Gecarcinus Leach 1814

Genus Gecarcinus Leach, 1814 Gecarcinus Leach, 1814: 427. TYPE SPECIES. — Cancer ruricola Linnaeus, 1758. Neotype of Gecarcinus ruricola, a male from Cuba, MNHN-IU-2017-8392 (= MNHNB 13155), designated by Guinot et al. (2018). Monogeneric. DIAGNOSIS Carapace (Fig. 1D) Carapace much broader than long, globular, with hepatic, subhepatic and branchial regions markedly inflated, even vaultlike; widest part of carapace considerably ahead of anterolateral angles of mesogastric region; area lateral to orbits proportionally very large, with fronto-orbital distance in adults about two-fifths carapace width (near half the carapace width in half-grown individuals) (Guinot et al. 2025: figs 1, 4A-G). Dorsal surface with well-pronounced grooves: cervical groove very deep, terminating anteriorly in a pit near orbital angle; median (or urogastric) groove very deep; longitudinal mesogastric groove very deep, rising towards frontal margin, making certain regions well defined, such as e.g. the gastric and cardiac regions. Numerous striae along lateral margins of carapace (Guinot et al. 2025: figs 1, 2A). Fronto-orbital border half or less than half width of carapace. Front long, proportionally narrow and slightly widening underneath, strongly produced and deflexed, with concave lateral margins and upturned lower margin. Mesial lobe of infraorbital margin elongated, curved around ventrolateral edge of front, covered by front edge (Guinot et al. 2025: fig. 2B-D). Antero-lateral margins rounded; proximally with a short row of more or less marked granules, well pronounced in small and medium-sized male individuals and in females, then tending to largely disappear in very large-sized individuals; and so then margins smooth (Guinot et al. 2025: figs 1, 2A). Cephalic structures (Fig. 3C) Antennules very small, folded obliquely. Antenna very short, visible but markedly recessed.Orbits small and deep, englobed in carapace; outer angle not marked. Eyestalks relatively short, curved (Guinot et al. 2025: figs 2B-D, 6). Proepistome (Fig. 3C) Proepistome small, not completely covered by triangular median process of subfrontal plate, thus visible. Subhepatic region with many striae; pterygostomial region glabrous, naked (Guinot et al. 2025: figs 2B-D, 6). Buccal frame, epistome, mxp3 and pterygostome (Figs 3C; 4B) Buccal frame rhomboid to subcircular, not circumscribed laterally, with anterior border being the anterior limit of linear margin of epistome (Guinot et al. 2025: fig. 6C). Mxp3 short; when applied well against buccal cavity, with anterior margin of merus reaching only epistome or advancing either to level of antennules or to frontal margin, thus showing phenotypic variations (Guinot et al. 2025: fig. 6); leaving between them narrow rhomboid gap, in which mandibles are exposed. Ischium and merus very unequal, their articulation clearly oblique; ischium smaller, merus elongated, both strongly directed obliquely; with marked longitudinal groove. Merus triangular, narrowing anteriorly, with anterior margin entire, not emarginate; meri of both sides more or less joining medially; palp with first article fused to merus internal surface and two mobile distal articles concealed, not visible (Guinot et al. 2025: figs 2B-D, 4D, 6). Exopod conspicuously reduced to more or less narrow suboval plate, entirely concealed; apex not reaching ischium-merus articulation, with lateral dense setae; without flagellum (Tavares 1989: fig. 14f; Guinot et al. 2025: fig. 11A). Subhepatic region with many striae; pterygostomial region glabrous, naked (Guinot et al. 2025: figs 2B-D, 6). Chelipeds Adult male chelipeds massive but not extraordinarily enlarged (Guinot et al. 2025: figs 1, 3A, B, 4B, Gecarcinus ruricola), equal or slightly to moderately subequal, possibly distinctly unequal in large individuals; narrowly gaping; heterochely and heterodonty usually not or slightly marked, possibly occasionally pronounced (Guinot et al. 2025: fig. 1B); occlusal margins of fingers with small, spaced teeth on both sides; in the rare cases of greatly uneven chelipeds (heterochely), a more pronounced gap and pronounced heterodonty (Guinot et al. 2025: fig. 1B). Merus with curved internal surface and with marked denticles on inner lower margin; carpus with conspicuous denticles on upper margin. Sexual dimorphism moderate. Ambulatory legs Very spiny. P3 propodus with lateral carinae bearing four rows of prominently and similarly developed spines; dactylus with lateral carinae bearing six rows of prominently and similarly developed spines (Guinot et al. 2025: fig. 1, Gecarcinus ruricola). Sterno-pleonal cavity and male pleon (Fig. 5B, D) Sterno-pleonal cavity long, its extremity ending either very close to suture 2/3 and even almost exceeding it, or slightly distant; relatively shallow, completely glabrous; ridge on sternite 4 marked, surrounding telson only (Guinot et al. 2025: fig. 2E). Male pleon rather long, with all somites free plus telson; margins only with sparse setae; somite 6 with convex margins (Guinot et al. 2025: fig. 3C). Female pleon (Fig. 9A) See below, Female pleon and pleopods. Thoracic sternum, locking pleonal structures and setal tufts (Fig. 5B, D) Thoracic sternum wide (especially at level of somite 5); sternite1 as small triangular tooth, not separated by suture from sternite 2, not recessed; sternite 2 semi-ovate with convex margins; suture 2/3 present, V-shaped; suture 3/4 absent, without lateral traces; sternites 3 + 4 completely fused, with convex, obliquely directed margins, thus not restricted at level of P1; 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; another weak median bridge at level of suture 6/7; deep median line only along sternite 7 (Guinot et al. 2025: fig. 2E). Locking pleonal structure as rather large prominence about in middle of sternite 5, but pleonal sockets not delineated, so no longer functional. Setal tufts of more or less dense hydrophilic setae located along first pleonal somites margins and at junction of pleon with carapace (Guinot et al. 2025: fig. 3C). Male gonopore and penis (Fig. 5D) Male gonopore widely sternal and penis emerging rather far from P5 coxo-sternal condyle, just below thoracic sternal suture 7/8 (Guinot et al. 2025: fig. 3D). G1 and G2 G1 relatively short, thick then tapering, with unequal setae at its tip; apex rather long (called ‘palp’, see Türkay 1970: 336, fig. 1a-f); laterally, a very long, narrow horny tube (called ‘Terminalanhang’ by Türkay 1970: 334, or appendix) completely rolled up on itself, with the opening displaced towards its distal extremity, and clearly exceeding G1 tip; several horny setae at its base; horny tube reaching suture 5/6 (Tavares 1989: fig. 18d; Guinot et al. 2025: fig. 3E, F). G2 tiny, without flagellum. Vulvae Protruding, obliquely directed, normally occluded by a rigid calcified immobile operculum (see Hartnoll 1968; Guinot et al. 2025: fig. 3G). BIOLOGY For the biology of Gecarcinus ruricola, see Guinot et al. (2025).