Lophoplax Tesch, 1918

Lophoplax Tesch, 1918 View in CoL

Lophoplax Tesch, 1918: 196 View in CoL (part); Ng, 1987: 100 (part); Trivedi et al., 2022: 595 (part); Poore & Ahyong 2023: 667, 670, 680 (part), fig. 14.112u; Ng & Rahayu, 2023: 429 (part).

Type species. Lophoplax bicristata Tesch, 1918 View in CoL , by original designation. Gender of genus feminine.

Included species. Lophoplax bicristata Tesch, 1918 View in CoL ; L. takakurai Sakai, 1935a View in CoL .

Diagnosis. Carapace subquadrate; only protogastric and intestinal areoles visible; epigastric region gently convex, smooth but setose, not areolate; cardiac region raised but not areolate, granulate and setose; hepatic areole absent, with only tubercle present; intestinal region forming small areole, adjacent areas slightly raised, granulate and setose, no visible intestino-branchial areole; surface adjacent to posterior carapace margin gently concave, not raised, not forming ridge, no transverse channel visible; orbit clearly visible in dorsal view, supraorbital margin breaking general carapace outline, forming distinct concavity; eyes freely movable, ocular peduncle relatively long; lateral lobe of frontal margin low, indistinct; anterolateral margin with 4 teeth (including external orbital tooth), with 2 tubercles on posterolateral margin; posterolateral margins gently converging towards posterior carapace margin. Endostome not obliquely recessed posteriorly into buccal cavern, subparallel to adjacent pterygostomial surface, low, short endostomial ridges present. Posterior margin of epistome with distinct median lobe, separated from lateral lobes by distinct fissure. Third maxilliped ischium subquadrate, length about 1.2× width. Adult male chelipeds slightly asymmetrical; dorsal surface of carpus with quadrate areole with raised margins; chela relatively enlarged with external median surface smooth. Ambulatory legs elongate, slender, P4 longest, about 1.8× carapace width; merus without obvious crest on extensor margin, instead P2–4 lined with low, broad well-spaced setose prominences or low tooth; P5 extensor margin smooth, outer surface smooth, without medio-longitudinal swelling; P4 merus length about 5× height, distinctly longer than half carapace width. G1 sinuous, relatively stout, tip strongly recurved.

Remarks. Lophoplax sensu stricto is distinct in its carapace ornamentation and the length and slenderness of the ambulatory legs. Lophoplax is separable from Myopilumnus and Hosekia , new genus, in the following characters: the hepatic surface of the carapace is ornamented only with a tubercle ( Fig. 2A View Fig ) (versus with a large, prominent areole; cf. Figs. 6A View Fig , 7C, D View Fig ); the intestino-branchial areole is absent with only a low intestinal areole visible ( Fig. 2A View Fig ) (versus intestino-branchial areole large and distinct; cf. Figs. 6A View Fig , 7C, D View Fig ); the surface adjacent to the posterior carapace margin is relatively wide, not forming a channel as no intestino-branchial areole is present ( Fig. 2A View Fig ) (versus with distinct channel between intestino-branchial areole and posterior carapace margin; cf. Figs. 6A View Fig , 7C, D View Fig ); and the absence of a dorsal crest on the ambulatory merus ( Figs. 2A, B View Fig , 3C, D View Fig ) (versus with prominent crest; cf. Ng & Rahayu, 2023: fig. 7G–L). The adult male chela is relatively more enlarged, with the external median surface almost smooth ( Fig. 2C, D View Fig ) (versus chelae not enlarged and the outer surface of the chela densely covered with granules; cf. Fig. 6F View Fig ). The ambulatory legs are also comparatively slender (P4 merus length about 5× height) and more elongated (P4 length about 1.8× carapace width), with the merus surface lacking a medio-longitudinal swelling or ridge ( Figs. 2A View Fig , 3C, D View Fig ) (versus median part swollen, P4 merus length 3.0–3.5× height; cf. Ng & Rahayu, 2023: fig. 7G–L). Lophoplax , like Myopilumnus , further differs from Hosekia , in having four instead of three anterolateral carapace teeth ( Figs. 6A View Fig , 7C, D View Fig versus Figs. 1A View Fig , 2A View Fig ).

Lophoplax superficially resembles Striacoeloma , new genus, in the slender ambulatory legs (albeit with P4 merus length about 5× height, versus 4× height) and in having blunt prominences along most of the lateral margins of the carapace, but is immediately distinguished by the position of the orbits: on the anterior margin and fully visible dorsally in Lophoplax , versus submarginal and scarcely visible dorsally in Striacoeloma (resembling the condition in Cryptocoeloma Miers, 1884 ).

Serène & Soh (1976: 4, 21) recorded a small male specimen of “? Lophoplax takakurai ” from the Andaman Sea stating only that “Like takakurai the present specimen has no elevation, but it differs by its anterolateral teeth much less clearly separated one from the other”. On this basis, it seems unlikely to be Myopilumnus andamanicus , which was also described from the Andaman Sea ( Deb, 1989; see also Trivedi et al., 2022). Serène & Soh’s (1976) record was subsequently repeated in the checklists of the Thai fauna by Naiyanetr (1980, 1998, 2007) and Ng & Davie (2002); it requires confirmation.