Islands Collected by Dredging, Bulletin of the National Museum of Nature and Science. Series A, Zoology 49 (1), pp. 7-42 : 11

Thusaenys calvarius (Alcock, 1895)

( Figs. 1A–C , 2 )

Hyastenus calvarius Alcock, 1895 , p. 213.─Alcock and Anderson, 1895, pl. 21 fig. 2.─ Balss, 1938, p. 23.

Thusaenys calvarius (Alcock) : Griffin and Tranter, 1986, p. 193, figs. 65d, 66a–b.

Material examined. Off Kwannon, Koror I., Palau Is., dredged; 1 ♂ (cb 6.7×pcl 10.5 mm excluding posterior tubercle; rl 5.9 mm), NSMTCr 30959; June 17, 1980; K. Baba leg. ─ Entrance to Toagel Mid in Arangel Channel, Babelthuap I., Palau Is., dredged, 40–50 m in depth; 1 ♂ (5.4×

8.8 mm; 4.5 mm), NSMT-Cr 30960; June 19, 1980; K. Baba leg.─ Same locality, ca. 40 m in depth, coralline algae; 1 ♂ (5.4× 8.2 mm; 4.7 mm), NSMT-Cr 30961; June 22, 1980; K. Baba leg.

Remarks. The shape and armature of the carapace and rostrum agree in the two males examined without remarkable differences, being most characterized by the oblong, narrow and smooth carapace only with a vestigial epibranchial tubercle and a dorsally curved intestinal tubercle. This species was well explained as the type species of the new genus Thusaenys by Griffin and Tranter (1986), with the figures of the orbital region in ventral view and the G1.

The brief diagnostic notes on the present specimens are given in the following lines. The carapace surface is dorsally convex ( Fig. 1A, C ) and shallowly separated into the gastric, cardiac and branchial regions. The rostral spines ( Fig. 1A–C ) are only slightly longer than half the length of pcl, straight, horizontally directed forwards, and moderately divergent from the base. The supraorbital eave ( Fig. 2A ) is thick, weakly curved dorsally and shallowly concave along the margin, with the anterior and posterior ends obtusely angulated. There are a deep U-shaped sinus and a prominent convexity behind the supraorbital eave ( Figs. 1A , 2A ); the postocular tooth is large and deeply cupped in dorsal, lateral and ventral views ( Fig. 2A–C ). The antennal basal segment ( Fig. 2B ) is thick and armed with an anterolateral tubercle, with the strongly convex outer margin. The third maxilliped ( Fig. 2D ) has several strong teeth along the ischium inner margin and the strongly developed antero-external angle of the merus. The pterygostomial region is armed with two large teeth ( Fig. 1C ).

The G1 ( Fig. 2E ) is slender, with a row of long hairs, especially at the subterminal part. The terminal part ( Fig. 2F ) is elongated, without hairs, agreeing well with the figures given by Griffin and Tranter (1986: fig. 66a–b).

Distribution. Known from some localities in Indonesian waters, Fiji, and the Andaman Islands; 36–120 m in depth. New to the Palau Islands.

Figures

Fig. 1. A–C: Thusaenys calvarius (Alcock), ♂ (NSMT-Cr 30959; cb 6.7×pcl 10.5mm excluding posterior tubercle). D: Naxioides hirta A. Milne-Edwards, ♂ (NSMT-Cr 30958; cb 17.0mm excluding branchial spines× pcl 25.2mm including posterior tubercle). E: Hyastenus sebae White, ♀ (NSMT-Cr 30957; cb 10.5×pcl 13.8mm).

Fig. 2. Thusaenys calvarius (Alcock), ♂ (NSMT-Cr 30959; cb 6.7×pcl 10.5mm excluding posterior tubercle), Right orbital region in dorsal view (A), left orbital region in ventral view, showing the basal antennal segment (B), right orbital region in lateral view (C), left third maxilliped (D), and left G1 in ventral (E) and sternal (F) views. Scales for AD=1mm, BC=2mm, E=1mm, F=0.5mm.