Amboplax 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
Amboplax

description: Genus Amboplax n. gen.
Chasmocarcinus Coelho & Coelho 1998: 799 (part); Melo 1996: 419 (part).
Type species. Chasmocarcinus peresi Rodrigues da Costa, 1968 (by monotypy, gender feminine)
Diagnosis. Carapace (Fig. 5 A, B; Rodrigues da Costa 1968: unnumb. pl., fig. 1) subtrapezoidal, high; front bilobed, with shallow median cleft; anterolateral margins arcuate, with conspicuous granular ridge. Epistome (Fig. 22 F) slightly depressed, posterior margin with salient, subtriangular median lobe with median fissure, lateral margins broad, straight. Antennular articles relatively short (Fig. 22 F). Orbits proportionally wide, proportionally short eye peduncles (Fig. 22 F, G) that do not fill orbit. Suborbital ridge (Fig. 22 G) below orbital margin. Third maxillipeds (Fig. 28 H) do not fill buccal cavern when closed; merus broadly ovate; ischium broad, quadrate, slightly longer than merus. Pterygostomial region bulging, clearly visible from dorsal view. Chelipeds subequal in length, slightly dissimilar in female, heteromorphic in males. Fingers of major chela (Fig. 37 E; Rodrigues da Costa 1968: unnumb. pl., fig. 3) proportionally short, slightly shorter than propodus in dorsal view; pollex as long as dactylus; cutting margins with several low teeth, outer submarginal part with stiff setae which do not obscure margin. Minor male chela (Fig. 37 F) slender; fingers subcircular in cross-section, scissor-like; cutting margins blade-like, with 3 short spines on distal quarter, short submedian spine on each finger, submarginal part lined with dense row of short setae that obscure margin. Female major chela more slender than male, submarginal row of setae less evident; female minor chela similar to male except short spines on cutting margin of pollex relatively smaller. Inner margin of cheliped carpus smooth (Fig. 5 A, B; Rodrigues da Costa 1968: unnumb. pl., fig. 1); ventral surface of cheliped merus smooth, without teeth. Proportionally short ambulatory legs, folded P5 merus only reaching middle portion of anterolateral margin (Fig. 5 A, B; Rodrigues da Costa 1968: unnumb. pl., fig. 1), meri with microscopic granules along posterior margin, unarmed. P5 dactylus upcurved, spur-like subdistal extension absent. Fused thoracic sternites 1, 2 (Figs. 50 G; 85C) broadly triangular, short; fused thoracic sternites 3, 4 (Figs. 50 G; 85C) relatively broad. Male pleon (Figs. 50 G; 57D; Rodrigues da Costa 1968: fig. 2) with rectangular somite 6, fused somites 3–5 convex; telson conspicuously proportionally long, narrow. Sterno-pleonal cavity of male deep, Press-button for pleonal holding as small, short tubercle posterior to thoracic sternal suture 4/5 at edge of sternopleonal cavity. Male thoracic sternite 8 (Fig. 57 D) short, quadrate; “supplementary plate” short, rectangular; structures tightly appressed with most of penis concealed. G1 (Fig. 66 D, E; Rodrigues da Costa 1968: fig. 4) stout, straight, distal segment with spinules. G2 (Fig. 66 F) about 3/4 G1 length, straight, slender, distal segment long, slightly curved. Somites of female pleon (Fig. 85 C) with convex lateral margins; telson proportionally short. Sterno-pleonal cavity of female (Fig. 89 I) shallow, vulvae conspicuously close together on anterior margin of thoracic sternite 6 close to suture 5/6.
Etymology. The name is derived from the arbitrary combination of ambon, Greek for “crest” and “ridge”, in reference to the suborbital ridge of the type species, a character unique for Chasmocarcinidae.
Remarks. Amboplax n. gen. is described for Chasmocarcinus peresi Rodrigues da Costa, 1968, a very distinctive species that can be easily separated from other chasmocarcinids by several important characters. One is the presence of a suborbital ridge (“suborbital carina” of Coelho & Coelho 1989: 813) below and subparallel to the suborbital margin of the orbit. This suborbital ridge is actually an extension of the inner edge of the suborbital margin that forms a separate, narrow transverse shelf-like structure on the suborbital region (Fig. 22 G). Also unique are the short and stout G1 (Fig. 66 D, E; Rodrigues da Costa 1968: fig. 4), vulvae that are located close together (Fig. 89 I) in contrast to being farther apart as in other chasmocarcinines, orbits that are slightly wider (“oblique” in Rodrigues da Costa’s 1968 description) (Fig. 22 F) than in Chasmocarcinus, and the long, straight lateral margins of the epistome (Fig. 22 F, G), typically circular in Chasmocarcinus. The press-button for pleonal holding of Amboplax peresi sits right at the edge of the anterior part of sternite 5, just before suture 4/5, in contrast to Chasmocarcinus, where the press-button is inside the sterno-pleonal cavity below the rim of the cavity.
The structure of the minor chela in A. peresi closely resembles that of some species of Camatopsis (e.g., C. rubida and C. africana n. sp.; Fig. 38 B, D, F, H, J) in that the cutting margins are blade-like, with sharp teeth along the distal quarter and lined with short setae, and the distal part of at least the pollex has sharp spines (e.g., Fig. 37 F). The minor chela of A. peresi superficially resembles that of Chasmocarcinus chacei in that the cutting margin of the pollex only has low teeth and the distal quarter has a few small spines (Fig. 35 J). The margins and submarginal parts, however, are not lined with setae in C. chacei.
Distribution. Western Atlantic region: Brazil.
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 27-28, DOI: 10.11646/zootaxa.4209.1.1, http://zenodo.org/record/272646