Deltopelta, Ng, Peter K. L. & Castro, Peter, 2016

Ng, Peter K. L. & Castro, Peter, 2016, Revision of the family Chasmocarcinidae Serène, 1964 (Crustacea, Brachyura, Goneplacoidea), Zootaxa 4209 (1), pp. 1-182 : 30-31

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.

Figures

FIGURE 5. Overall dorsal view of species of Chasmocarcinus, Amboplax n. gen. and Deltopelta n. gen. A, Amboplax peresi, Brazil: male 5.1 × 6.4 mm (ZRC 2015.217); B, Amboplax peresi, Brazil: ovigerous female 4.9 × 6.2 mm (ZRC 2015.217); C, Chasmocarcinus gemmatus n. sp., Costa Rica: male 7.7 × 11.6 mm (SIO C 1205); D, Chasmocarcinus latipes, Ecuador: male 10.5 × 14.5 mm (AMNH 1941 - 10668); E, Deltopelta obliquus, A, Bahamas: holotype male 5.2 × 5.1 mm (USNM 20509); F, Deltopelta obliquus, Belize: female 10.7 × 13.0 mm (PMNH, holotype of Hephthopelta superba); G, Deltopelta obliquus, Belize: female 10.7 × 13.0 mm (PMNH, holotype of Hephthopelta superba, after Boone 1927: fig. 5).

FIGURE 22. Frontal view of cephalothorax of species of Chasmocarcinus, Amboplax n. gen. and Deltopelta n. gen. A, Chasmocarcinus latipes, Mexico: holotype female 12.4 × 14.4 mm (USNM 21592); B, Chasmocarcinus latipes, Mexico: female 9.2 × 13.0 mm (AMNH 16262, holotype female of Chasmocarcinus ferrugineus Glassell, 1936); C, Chasmocarcinus longipes, Colombia: paratype male 5.5 × 6.4 mm (USNM 78785); D, Chasmocarcinus meloi, Brazil: holotype female 5.3 × 6.2 mm (after Coelho & Coelho 1998: fig. 8 b); E, Chasmocarcinus mississippiensis, Gulf of Mexico: holotype male 8.3 × 10.4 mm (USNM 64074); F, G, Amboplax peresi, Brazil: male 5.1 × 6.4 mm (ZRC 2015. 217); H, Deltopelta obliquus, Bahamas: holotype male 5.2 × 5.1 mm (USNM 20509).

FIGURE 29. Third maxilliped of species of Chasmocarcinus and Deltopelta n. gen. A, Chasmocarcinus gemmatus n. sp., Costa Rica: male 7.7 × 11.6 mm (SIO C 1205); B, C. hirsutipes, Brazil: holotype male 6.6 × 7.0 mm (after Coelho & Coelho 1998: fig. 6 c); C, Chasmocarcinus latipes, Mexico: holotype female 12.4 × 14.4 mm (USNM 21592); D, Chasmocarcinus latipes, Ecuador: male 10.5 × 14.5 mm (AMNH 1941 - 10668); E, Chasmocarcinus latipes, Mexico: female 9.2 × 13.0 mm (AMNH 16262, holotype female of Chasmocarcinus ferrugineus Glassell, 1936); F, Chasmocarcinus longipes, Colombia: paratype male 5.5 × 6.4 mm (USNM 78785); G, C. meloi, Brazil: holotype male 5.3 × 6.2 mm (after Coelho & Coelho 1998: fig. 8 c); H, Chasmocarcinus mississippiensis, Gulf of Mexico: male 7.2 × 8.8 mm (USNM 101082); I, Deltopelta obliquus, Bahamas: holotype male 5.2 × 5.1 mm (USNM 20509).

FIGURE 37. Chelae of species of Chasmocarcinus, Amboplax n. gen. and Deltopelta n. gen. A, B, Chasmocarcinus meloi, Brazil: holotype male 5.3 × 6.2 mm (after Coelho & Coelho 1998: fig. 9 a, b); C, D, Chasmocarcinus mississippiensis, Gulf of Mexico: holotype male 8.3 × 10.4 mm (USNM 64074); E, F, Amboplax peresi, Brazil: male 5.1 × 6.4 mm (ZRC 2015.217); G, Deltopelta obliquus, Bahamas: holotype male 5.2 × 5.1 mm (USNM 20509).

FIGURE 50. Anterior thoracic sternum and male pleon of species of Chasmocarcinus, Amboplax n. gen. and Deltopelta n. gen. A, C, Chasmocarcinus latipes, Ecuador: male 10.5 × 14.5 mm (AMNH 1941 - 10668); B, Chasmocarcinus longipes, Colombia: paratype male 5.5 × 6.4 mm (USNM 78785); D, Chasmocarcinus latipes, D, Mexico: male 8.3 × 11.0 mm (SIO C 319); E, Chasmocarcinus meloi, Brazil: holotype male 5.3 × 6.2 mm (after Coelho & Coelho 1998: fig. 9 d); F, Chasmocarcinus mississippiensis, Gulf of Mexico: male 7.2 × 8.8 mm (USNM 101082); G, Amboplax peresi, Brazil: male 5.1 × 6.4 mm (ZRC 2015. 217); H, Deltopelta obliquus, Bahamas: holotype male 5.2 × 5.1 mm (USNM 20509).

FIGURE 57. Thoracic sternites 7 and 8 and male pleon of species of Chasmocarcinus, Amboplax n. gen. and Deltopelta n. gen. A, Chasmocarcinus latipes, Ecuador: male 10.5 × 14.5 mm (AMNH 1941 - 10668); B, Chasmocarcinus longipes, Colombia: paratype male 5.5 × 6.4 mm (USNM 78785); C, Chasmocarcinus mississippiensis, Gulf of Mexico: holotype male 8.3 × 10.4 mm (USNM 64074); D, Amboplax peresi, Brazil: male 5.1 × 6.4 mm (ZRC 2015. 217); E, Deltopelta obliquus, Bahamas: holotype male 5.2 × 5.1 mm (USNM 20509).

FIGURE 89. Vulvae of species of Chasmocarcinus, Amboplax n. gen. and Deltopelta n. gen. A, Chasmocarcinus typicus, Trinidad: paratype female 6.5 × 6.8 mm (USNM 6901); B, Chasmocarcinus arcuatus, Brazil: female 4.4 × 6.2 mm (ZRC 2015.222); C, Chasmocarcinus chacei, Gulf of Mexico: female 7.8 × 9.5 mm (USNM 61489); D, Chasmocarcinus cylindricus, Puerto Rico: female 6.9 × 9.2 mm (USNM 24551); E, Chasmocarcinus latipes, Mexico: holotype female 12.4 × 14.4 mm (USNM 21592); F, C. latipes, Mexico: female 9.2 × 13.0 mm (AMNH 16262, holotype female of Chasmocarcinus ferrugineus Glassell, 1936); G, Chasmocarcinus longipes, Colombia: paratype female 5.6 × 6.8 mm (USNM 78785); H, Chasmocarcinus mississippiensis, Gulf of Mexico: female 9.6 × 12.4 mm (USNM 150090); I, Amboplax peresi, Brazil: ovigerous female 4.9 × 6.2 mm (ZRC 2015. 217); J, Deltopelta obliquus, Belize: female 10.7 × 13.0 mm (PMNH, holotype of Hephthopelta superba).