Chasmocarcinus Rathbun, 1898

Ng, Peter K. L. & Castro, Peter, 2016, Revision of the family Chasmocarcinidae Serène, 1964 (Crustacea, Brachyura, Goneplacoidea), Zootaxa 4209 (1), pp. 1-182 : 12-13

Genus Chasmocarcinus Rathbun, 1898

Chasmocarcinus Rathbun, 1898a: 284 ; 1901: 9 [diagnosis]; 1918: 54 [diagnosis].— Serène, 1964a: 257; 1964b: 186 [in list].— Ng et al. 2008: 76 [in list].— De Grave et al. 2009: 32 [in list].—Castro et al. 2010: 41.— Guinot et al. 2013: 44, 113, 114.

Type species. Chasmocarcinus typicus Rathbun, 1898 (gender masculine) (= Chasmocarcinus rathbuni Bouvier, 1917 )

Other species included:

Chasmocarcinus arcuatus Coelho & Coelho, 1998

Chasmocarcinus chacei Felder & Rabalais, 1986

Chasmocarcinus cylindricus Rathbun, 1901

Chasmocarcinus gemmatus n. sp.

Chasmocarcinus hirsutipes Coelho & Coelho, 1998

Chasmocarcinus latipes Rathbun, 1898 (= Chasmocarcinus ferrugineus Glassell, 1936 )

Chasmocarcinus longipes Garth, 1940 (= Chasmocarcinus panamensis Serène, 1964 )

Chasmocarcinus meloi Coelho & Coelho, 1998

Chasmocarcinus mississippiensis Rathbun, 1931

Diagnosis. Carapace subtrapezoidal, high; front bilobed, with shallow to well-defined median cleft; anterolateral margins arcuate, variously granular, without distinct lobes or teeth. Epistome slightly depressed; semicircular or widely rectangular median lobe with median fissure, semicircular or straight lateral margins with or without lower margin. Eye peduncle filling orbit, short, mobile; cornea reduced, pigmented. Third maxillipeds not filling buccal cavern when closed; merus oval to broadly oval, outer margin rounded, even or slightly elevated anteriorly, anteroexternal angle rounded; ischium quadrate or rectangular, about same length or longer than merus. Pterygostomial region bulging, clearly visible from dorsal view ( Fig. 5 C) or only gently convex, not visible from dorsal view ( Fig. 5 D). Chelipeds subequal in length, slightly dissimilar in female, heteromorphic in males; cutting margins of major chela of both sexes with broad, blunt teeth, largest most proximal usually on upper, lower cutting margins; fingers of minor chela of both sexes subcircular in cross-section, scissor-like, with small, often pointed teeth; inner margin of cheliped carpus with or without tooth; ventral surface of cheliped merus smooth, without teeth, in both sexes. P2, P3 meri with short teeth or granules along posterior margin, meri of other ambulatory legs with microscopic granules, unarmed. P5 dactylus upcurved, often with spur-like subdistal extension. Fused thoracic sternites 1, 2 broadly triangular, long; fused thoracic sternites 3, 4 relatively broad. Male thoracic sternite 8 short, quadrate; “supplementary plate” short, rectangular; structures tightly appressed with most of penis concealed. Male pleon with rectangular somite 6, fused somites 3–5 nearly straight; telson proportionally short; postero-lateral regions as in rest of somite or clearly 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. G1 stout, distal segment broad or straight, with spinules. G2 straight, long, slender distal segment; shorter than G1. Somites of female pleon with convex lateral margins; telson proportionally short. Sterno-pleonal cavity of female moderately shallow, vulvae close together in shallower portion of thoracic sternite 6.

Remarks. There appears to be several groups of species in Chasmocarcinus as defined at present, although only C. obliquus Rathbun, 1898 , and C. peresii Rodrigues de Costa, 1968, possess distinct suites of carapace, male pleonal, gonopodal, and/or cheliped characters that warrant their transfer to their own genera ( Deltaplax n. gen. and Amboplax n. gen., respectively). Two species, C. typicus Rathbun, 1898 , and C. gemmatus n. sp. are distinct in having a carapace in which the lateral margins are not distinctly cristate, with the carapace appearing subovate from frontal view; and the third maxilliped ischium is relatively more elongated and slender, leaving a large gap between them even when closed, with the mandibles more clearly exposed ( Figs. 28 A, B; 29A). The antennular articles of C. typicus are also proportionately much longer ( Fig. 21 A) than any of the other Chasmocarcinus species (including C. gemmatus n. sp., Fig. 21 G). These character states are not clear-cut, however, with species like C. chacei Felder & Rabalais, 1986 , and C. longipes Garth, 1940 , possessing intermediate character states in the carapace shape and third maxillipeds. Chasmocarcinus latipes Rathbun, 1898 , is also unusual in that its ambulatory legs are proportionately short, the posterior margin of the epistome has a subtriangular median lobe and the pterygostomial region is not prominently bulging, not being clearly visible from dorsal view ( Fig. 5 D). In other Chasmocarcinus species, the ambulatory legs are proportionately longer, the posterior margin of the epistome has a more rounded or subtruncate median lobe and the pterygostomial region is bulging and clearly visible from dorsal view (e.g., Fig. 5 C). We nevertheless do not regard this as useful generic characters. Certainly the taxonomically important G1 and G2 structures do not show any distinct pattern to support separating these species into separate genera for the time being.

The major chelae of adult male Chasmocarcinus typicus and C. gemmatus n. sp. are unusual compared to all other congeners in that the fingers are conspicuously elongated and curved, with the submarginal parts of the cutting edge of the dactylus lined with dense short setae ( Figs. 35 A, C; 36A). The adult male fingers are also distinctly curved in Chasmocarcinus arcuatus , forming a large gap when closed ( Fig. 35 E), but they are relatively shorter than those of C. typicus and C. gemmatus n. sp.

A prominently upcurved P5 dactylus in present in all Chasmocarcinus species, although this character is also present in Indo-West Pacific genera like Camatopsis , Chasmocarcinops, Chinommatia n. gen., Statommatia n. gen., and Notopelta n. gen. A subdistal spur on the P5 dactylus appears in some species ( C. chacei , C. cylindricus , C. latipes , and C. mississippensis ; see Felder & Rabalais 1986) but not all species of Chasmocarcinus . This spur is, however, absent in all other chasmocarcinid genera. Its size varies among individuals, however, tending to be more developed in males, and seemingly eroded or broken off in some specimens.

Distribution. Western Atlantic and Tropical Eastern Pacific regions.

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 21. Frontal view of cephalothorax of species of Chasmocarcinus. A, C. typicus, Trinidad: holotype male 7.8 × 8.7 mm (USNM 6901); B C. arcuatus, Brazil: male 5.7 × 7.7 mm (ZRC 2015.221); C, D, C. arcuatus, Brazil: male 3.7 × 5.1 mm (ZRC 2015.220); E, C. chacei, Gulf of Mexico: male 6.6 × 8.0 mm (USNM 61489); F, C. cylindricus, Puerto Rico: neotype female 4.6 × 6.1 mm (USNM 23765); G, C. gemmatus n. sp., Costa Rica: male 7.7 × 11.6 mm (SIO C 1205); H, C. hirsutipes, Brazil: holotype female 6.6 × 7.0 mm (after Coelho & Coelho 1998: fig. 5 c).

FIGURE 28. Third maxilliped of species of Chasmocarcinus and Amboplax n. gen. A, Chasmocarcinus typicus, Trinidad: holotype male 7.8 × 8.7 mm (USNM 6901); B, Chasmocarcinus typicus, Brazil: male 4.0 × 5.1 mm (MCZ CRU- 10993, holotype of Chasmocarcinus rathbuni); C, Chasmocarcinus typicus, Brazil: male 9.4 × 12.1 mm (ZRC 2015.211); D, Chasmocarcinus arcuatus, Brazil: male 5.7 × 7.7 mm (ZRC 2015.221); E, Chasmocarcinus arcuatus, Brazil: male 3.7 × 5.1 mm (ZRC 2015.220); F, Chasmocarcinus chacei, Gulf of Mexico: female 7.8 × 9.5 mm (USNM 61489); G, Chasmocarcinus cylindricus, Puerto Rico: female 6.9 × 9.2 mm (USNM 24551); H, Amboplax peresi, Brazil: male 5.1 × 6.4 mm (ZRC 2015.217).

FIGURE 35. Chelae of species of Chasmocarcinus. A, C. typicus, Trinidad: holotype male 7.8 × 8.7 mm (USNM 6901); B, C. typicus, Brazil: male 4.0 × 5.1 mm (MCZ CRU- 10993, holotype of Chasmocarcinus rathbuni); C, D, C. typicus, Trinidad: paratype male 7.1 × 9.4 mm (USNM 6901); E, F, C. arcuatus, Brazil: holotype male 5.5 × 6.8 mm (after Coelho & Coelho 1998: figs. 2 d, 3 e); G, H, C. arcuatus, Brazil: male 3.7 × 5.1 mm (ZRC 2015.220); I, J, C. chacei, Gulf of Mexico: male 6.6 × 8.0 mm (USNM 61489); K, L, C. cylindricus, Jamaica: male 3.6 × 4.9 mm (USNM 42942).