Chasmocarcinus typicus 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 : 14-17

Chasmocarcinus typicus Rathbun, 1898

( Figs. 2 A‒D; 21A; 28A‒C; 35A‒D; 49A, B; 56A; 63A‒C; 84A; 89A)

Chasmocarcinus typicus Rathbun, 1898a: 285 , pl. 7, figs. 3–5 [type locality: north of Trinidad]; 1898b: 602 [ Brazil] (part); 1918: 55 [in key], 55, fig. 23 (part).— Holthuis 1959: 235 [ Suriname].— Serène 1964a: 258 [in list].— Coelho & Ramos 1972: 165 [in list].— Fausto-Filho & Sampaio Neto 1976: 69 [ Brazil].— Blow & Bailey 1992: 176 [in list], pl. 1, figs. 1, 2, pl. 2, fig. 1 [ Trinidad].— Melo et al. 1989: 18 [ Brazil].— Melo 1996: 419 [in key], 423, unnumb. fig. [ Brazil]; 1998: 494 [in list]; 2008: 13 [ Brazil].— Coelho et al. 2008: 12 [ Brazil].—Ng et al. 2008: 76 [in list].—Castro et al. 2010: 51 [ Brazil].— Guinot et al. 2013: 113, 279, fig. 24C [ Brazil].

Chasmocarcinus rathbuni Bouvier, 1917: 391 [ Brazil]—A. Milne Edwards & Bouvier 1923: 342, fig. 5, pl. 8, fig. 1, pl. 10, fig. 1 [ Brazil].— Coelho & Ramos 1972: 165 [in list; Brazil].— Melo 1996: 419 [in key], 422, unnumb. fig. [ Brazil]; 1998: 494 [in list].—Coelho & Coelho 1998: 813 [in list].—Ng et al. 2008: 76 [in list].

Type material. Holotype male (7.8 × 8.7 mm) (USNM 6901), Trinidad, Albatross, stns 2121, 2122, 10°37’40”‒10°37’00”N to 61°42’40‒61°44’22” W, 57– 62 m.

Paratypes: 74 males, 59 females (USNM 6901) (part), type locality; 2 males, 1 female (as “co-types”) (MNHN-IU-2014-12795, ex MNHN-B10127), type locality.

Other material examined. Anguilla. 1 female molt 8.3 × 10.1 mm ( USNM 228470 ), Oregon, stn 5913, 18°11’N, 63°15’W, 296 m, 25.02.1966.

Trinidad. 1 male ( ZRC, ex USNM 139265 ), Gulf of Paria, 9 m, J.M. Stanley coll., 19.01.1971.

Colombia. 1 male, 1 female ( USNM 1071474 ), Caribana Point, 160–380 m, R. Lemaitre & N.H. Campos coll., 0 4.08.1995.

Suriname. 1 female ( USNM 1199102 ) ( USNM 1199102 ), Oregon, stn 4304, 07°30’N, 55°00’W, 183 m, 24.03.1963.

Brazil. 1 male (4.0 × 5.1 mm) ( MCZ CRU- 10993 ) (holotype of Chasmocarcinus rathbuni Bouvier, 1917 , Hassler, off Rio Grande do Sul, 32°00’S, 50°15’W, 128 m, 12.1871‒08.1872.— 1 male (9.4 × 12.1 mm) , 1 female (8.6 × 10.5 mm) (ZRC 2015.211, ex MZUSP 4114), Rio Grande do Sul, GEDIP, stn 1697, 30.01.1972.— 3 males , 1 female ( USNM 220773 ), Albatross, off Cape Frio , stn 2762, 23°08’S, 41°34’W, 106 m , 30.12.1887.—1 juvenile male (5.5 × 7.6 mm) (ZRC 2015.266, ex MZUSP 12152), GEDIP - R.G. Sul, Brazil, 26.06.1968 .

Diagnosis. Carapace ( Fig. 2 A‒D) subtrapezoidal, anterior third proportionally narrow, 1.1‒1.2 wider than long; front bilobed, with well-defined median cleft; anterolateral margins arcuate, minutely granular, without distinct lobes or teeth; low rows of granules along anterolateral margins of carapace but not distinctly cristate ( Fig.

2A‒D; Blow & Bailey 1992: fig. 1a). Prominently swollen epigastric region. Orbits short, proportionally long eye peduncles, slight ridge of granules along anterior, anterolateral margins of carapace. Bulging pterygostomial region, clearly visible from dorsal view. Epistome ( Fig. 21 A) slightly depressed; short, semicircular median lobe with median fissure, lateral margins straight. Antennular articles usually long ( Fig. 21 A). Eye peduncle nearly filling orbit, long, much longer than cornea ( Fig. 21 A; Blow & Bailey 1992: fig. 2), mobile; cornea reduced. Third maxillipeds ( Fig. 28 A‒C) not filling buccal cavern when closed, leaving wide gap; merus broadly ovate, outer margin strongly curved, anteroexternal angle rounded, elevated; ischium elongated, slightly longer than merus. Chelipeds ( Figs. 2 A‒C; 35A‒D) subequal in length, slightly dissimilar in female, heteromorphic in males, especially largest males; fingers of major chela ( Fig. 35 A) proportionally long, much longer than propodus, dactylus curved in large males; pollex much longer than dactylus in large males; fingers of minor chela of both sexes ( Fig. 35 B) subcircular in cross-section, scissor-like; with small, often sharp teeth. Anterior margin of ventral surface of propodus, proximal portion of pollex of major chela of large males with conspicuous tufts of long setae ( Fig. 35 A, C). Inner margin of cheliped carpus often with acute tooth ( Fig. 2 B, D). Ventral surface of cheliped merus smooth in both sexes. Ambulatory legs ( Fig. 2 A‒D) proportionally short, P5 merus not reaching front when folded; no subdistal spur on P5 dactylus. Meri of P2–P5 with short granules along posterior margin, unarmed. P5 dactylus ( Fig. 2 A) upcurved. Fused thoracic sternites 1, 2 ( Fig. 49 A, B) broadly triangular, proportionally narrow, long; fused thoracic sternites 3, 4 ( Figs. 49 A, B; 89A) relatively broad. Male pleon ( Figs. 49 A, B; 56A) with rectangular somite 6, fused somites 3–5 nearly straight; telson proportionally long, narrow; postero-lateral regions 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. G1 ( Fig. 63 A, B) stout, distal part thickened, distal segment straight with spinules. G2 ( Fig. 63 C) about 3/4 G1 length, straight, slender, distal segment long, curved. Somites of female pleon ( Fig. 84 A) with convex lateral margins; telson proportionally long. Sterno-pleonal cavity of female ( Fig. 89 A) moderately shallow, vulvae relatively close together.

Remarks. Diagnostic of C. typicus is the major chela of large males, which has distinctively curved fingers and conspicuous tufts of long setae on the proximal portion of the pollex ( Fig. 35 A, C). The anterior third of the carapace is proportionally narrow, with proportionally short orbits and long eye peduncles ( Fig. 21 A). Also diagnostic is a rectangular third maxilliped ischium that is slightly longer than the merus ( Fig. 28 A‒C), and fused thoracic sternites 1, 2 that are rectangular, with parallel sides, and proportionally narrow and long ( Fig. 49 A). There are only a few rows of low granules along the anterolateral margins of the carapace. A tooth is present on the inner angle of the cheliped carpus.

The holotype of C. rathbuni Bouvier, 1917 , a juvenile male (4.0 × 5.1 mm, MCZ CRU-10993) from Brazil, clearly shows the diagnostic characters of C. typicus : fused thoracic sternites 1, 2 broadly triangular with parallel sides; a wide gap between the third maxillipeds; ischium of the third maxillipeds rectangular and slightly longer than the merus; fingers proportionally long, much longer than the cheliped propodus; and epistome with straight lateral margins. These morphological characters are also present in the juvenile paratypes of C. typicus (male 4.1 × 4.7 mm, 2 females 4.0 × 4.8 mm, 4.3 × 5.6 mm, USNM 6901). In the holotype of C. rathbuni , there is a tooth on the inner margin of the cheliped carpus ( Fig. 2 C), which may be low or even absent in large males of C. typicus (e.g., Fig. 2 A) but this structure is sometimes present (e.g., Fig. 2 B, D), and as such, is too variable to be useful. There also appears to be some variation in the relative length of the antennular articles. In large males with a strongly enlarged chela, the article after the basal one is long ( Fig. 21 A), with smaller specimens having a relatively shorter structure. The holotype of C. rathbuni and material from Brazil have proportionately shorter antennular articles, even for some large specimens. The larger specimens from Brazil, however, are otherwise identical to the types of C. typicus , including the structure of the G1. As such, we believe the length of the antennular articles is not a reliable character in separating the species, at least on the basis of the material on hand. The G1 and G2, absent in the holotype of C. rathbuni , were also missing in the juvenile male of C. typicus examined. It is noteworthy mentioning that the label accompanying the holotype of C. rathbuni , most probably written by F.A. Chace, Jr. because of his characteristic handwriting, reads “ Chasmocarcinus typicus Rathbun , 1 young, holotype of C. rathbuni Bouvier ”. Thus, C. rathbuni Bouvier, 1917 , is almost certainly a junior subjective synonym of C. typicus Rathbun, 1898 . In any case, Chasmocarcinus rathbuni has only been reported from Brazil thus far (see Melo 1996: 422).

A juvenile female that is part of the type material of C. typicus from Trinidad ( USNM 6901) actually represents C. cylindricus . Both species, which appear to be sympatric at some localities, may be easily confused, particularly small males and females. They can be separated by the relative size of the anterior portion of the carapace, which is narrower in C. typicus ( Fig. 2 A‒D) than in C. cylindricus ( Fig. 3 C‒E).

Distribution. West Indies ( Anguilla) to Brazil (south to Rio Grande do Sul). Depth: 9‒ 380 m.