Deltopelta Ng & Castro, 2016, n. gen.
Creators: name: Ng, Peter K. L.
name: Castro, Peter
Publisher: name: Zenodo
Year: 2016
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Chasmocarcinidae
Deltopelta

description: Genus Deltopelta n. gen.
Chasmocarcinus — Rathbun, 1898a: 286 (part); 1918: 54 (part)— Serène 1964a: 257 (part).— Blow & Bailey 1992: 176 [in list] (part).—Coelho & Coelho 1998: 799 (part).—Ng et al. 2008: 76 [in list] (part).—Felder et al. 2009: 1081 [in list] (part). Hephthopelta — Boone, 1927: 16 (part).
Type species. Chasmocarcinus obliquus Rathbun, 1898 (by monotypy, gender feminine).
Diagnosis. Carapace (Fig. 5 E–G; Boone 1927: fig. 5, as Hephthopelta superba) subtriangular, anterior portion much narrower than posterior portion, globose, 1.0‒1.2 wider than long; front bilobed, with well-defined median cleft; anterolateral margins oblique, smooth, without distinct lobes, teeth or granular ridges (Fig. 5 E–G; Boone 1927: fig. 5, as Hephthopelta superba). Pterygostomial region gently convex. Epistome (Fig. 22 H) slightly depressed; short, posterior margin with semicircular median lobe with median fissure, lateral margins straight. Eye peduncle (Fig. 22 H) nearly filling short orbit, proportionally short, longer than cornea, mobile; cornea not reduced. Third maxillipeds (Fig. 29 I) not filling buccal cavern when closed, leaving wide gap; merus ovate, outer margin strongly curved, anteroexternal angle rounded, flat; ischium rectangular, longer than merus. Chelipeds (Figs. 5 E– G; 37G) subequal in length; fingers proportionally long, much longer than propodus, dactylus slightly curved; pollex as long as dactylus. Major chela of holotype male (Fig. 37 G) with teeth of varying sizes on cutting margin; small teeth in minor chela. Inner margin of cheliped carpus with conspicuous tooth (Fig. 5 E–G; Boone 1927: fig. 5, as Hephthopelta superba). Ventral surface of cheliped merus with teeth. Ambulatory legs (Fig. 5 G; Boone 1927: fig. 5, as Hephthopelta superba) proportionally long. Meri of extant ambulatory legs unarmed. P5 dactylus gently upcurved (Fig. 5 G; Boone 1927: fig. 5, as Hephthopelta superba). Fused thoracic sternites 1, 2 (Fig. 50 H) triangular, long; fused thoracic sternites 3, 4 (Figs. 50 H; 89J) relatively narrow. Male pleon (Figs. 50 H, 57E) with quadrate somite 6, outer margins of fused somites 3–5 straight; telson proportionally short, narrow; postero-lateral regions flat as in rest of somite. Sterno-pleonal cavity 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 (Fig. 57 E) short, quadrate; “supplementary plate” subtriangular, proportionally long at exterior margin, wider along margin bordering episternite 7; structures tightly appressed with most of penis concealed. G1, G2 of adult male unknown. Sterno-pleonal cavity of female (Fig. 89 J) moderately shallow, vulvae conspicuously close together.
Etymology. From delta, the Greek capital letter (Δ), which is shaped like a triangle, alluding for the triangular shape of the carapace, and pelta, which is derived from the generic name Hephthopelta, the original generic name of the only species included in the new genus.
Remarks. The subtriangular shape of the carapace is unique among chasmocarcinids and sharply contrasts with the subtrapezoidal carapace of other members of the family. Other important differences are a male pleon which is proportionately more narrower, with nearly straight sides of the fused somites 3‒5 (Fig. 57 E), and the posterior portion not conspicuously wider than the anterior portion, or T-shaped as in other genera; the presence of teeth along the ventral surface of each cheliped merus (smooth in other genera); and a complete, non-reduced cornea, which is reduced to varying degrees in the remaining chasmocarcinines.
The chela of D. obliqua is relatively delicate but of particular interest because the base of the dactylar finger has a stout and gently curved tooth that fits into a concavity at the base of the pollex when closed (Fig. 37 G). This is not unlike the condition seen in many species of Calappidae and some Portunidae, and is probably used to open small gastropods (see Ng & Tan 1984, 1985). Such a “shell-peeling” structure is not known in any other Chasmocarcinus species.
Distribution. Tropical Western Atlantic region.
description: Published as part of Ng, Peter K. L. & Castro, Peter, 2016, Revision of the family Chasmocarcinidae Serène, 1964 (Crustacea, Brachyura, Goneplacoidea), pp. 1-182 in Zootaxa 4209 (1) on pages 30-31, DOI: 10.11646/zootaxa.4209.1.1, http://zenodo.org/record/272646