Lophoplax Tesch, 1918

Lophoplax Tesch, 1918: 196; Takeda 1977: 120; Ng 1987: 100; Poore & Ahyong 2023: 667, 670, 680, fig. 14.112i.

Myopilumnus Deb, 1989 (type species Myopilumnus andamanicus Deb, 1989, by monotypy; gender masculine)

Type species. Lophoplax bicristata Tesch, 1918, by original designation and monotypy; gender feminine.

Remarks. The presence of prominent areolets on the carapace is a character found in all species of Lophoplax but the number, size and strength vary considerably. In the type species, L. bicristata, the transverse protogastric areolet is especially distinct, with the epigastric areolet relatively low and not prominent, with all the other areolets not obvious, notably, the hepatic one is only represented by a tubercle. In addition, the dorsal surface of the carpus of the cheliped has one raised plate, without areolets (cf. Tesch 1918: pl. 12 fig. 2; unpublished photographs of syntype male). On the other hand, the other species included in Lophoplax by Tesch (1918), L. sculpta, has well developed and very prominent smooth epigastric, protogastric and hepatic areolets (Fig. 3A–C). The same is true for L. sordida n. sp. and L. pannosa n. sp. (Figs. 3D–F, 4D). In L. takakurai, there are no prominent areolets, with the gastric regions only possessing clusters of granules (cf. Sakai 1935: pl. 7 fig. 2; Sakai 1976: pl. 191 fig. 4). In L. sextuberculata, the epigastric, protogastric and hepatic areolets are present but smaller and arranged more or less in a subtransverse row (Takeda & Kurata 1984: fig. 14), but this condition varies, with areolets varying in number and positioning (Takeda & Marumura 1995: fig. 1). Lophoplax vermiculata has all the areolets of L. sculpta except that these are prominently rugose and eroded, very different from the almost smooth surfaces of L. sculpta (Fig. 4A–C). The same is true of the areolets on the carpus of the cheliped in L. vermiculata and L. pannosa n. sp., they are eroded and not smooth (Fig. 6D–F).

In L. symmetrinuda, the areolets are smooth and prominent, being relatively even larger than those of L. sculpta (Fig. 3A–C).

All but one of the Lophoplax species have a prominent raised and rounded longitudinal ridge on the posterior part of sub-branchial region which is an extension of the posterior carapace margin (Figs. 3, 4). It forms a channel with the carapace margin (Fig. 9A). This structure on the posterior part of the sub-branchial region is similar to the condition reported in Antrocarcinus Ng & Chia, 1994 (Antrocarcinidae Ng & Chia, 1994) and Rathbunaria Ward, 1933 (Planopilumnidae Serène, 1984) (see Ng & Chia 1994; Ng 2010). Ng & Chia (1994) suggested that it may assist in respiration when the animals are partially covered by silt and sand. The condition in these species of Lophoplax, however, is not as pronounced as those of the above genera. Only in L. bicristata is the intestinal region not distinctly raised, with the area appearing almost flat (cf. Tesch 1918: pl. 12 fig. 2; unpublished photographs of syntype male).

Lophoplax can also be approximately divided into two groups of species on the basis of their adult ambulatory legs. The species in the first group (with L. bicristata, L. sextuberculata and L. takakurai) have proportionately longer legs with the merus slender and more elongate (cf. Tesch 1918; Sakai 1935; Takeda & Kurata 1984; Takeda & Marumura 1995; Marumura & Kosaka 2003). The figure of L. bicristata in Tesch (1918: pl. 12 fig. 2) is accurate; the authors have examined photographs of a syntype male in Leiden and the leg characters agree. Species in the second group (with L. sculpta, L. andamanica, L. sordida n. sp., L. pannosa n. sp., L. vermiculata and L. symmetrinuda) have proportionately shorter legs, with the merus distinctly wider and stouter (Figs. 7G–L, 12C). The ambulatory legs of L. symmetrinuda are very characteristic, being much stouter and shorter than in all the other species (Fig. 12C). The adult ambulatory legs of L. vermiculata are intermediate in length to L. symmetrinuda, and L. sculpta, L. andamanica, L. sordida n. sp. and L. pannosa n. sp. (Fig. 7J). It must be noted, however, that subadult specimens of L. vermiculata have proportionately more slender and longer ambulatory legs (Fig. 7K).

The carapace shape of most of the species varies from almost quadrate (e.g., L. bicristata and L. sculpta; Tesch 1918: pl. 12 fig. 2; Fig. 3A–C), to transversely hexagonal or subquadrate (e.g., L. pannosa n. sp., Fig. 4D), being proportionately widest in L. vermiculata (Fig. 4A, C), with all of them possessing well defined anterolateral teeth. Lophoplax bicristata, L. sculpta, L. takakurai, L. andamanica, L. sordida n. sp. and L. pannosa n. sp. have distinct tubercles on the posterolateral margin of the carapace, just posterior of the last anterolateral tooth (e.g., Fig. 3A–C); being absent in L. sextuberculata (cf. Takeda & Kurata 1984: fig. 14), and in adult L. vermiculata, only sharp granules or tubercles are present instead (Fig. 4A, C), being unarmed in subadult specimens (Fig. 4B). Lophoplax symmetrinuda is in a group on its own, with the carapace transversely ovate, the anterolateral margin almost entire with only low rounded lobes and the posterolateral margin smooth (Fig. 12A).

The male anterior thoracic sternum in most of the species of Lophoplax is relatively narrow transversely (e.g., Fig. 9B, E) but in adult L. vermiculata, sternites 1–4 are distinctly wider (Fig. 9G), although in subadult males, it is relatively less obvious and more like those of other species of Lophoplax (Fig. 9F).

The females of all the species are relatively consistent in their characters; the pleons being longitudinally ovate (e.g. Fig. 8A), and the vulvae are large, ovate, obliquely positioned and occupy most of sternite 6 without any visible operculum (Fig. 8B–D).

In summary, the composition of Lophoplax remains imperfect. Looking at the series of characters of the recognised species of Lophoplax, it can be argued that the genus is heterogeneous, and should perhaps be separated into three or even four genera. Certainly, there are sufficient differences between L. bicristata (the type species), and L. sculpta, L. andamanica, L. sordida n. sp. and L. pannosa n. sp., to treat the latter group as a separate genus. A case can also be made for L. vermiculata and L. symmetrinuda to be recognised as two additional monotypic genera, especially the latter species which has many peculiar features. Recognising new genera now, however, may not be the best approach, especially since we have not had the opportunity to examine specimens of L. takakurai and L. sextuberculata. The resolution of the this matter will need to be deferred to another time.