Bioacoustics of Gecarcinidae

FAMILIAL DESCRIPTION Carapace ( Figs 1 D-F; 14A, B) Carapace transversally ovate, strongly longitudinally and transversally convex, widest in anterior half. Dorsal surface more or less flat, with certain regions well-delineated, such as gastric and cardiac regions; branchial regions usually greatly swollen, even vaulted-like expanded ( Gecarcinu s); protogastric area and branchial regions becoming conspicuously swollen in large individuals (e.g. Johngarthia cocoensis ); all grooves similar or certain grooves more pronounced, in particular cervical groove deep or moderately developed, ending anteriorly in a pit near orbital angle; median (or urogastric) groove deep; longitudinal mesogastric groove very deep, rising more or less towards frontal margin. Fronto-orbital border half or less than half width of carapace. Antero-lateral borders rounded, either not well delimitated, with only a marginal line on anterior part not distinctly joining orbital border ( Gecarcinus , Gecarcoidea , Hartnollius , and Johngarthia pro parte ) or lined by 6-16 teeth ( Johngarthia cocoensis , see Perger et al. 2011 : figs 2, 3A), more marked in females ( Perger et al. 2011 : fig. 3B). Front long or short, more or less narrow, usually nearly straight or slightly arcuate, variously produced and deflexed. Cephalic structures ( Figs 2 D-F; 3B, C) Antennular fossae narrow (see Tavares 1989 : fig. 7, Gecarcoidea lalandii ). Antennules very small, folded obliquely, hidden under the front. Antenna very small, visible but markedly recessed ( Gecarcinus ) or completely exposed ( Hartnolliu s), in orbital hiatus ( Gecarcoidea humei only) or not ( Gecarcinus , Gecarcoidea , Hartnollius , Johngarthia ). Orbits small and deep, englobed in carapace ( Gecarcinus ) or small and more or less oblique; outer angle usually obtuse and not prominent, or with tooth ( Johngarthia cocoensis , see Perger et al. 2011 : figs 2, 3). Eyestalks small, more or less curved. Stridulatory apparatus Absent. No suborbital crest acting as pars stridens. Subhepatic and pterygostomial striae acting as pars stridens and inner surface of palm of chelae used as stridulatory plectrum, at least in Hartnollius species, probably also in Gecarcinus that has similar striae. Not documented in other gecarcinid species. Proepistome ( Figs 2 D-F; 3B, C) Proepistome very small, not completely covered by subfrontal plate, thus visible ( Gecarcinus , see Guinot et al. 2025 : fig. 2C), or completely covered by subfrontal plate, thus hardly discernible ( Hartnollius , see Guinot et al. 2025 : figs 10B, C, 12A-C; Gecarcoidea , see Tavares 1989 : figs 7, 8; Johngarthia , see Tavares 1989 : fig. 9, as Gecarcinus lagostoma ). Buccal frame, epistome, mxp3 and pterygostome ( Figs 2 D-F; 3B, C; 4B) Buccal frame short, quadrate, subcircular or rhomboid, not delineated laterally, with its anterior border being the anterior limit of linear margin of epistome. Mxp3 relatively short, reaching front or antennular fossae ( Gecarcinus ) or just about reaching epistome ( Johngarthia pro parte ) or not reaching epistome ( Hartnollius ), even very far from setose epistome ( Gecarcoidea , see Tavares 1989 : fig. 7), from until leaving gap between lateral borders of buccal frame and exposing epistomial tooth ( Johngarthia cocoensis ); mxp3 leaving between them rhomboidal gap, either proportionally narrow ( Gecarcinus , Hartnolliu s), or more or less wide ( Gecarcoidea , Johngarthia pro parte ), or very wide (e.g. Johngarthia cocoensis ), in which mandibles are exposed. Ischium and merus either subequal ( Gecarcinus , Johngarthia ) or merus shorter than ischium ( Gecarcoidea ). Anterior margin of merus usually emarginate, sometimes deeply fissured, or with V-shaped emargination ( Johngarthia cocoensis , see Perger et al. 2011 : fig. 3), or not emarginate ( Gecarcinus , see Guinot et al. 2025 : figs 2B-D, 6). Palp with first article fused to merus internal surface and with only two distal articles being mobile; palp either completely concealed ( Gecarcinus ), or with last two mobile articles more or less externally visible (other genera); meri of both sides joining medially ( Gecarcinus , Johngarthia pro parte ). Exopod completely concealed, as narrow plate, proportionally short or long, its apex reaching or not ischium-merus articulation; without flagellum. Pterygostomial area not setose, glabrous. Chelipeds Adult male chelipeds rather massive but not conspicuously enlarged, equal or slightly to moderately subequal, possibly distinctly unequal in large individuals; narrowly gaping; heterochely and heterodonty usually slightly marked or not marked ( Fig. 14A, B ), more marked in old males ( Hartnollius ). Sterno-pleonal cavity and male pleon ( Figs 5B, D ; 6C, D ; 7C, D ) Sterno-pleonal cavity relatively shallow, long, its apex rather close to suture 2/3 ( Gecarcinus ), or short and not close to sternal suture 2/3 ( Hartnollius , Gecarcoidea , Johngarthia ); ridge on sternite 4 marked, surrounding telson only. Male pleon long, with all somites free plus telson; somite 6 with convex margins ( Hartnollius lateralis , Johngarthia cocoensis ); bordered by narrow fringe of setae along margins. Female pleon ( Fig. 9 A-E) Female pleon very wide (narrower only in Johngarthia , fig.9B), discoid, with telson in the form of broad triangle, leaving laterally and further forward, a sternal portion visible in addition to the episternites; bordered by narrow fringe of setae along margins. See below, Female pleon and pleopods. Thoracic sternum, locking pleonal structures and setal tufts ( Figs 5B, D ; 6C, D ; 7C, D ) Thoracic sternum wide, wider (especially at level of somite 5) in Gecarcoidea than in Gecarcinus and Johngarthia . Sternite 1 as small triangular tooth, not separated by suture from sternite 2, not recessed; sternite 2 semi-ovate or trapezoidal; suture 2/3 present, straigh or slighly concave ( Hartnollius ) or V-shaped ( Gecarcinus ); suture 3/4 absent, without lateral traces; sternites 3 + 4 completely fused, margins straight more or less sinuous ( Gecarcoidea , Johngarthia pro parte ), sligthly convex ( J. cocoensis , see Perger et al. 2011 : fig. 4C), obliquely directed ( Gecarcinus ), or concave ( Hartnollius ); sutures 4/5 to 7/8 interrupted; sternites 5-7 similarly shaped, sutures well defined; suture 7/8 rather short; sternite 8 not developed medially, totally hidden when pleon is folded; posterior emargination reaching sternite 7 at level of narrow median bridge at level of suture 7/8; deep median line along sternite 7 in all genera ( Guinot et al. 2018 : fig. 7B, C; Guinot et al. 2025 : fig. 3D, Gecarcinus ), except in Gecarcoidea lalandii ( Guinot et al. 2018 : fig. 7F, G), where it slightly extends on sternite 6, and in G. humei . Locking structure as relatively small glabrous prominence ( Gecarcinus ), or as thickened oblique prominence covered by setae (present all along slopes of the cavity) ( Hartnollius lateralis , H. quadratus ), or as less prominent crest ( Gecarcoidea lalandii , Johngarthia planata ) but pleonal sockets not delineated, so pleonal locking non-functional. Setal tufts of more or less dense hydrophilic setae located along first somites margins of pleon, at its junction with carapace, on posterior margin of sternite 7, and slightly on P5 coxae ( Hartnollius lateralis , see Bliss 1963 : fig. 20; 1968: fig. 14; 1979: fig. 4, as Gecarcinus lateralis ). Male gonopore and penis ( Figs 5D ; 7C, D ) Male gonopore widely sternal; penis emerging rather far from P5 coxo-sternal condyle, very close to thoracic sternal suture 7/8 ( Johngarthia planata , see Guinot 1979 : fig. 54D, as Gecarcinus planatus ; Guinot & Bouchard 1998 : fig. 25A, as Gecarcinus planatus ; Johngarthia weileri , see N. K. Ng et al. 2007: 246 , fig. 4H, as Gecarcinus weileri ). G1 and G2 ( Figs 5F ; 7D ) G1 varying from relatively short to longer and from straight to gently curved.With very long, narrow horny tube, either completely rolled up on itself ( Tavares 1989 : fig. 18d, Gecarcinus ; Guinot et al. 2025 : fig. 3E, F, Gecarcinus ) or not folded and open all along ( Tavares 1989 : fig. 18e, as Gecarcinus lateralis ; Guinot et al. 2025 : fig. 13A and 13C, Hartnollius lateralis , H. quadratus , respectively). Pectinate distal part either bent c. 75-80° from vertical ( Lai et al. 2017 : fig. 8A, Gecarcoidea lalandii ) or bent c. 45° from vertical ( Tavares 1989 : fig. 17c, G. natalis ; Lai et al. 2017 : fig. 8C and 8B, D, G. humei , G. natalis , respectively). Caudal process shorter than prominently developed mesial process, with end piece at level of terminal setae ( Perger et al. 2011 : fig. 4A, B, Johngarthia cocoensis ). Vulvae On sternite 5, protruding, more or less close to suture 5/6, normally occluded by rigid calcified, immobile operculum. Slightly obliquely directed ( Gecarcinus , see Hartnoll 1968 ; Guinot et al. 2025 : fig. 3G), or oriented nearly horizontally ( Guinot et al. 2025 : fig 13B, Hartnollius ). Sternal cover narrow on inner margin ( Gecarcoidea , Johngarthia ).