Discoplax A. Milne-Edwards 1867
Creators: name: Guinot, Danièle
name: Ng, Ngan Kee
name: Rodríguez Moreno, Paula A.
Publisher: name: Zenodo
Year: 2018
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Gecarcinidae
Discoplax

description: Genus Discoplax A. Milne-Edwards, 1867
TYPE SPECIES. — Discoplax longipes A. Milne-Edwards, 1867 (see N. K. Ng et al. in press: fig. 2C).
OTHER SPECIES. — Discoplax gracilipes Ng & Guinot, 2001 (see N. K. Ng et al. in press: fig. 2B); D. michalis Ng & Shih, 2015 (not examined).
DIAGNOSIS
Carapace not inflated (Fig. 4D, E), the maximum recorded size being 61.7× 71.9 mm for D. longipe s; dorsal surface with regions distinctly marked, covered with granules; posterolateral regions with pronounced striae (Ng & Shih 2015: fig. 6); anterolateral margin well delimited, with a marked cleft (so “anterolateral margins unarmed”, in the diagnosis of Gecarcinidae by Davie et al. 2015c: 1117, must be corrected); proepistome small, narrow, subquadrate; subhepatic and subbranchial areas heavily striated (Fig. 6A) (Ng & Shih 2015: fig. 7); orbit not closed laterally, with conspicuously granulated suborbital area; suborbital crest long, consisting of very small, close granules (Fig. 6A) (Ng & Guinot 2001: figs 5B, 6C, 9C; Ng & Shih 2015: fig. 7), here considered stridulatory pars stridens, rubbing on a thickened ridge (plectrum) located on inner magin of P1 merus (Fig. 11A), present in both sexes; male gonopore close to P5 coxo-sternal condyle and opening in middle of sternite 8 thus far from suture 7/8; penis proximally narrow, then more expanded (Fig. 5C); thoracic sternum not inclined posteriorly, wide, especially at level of sternite 4 (Fig. 5A, B) (Ng & Guinot 2001: fig. 4A; Ng & Shih 2015: figs 1G, 4D); sternites 1-3 forming a low plate; sternite 1 triangular, separated from sternite 2 by thick ridge anteriorly lined with granules (suture 1/2); sternite 2 short, flattened, separated from sternite 3 by thin, straight, complete sulcus (suture 2/3); no suture 3/4, without lateral trace; sternite 4 short and wide, with weakly concave lateral margins, thus only weakly restricted between the P1; very weak rim delineating margin of the sterno-pleonal cavity at level of telson; episternites 4-6 narrow, longitudinally directed; posterior sternites on same plane as preceding sternites; sternite 8 short but developed medially, the posterior emargination being far from reaching sternite 7; suture 7/8 rather short; thick and wide bridge at level of suture 6/7 (Fig. 5B, C) (Ng & Guinot 2001: fig. 4A); median line on sternite 8 and extending on sternite 7 only, not extending on sternite 6 (Fig. 5C); no visible portion of sternite 8 when pleon is folded; on sternite 5 acute pressbuttons close to suture 4/5, observed in D. longipes (Fig. 5B) and D. gracilipes (character not mentioned in the description of D. michalis), however hardly visible due to dense setal cover (a trace in female D. longipes 32.6× 37.0 mm) (Köhnk et al. 2017: fig. 19a), thus pleonal locking perhaps no longer functional over a certain size; socket on pleonal somite 6 not easily noticeable, except for small cuticular border (Köhnk et al. 2017: fig. 19b); sterno-pleonal cavity wide; pleon long, reaching sternite 3, subdistally wide; pleonal somite 4 especially broad; telson short, rounded (Fig. 5A) (Ng & Shih 2015: figs 8, 9); heterochely very weak, male chelipeds may be only slightly subequal; P2, P3 elongated and slender, covered with distinct granules and/or striae, appearing very rugose; dorsal margins of merus granulated; only lateral margins of propodus and dactylus with short stiff spines or setae (Ng & Shih 2015: fig. 10); G1 proportionally short (Ng & Shih 2015: figs 11, 13-15, D. longipes and D. michalis).
REMARKS
A particular character not previously noted in Discoplax is the presence of a large, laterally opened orbit, with the suborbital crest forming a stridulating pars stridens (Fig. 6A); a thickened ridge close to the inner margin of the cheliped merus represents the plectrum (Fig. 11A). This disposition has been observed in the both sexes of D. longipes and D. gracilipes of the MNHN collection. Given the similar morphology of the orbital area of D. michalis (Ng & Shih 2015: fig. 7G, H), a comparable stidulating apparatus may characterise this species. Note that stridulatory structures occur only in Discoplax and are lacking in Cardisoma sensu stricto and in Tuerkayana n. gen. (see below and Table 1).
The presence of a pleonal locking mechanism in Discoplax was a character overlooked by Ng & Guinot (2001), Ng & Davie (2012), Ng & Shih (2014, 2015) and subsequent authors. A structure is tangible in D. longipes (Fig. 5B) and D. gracilipes, i.e., a button close to suture 4/5 and surrounded by dense setae that obscure it (Köhnk et al. 2017: fig. 19a) (to be confirmed in D. michalis). This dense setal cover on the area probably makes the pleonal locking system non-functional in adults.
description: Published as part of Guinot, Danièle, Ng, Ngan Kee & Rodríguez Moreno, Paula A., 2018, Review of grapsoid families for the establishment of a new family for Leptograpsodes Montgomery, 1931, and a new genus of Gecarcinidae H. Milne Edwards, 1837 (Crustacea, Decapoda, Brachyura, Grapsoidea MacLeay, 1838), pp. 547-604 in Zoosystema 40 (26) on page 563, DOI: 10.5252/zoosystema2018v40a26, http://zenodo.org/record/4336858