Striacoeloma, Ng & Ahyong & Shane T., 2024

Ng, Peter K. L., Ahyong, & Shane T., 2024, Redefining Lophoplax Tesch, 1918: recognising Myopilumnus Deb, 1989, and the establishment of three new genera, Hosekia, Takedaplax, and Striacoeloma (Crustacea: Brachyura: Pilumnidae), Raffles Bulletin of Zoology 72, pp. 335-355 : 351

Striacoeloma , new genus

Type species. Striacoeloma tubur , new species, by present designation. Gender of genus neuter.

Included species. Only Striacoeloma tubur , new species.

Diagnosis. Carapace subhexagonal; frontal and anterolateral margins with scattered tufts of short and long setae, not brush-like; surface appearing irregularly transversely corrugated, partly eroded; gastric and cardiac regions prominently raised, forming transverse swellings extending to branchial region, almost to lateral margin; surface adjacent to posterior carapace margin raised, with prominent wide rounded transverse ridge adjacent to the posterior carapace margin, forming channel extending to sub-branchial region; orbit submarginal, supraorbital margin almost confluent with general carapace outline, eyes only partially visible in dorsal view; eyes scarcely movable, ocular peduncle short, filling orbit, scarcely movable; lateral lobe of frontal margin small but distinct; anterolateral margin with 4 distinct teeth (including external orbital tooth), first 2 broad, lobiform, with 2 additional tubercles on posterolateral margin; posterolateral margins distinctly converging towards posterior carapace margin. Epistome with shallow biconcave posterior margin, median lobe subtruncate. Endostome short, recessed posteriorly into buccal cavern oblique to plane of adjacent pterygostomial surface, endostomial ridges low, short. Third maxilliped ischium short, quadrate, length 1.2× width; merus with anterolateral angle distinctly auriculiform. Adult female chelipeds symmetrical or almost so, stout, outer surface of chela with margins rugose and covered with granules, median part smoother; margins not lined with dense brush-like setae. Ambulatory legs short, P4 longest, about 1.3× carapace width; merus slender, length about 4× height, extensor margin not cristate, with evenly, widely spaced row of blunt, prominent nodules, outer surface slightly raised medio-longitudinally; P4 merus length about 4× height, length half carapace width.

Etymology. The name is an arbitrary combination of the Latin word “stria”, for ridge, in arbitrary combination with part of the genus name, Cryptocoeloma , in recognition of the ridged carapace surface and close relationship to the pilumnid genus. Gender is feminine.

Remarks. Whereas Striacoeloma , new genus, is superficially similar to various planopilumnids (now in the Pseudozioidea Alcock, 1898) (cf. Ng, 2010), it is clearly a pilumnid based the form of its carapace and chelipeds. The new genus is morphologically more similar to Cryptocoeloma Miers, 1884 than Lophoplax , Myopilumnus , or Hosekia , new genus. In Striacoeloma , like Cryptocoeloma , the orbits are submarginal, so the supraorbital margin does not form a strong concavity that noticeably breaks the general carapace outline. The eyes too, are relatively smaller in Striacoeloma and scarcely movable, compared with the proportionally larger, freely movable eyes of Lophoplax , Myopilumnus , and Hosekia . The chelae of Striacoeloma and Cryptocoeloma are also stouter than in Lophoplax and allies, and both have a shallow biconcave posterior margin of the epistome with the median lobe broader and subtruncate ( Fig. 10D ) (versus median lobe broadly triangular; Ng et al., 2022: fig. 3F). Striacoeloma , however, differs from Cryptocoeloma in having the dorsal surface of carapace ornamented with transverse, corrugated swellings ( Fig. 10A–C ) (versus surface granulate and rugose but without swellings and regions well demarcated; Ng et al., 2022: fig. 2); the frontal and anterolateral margins have tufts of short and long setae but not dense or brush-like ( Fig. 10A, B ) (versus margins evenly lined with dense short and long setae, appearing brush-like; Ng et al., 2022: figs. 1A, B, 2A, B, 5A, B, 7A, B); the outer surface of the chelae are covered with scattered long and short setae, not dense or brush-like ( Fig. 10A ) (versus lined with dense brush-like setae; Ng et al., 2022: figs. 1A, B, 2A, B, 5A, B, 7A, B); the ischium of the third maxilliped is subquadrate and shorter ( Fig. 10E ) (length 1.2× width versus 1.3×; Ng et al., 2022: fig. 4A); and the ambulatory merus is dentate along the extensor margin ( Fig. 11D ) (versus smooth; Ng et al., 2022: figs. 2, 4B); and the vulva is proportionately larger, occupying most of the space of sternite 5 ( Fig. 11B ) (versus vulva smaller, occupying two-thirds the space of sternite 5; Ng et al., 2022: fig. 7E, F).

Figures

Fig. 10. Striacoeloma tubur, new genus and species, holotype female (8.6 × 6.3 mm) (AM P107758), Northern Territory, Australia. A, dorsal habitus (right side denuded); B, dorsal view of carapace (right side denuded); C, subfrontal view of cephalothorax (right side denuded); D, frontal view of cephalothorax (right side denuded); E, right third maxilliped.

Fig. 11. Striacoeloma tubur, new genus and species, holotype female (8.6 × 6.3 mm) (AM P107758), Northern Territory, Australia. A, buccal cavity, anterior thoracic sternum and pleon; B, sternopleonal cavity and vulvae; C, right chela; D, right P4 and P5.