Megaesthesius yokoyai Sakai, 1939

Ng, Peter K. L. & Castro, Peter, 2016, Revision of the family Chasmocarcinidae Serène, 1964 (Crustacea, Brachyura, Goneplacoidea), Zootaxa 4209 (1), pp. 1-182 : 92

Megaesthesius yokoyai Sakai, 1939

( Figs. 19 C–F; 34B; 48E; 82G–I)

“? Megaesthesius sp.” Yokoya, 1933: 203, fig. 86.

Megaesthesius yokoyai Sakai, 1939: 577 [type locality: Japan] ; 1976: 552, fig. 299 [Japan].— Serène 1968: 92 [in list].— Takeda & Miyake 1969: 460, figs. 3, 4 [Japan]; 1972: 86 [East China Sea].—Ng et al. 2008: 76 [in list]. Megaesthesius sagedai [sic]— Serène 1964b: 176. Not Megaesthesius sagedae Rathbun, 1909 .

Type material. Depository unknown, Japan, Bungo Strait and Sata-Misaki.

Diagnosis. Small size. Carapace ( Fig. 19 C; Takeda & Miyake 1969: fig 3; Sakai 1976: fig. 299a) subquadrate; front bilobed, with deep median cleft; dorsal surface with low, spherical tubercles; post-orbital margins sloping to angular anteroexternal angle. Lateral margins straight, parallel to each other, each with 3 or 4 dented regions separated by deep notches, acute tooth on posterior third of margin. Posterior margin nearly straight. Long orbits, eye peduncle elongated ( Fig. 19 F; Takeda & Miyake 1969: 4a, b), immobile, cornea reduced, non-pigmented. Antennule greatly enlarged ( Fig. 19 C; Takeda & Miyake 1969: 3a; Sakai 1976: fig. 299b), particularly in males. Third maxillipeds ( Fig. 34 B; Takeda & Miyake 1969: 4c; Sakai 1976: fig. 299b) nearly fill buccal cavern when closed; merus subovate, ischium subovate, longer than merus; outer margins of merus, ischium unarmed. Chelipeds subequal in length, nearly similar in both sexes; fingers relatively short, not stout; cutting margin of major chela ( Fig. 48 E; Takeda & Miyake 1969: 4d) smooth; ventral margin of palm unarmed; ventral surface of cheliped merus with spiniform granules. Proportionally long ambulatory legs ( Fig. 19 E; Takeda & Miyake 1969: 4a; Sakai 1976: fig. 299a), P5 merus not reaching front when folded. Distal end of meri of ambulatory legs with minute spinules. Small, acute tooth on inner margin of cheliped carpus of female, ventral surface of cheliped merus smooth in both sexes. Male pleon ( Fig. 19 D; Takeda & Miyake 1969: 4f) with lateral margins of somite 6, fused somites 3–5 nearly straight; postero-lateral regions slightly swollen; telson proportionally long. G1 ( Fig. 82 G‒H; Takeda & Miyake 1969: 4h, i) slender, distal segment, straight, with spinules. G2, female pleon, thorax, vulvae undescribed.

Remarks. Although synonymised by Serène (1964b: 176), there are differences between M. sagedae and M. yokoyai (based on the illustrations by Yokoya 1933 and Takeda & Miyake 1969). The G1 of M. yokoyai is slightly less slender, with more spinules on the distal portion, and a more flared distal segment ( Fig. 82 G–I; Takeda & Miyake 1969: fig. 4 g –i) than in M. sagedae ( Fig. 82 A, B). The larger chela is thick, high, and the ventral margin and cutting margins are unarmed, with the pollex relatively slender and shorter than the palm in a male (3.5 × 3.8 mm, Fig. 48 E; Takeda & Miyake 1969: fig. 4d) but with the pollex stouter and relatively longer and the cutting margin lined with small teeth in the male specimen on hand of M. sagedae (2.5 × 3.0 mm, ZRC 1995.885; Fig. 48 C).

Distribution. Western Pacific Ocean ( Japan and East China Sea). Depth: 105–250 ( Sakai 1976).