Bioacoustics of Discoplax

DIAGNOSIS Carapace ( Fig. 1B ) Carapace more or less rounded, not inflated ( Guinot et al. 2018 : fig. 4D, E; N. K. Ng et al. 2019 : fig. 2B and 2C, Discoplax longipes , D. gracilipes , respectively; Kobayashi & Naruse 2023 : fig. 1A, D. gracilipes ), with the maximum recorded size being 61.7-71.9 mm for D. longipes . Dorsal surface with regions distinctly marked, covered with granules. Fronto-orbital border more than half width of carapace. Front relatively wide, not much deflexed. Anterolateral margin well delimited, with a marked cleft (in the diagnosis of Gecarcinidae sensu lato by Davie et al. 2015c: 1117 , “anterolateral margins unarmed” must be corrected). Subhepatic and subbranchial areas heavily striated ( Ng & Shih 2015 : fig. 7A-D and G, H, D. longipes and D. michalis , respectively; Guinot et al. 2018 : fig. 6A, D. longipes ); posterolateral regions with pronounced striae ( Ng & Shih 2015 : figs 1-3, 6A-C, D. longipes ; 4A, B, 5, 6D-F, D. michalis ). Cephalic structures ( Fig. 2A ) Orbits developed but not completely filled by eyestalks, not closed laterally. Suborbital area conspicuously granulated ( Ng & Guinot 2001 : figs 5B and 9C, D. longipes , D. gracilipes , respectively; Ng & Shih 2015 : figs 4C, 7G, H and 7A, B, D. michalis , D. longipes , respectively); with suborbital stridulatory crest, see below. Stridulatory apparatus ( Fig. 2A ) Suborbital crest long, forming stridulatory pars stridens consisting of very small, tightly packed granules ( Ng & Guinot 2001 : figs 5B and 9C, Discoplax longipes , D. gracilipes , respectively; Ng & Shih 2015 : fig. 7A, B and 7G, H, D. longipes , D. michalis , respectively; Guinot et al. 2018 : fig. 6A, D. longipes ; Kobayashi & Naruse 2023 : fig. 3A, B, D. gracilipes ). Plectrum consisting of thickened ridge located on inner margin of P1 merus ( Guinot et al. 2018 : fig. 11A, D. longipes ), present in both sexes. Proepistome ( Fig. 2A ) Proepistome small, narrow, subquadrate, low, recessed. Buccal frame, epistome, mxp3 and pterygostome ( Fig. 2A ) Buccal frame elongate-subquadrate, circumscribed laterally, with wide, thick, straight anterior margin joining anterior border of epistome as two concave edges joining medially. Mxp3 relatively long, leaving wide rhomboidal gap between them; merus with emargination on anterior border; palp normal, with three mobile articles externally visible. Exopod exposed, with concealed flagellum ( Ng & Shih 2015 : figs 1C, G, 7A, B and 4D, 7G, H, Discoplax longipes , D. michalis , respectively). Pterygostomial area setose ( Kobayashi & Naruse 2023 : fig. 1B). Chelipeds Male chelipeds sometimes only slightly subequal; heterochely very weak ( Ng & Shih 2015 : figs 1A, B, E and 3C, 4A, B, 5, Discoplax longipes , D. michalis , respectively; Kobayashi & Naruse 2023 : figs 2, 3B, D. gracilipes ). Ambulatory legs P2, P3 elongated and slender, especially in D. gracilipes ( Kobayashi & Naruse 2023 : fig. 1), covered with distinct granules and/or striae, appearing very rugose; dorsal margin of merus granular; only lateral margins of propodus and dactylus equipped with short stiff spines or setae ( Ng & Shih 2015 : fig. 10A, B, G and 10E, F, K, L, Discoplax longipes , D. michalis , respectively). Sterno-pleonal cavity and male pleon ( Figs 6A ; 7A ) Sterno-pleonal cavity deep, wide. Male pleon very wide (resembling a female pleon), long, reaching almost suture 2/3. Somite 6 very short, particularly broad and rounded, with convex margins; telson short, rounded; ridge on sternite 4 faint, surrounding telson only ( Ng & Shih 2015 : figs 1D, G, 8A-F and 4D, 9, D. longipes , D. michalis , respectively; Guinot et al. 2018 : fig. 5B, D. longipes ). Female pleon ( Fig. 8D ) Relatively narrow, without strong sexual dimorphic. See below, Female pleon and pleopods and The case of the genus Discoplax A. Milne Edwards, 1867 . Thoracic sternum, locking pleonal structures and setal tufts ( Figs 6A ; 7A ) Thoracic sternum wide, especially at level of sternite 4 ( Ng & Shih 2015 : figs 1D, G, 8A-E and 4D, 9, Discoplax longipes , D. michalis , respectively; Guinot et al. 2018 : fig. 5A, B, D. longipes ); sternites 1-3 forming low plate; sternite 1 triangular, separated from sternite 2 by thick ridge anteriorly lined with granules (suture 1/2); sternite 2 very short, flattened, separated from sternite 3 by thin, straight, complete sulcus (suture 2/3); no suture 3/4, without lateral traces; sternite 4 short and wide, with weakly concave lateral margins, thus only weakly restricted between the P1; faint rim delineating margin of sterno-pleonal cavity at level of telson; episternites 4-6 narrow, longitudinally directed; posterior sternites on same plane as preceding sternites; suture 7/8 rather short ( Guinot et al. 2018 : fig. 5B, C, D. longipes ); sternite 8 short but developed medially, the posterior emargination being far from reaching sternite 7; no visible portion of sternite 8 when pleon is folded. Median line on sternites 8 and 7, and reappearing on sternite 6 in front of thick and wide median bridge at level of suture 6/7 ( Guinot et al. 2018 : fig. 5B, C, D. longipes ). Press buttons on sternite 5 acute, close to suture 4/5, observed in D. longipes ( Guinot et al. 2018 : fig. 5B) and D. gracilipes (character not mentioned in the description of D. michalis ), however barely visible due to dense setal cover (a trace in a female D. longipes 32.6 × 37.0 mm) ( Köhnk et al. 2017 : fig. 19a); socket on pleonal somite 6 not easily noticeable, except for small cuticular border ( Köhnk et al. 2017 : fig. 19b), thus pleonal locking perhaps no longer functional, at least over certain size. Pleon bordered by thick fringe of setae along its margins; absence or strong reduction of hydrophilic setal tufts in other locations. Male gonopore and penis ( Fig. 7A ) Male gonopore close to P5 coxo-sternal condyle but penis emerging in middle of sternite 8 thus far from thoracic sternal suture 7/8; penis proximally narrow, then more expanded ( Guinot et al. 2018 : fig. 5C, D. longipes ). G1 and G2 G1 proportionally short, stout, almost straight, tip reaching sternal suture 4/5 ( Ng & Shih 2015 : figs 11, 14A-D, 15A, B, 16A, B and 14E-H, 15C, D, 16C, D, D. longipes , D. michalis , respectively; Kobayashi & Naruse 2023 : figs 4A-D and E, F and G, H, D, D. gracilipes , D. longipes , D. michalis , respectively). G2 short, spatulate, with pointed tip (see Ng & Shih 2015 : fig. 11A, D. longipes ; Kobayashi & Naruse 2023: 181 ). Vulvae Vulvae small, operculum moderately salient.

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.

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 ).