Microtopsis Komai, Ng & Yamada, 2012

Ng, Peter K. L. & Castro, Peter, 2016, Revision of the family Chasmocarcinidae Serène, 1964 (Crustacea, Brachyura, Goneplacoidea), Zootaxa 4209 (1), pp. 1-182 : 84-85

Genus Microtopsis Komai, Ng & Yamada, 2012

Microtopsis Komai, Ng & Yamada, 2012: 138 .

Type species. Microtopsis takedai Komai, Ng & Yamada, 2012 (gender masculine) Other species included:

Microtopsis teschi n. sp.

Diagnosis. Carapace subtrapezoidal; front nearly straight; anterolateral margins arcuate, minutely granular, without distinct lobes or teeth. Epistome slightly depressed; posterior margin with widely semicircular median lobe with median fissure, semicircular lateral margins without fissures. Eye peduncle filling orbit, long, mobile; cornea reduced, pigmented. Third maxillipeds fill buccal cavern when closed; merus subcircular or subquadrate, anteroexternal angle rounded; ischium quadrate, about same length but wider than merus. Chelipeds subequal in length, slightly dissimilar in female, heteromorphic in males; fingers of minor chela subcircular in cross-section, not distinctly laterally flattened, gently tapering to tip, scissor-like, cutting margins with distinct sharp teeth; cutting margins of both chelae in females, minor in males with dense row of short setae, largest chela of male high, with broad teeth on pollex, dentate dactylus. Inner margin of cheliped carpus with small tooth or smooth. Ventral surface of cheliped merus with short row of low microscopic tubercles on posterior outer margin in both sexes. Meri of ambulatory legs unarmed. Fused thoracic sternites 1, 2 proportionally broadly triangular, short; fused sternites 3, 4 relatively broad. Male pleon with lateral margins of somite 6, fused somites 3‒5 nearly straight; telson proportionally short; postero-lateral regions slightly swollen. Sterno-pleonal cavity of male deep, press-button for pleonal holding as small, short tubercle posterior to thoracic sternal suture 4/5 near edge of sterno-pleonal cavity. Male thoracic sternite 8 proportionally long, quadrate; “supplementary plate” conspicuously narrow, only reaching median portion of exposed thorax, short, slightly longer at rounded outer margin; structures tightly appressed with most of penis concealed. G1 stout, short, distal part slightly twisted, distinctly twisted medially, distal segment with short spinules, spinules. G2 slightly sinuous, slender, pointed distal segment, almost as long as G1. Somites of female pleon with convex lateral margins; telson proportionally short. Sterno-pleonal cavity of female moderately deep, vulvae far apart from each other on outer margins of cavity between sutures 5/6, 6/7.

Remarks. Microtopsis is allied to Camatopsis , and small specimens of the latter are difficult to distinguish with those of Microtopsis (see discussion for Camatopsis ). In fact, specimens identified as “ Camatopsis rubida ” by Tesch (1918) proved to belong to a new species of Microtopsis , M. teschi n. sp. (see remarks under C. rubida ). Komai et al. (2012) listed numerous characters to separate the two genera but the present study of a larger series of Camatopsis , including small-size species and juveniles suggest that not all the characters can be used for all size groups (see discussion under Camatopsis ).

The “supplementary plate” of Microtopsis is unique among chasmocarcinids in not reaching the width of the exposed male thoracic sternum, the parts not covered by the pleon. It reaches only about halfway across the exposed sternal surface ( Fig. 62 A; Komai et al. 2012: fig. 2F). It reaches to the edge of the sterno-pleonal cavity in all other chasmocarcinids.

Distribution. Indo-West Pacific region: western Pacific Ocean.

Figures

FIGURE 62. Thoracic sternites 7 and 8 and male pleon of species of Microtopsis and Megaesthesius. A, Microtopsis takedai, Japan: paratype male, carapace damaged (ZRC 2012.0146); B, Megaesthesius sagedae, Singapore: male 2.5 × 3.0 mm (ZRC 1995.885); C, Megaesthesius migmus n. sp., Philippines: holotype male 4.4 × 4.9 mm (NMCR); D, Alainthesius bertrandi n. sp., New Caledonia: holotype male 3.3 × 4.1 mm (MNHN-IU- 2013 - 9073).