Gecarcinus Leach, 1814

Genus Gecarcinus Leach, 1814 View in CoL new status

Gecarcinus Leach, 1814: 430 View in CoL . — Toledano-Carrasco et al. 2021 pro parte: 221, fig. 5.

Gecarcinus View in CoL of all authors, except for references to Gecarcinus ruricola View in CoL , see under this name.

Geocarcinus – Miers 1886: 217 (incorrect emendation of Gecarcinus Leach, 1814 ). — Young 1900: 236, 237.

TYPE SPECIES. — Cancer ruricola Linnaeus, 1758 , by subsequent designation by H. Milne Edwards (1837b: pl. 24), see Figs 1-4 View FIG View FIG View FIG View FIG , 6 View FIG ; 10A, D View FIG ; 11A View FIG ; 16-18 View FIG View FIG View FIG ; 21 View FIG , Table 1. View TABLE

OTHER INCLUDED SPECIES. — None.

EMENDED DIAGNOSIS

Carapace

Carapace much broader than long, globular, with hepatic, subhepatic and branchial regions markedly inflated, even vaultlike; widest part of carapace considerably ahead of antero-lateral angles of mesogastric region; area lateral to the orbits proportionally very large, with fronto-orbital distance in adults about two-fifths carapace width (near half the carapace width in half-grown individuals). 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. Front long, strongly produced and deflexed, proportionally narrow and slightly widening underneath, with concave lateral margins and upturned lower margin. Mesial lobe of infraorbital margin elongated, curved around ventrolateral edge of front, covered by front edge. 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; then margins smooth.

Cephalic structures

Antennules very small, folded obliquely. Antennae very short, visible but markedly recessed. Orbits small, englobed in carapace, deep; outer angle not marked. Eyestalks relatively short, curved.

Proepistome, epistome and pterygostome

Proepistome not completely covered by triangular median process of subfrontal plate, thus visible. Epistome developed, with one median crest and a lateral crest on each side. Buccal cavity rhomboid. Subhepatic and pterygostomial region glabrous, with many striae.

Mxp3

Mxp3 (when applied well against buccal cavity) with anterior margin of merus reaching only the epistome or advancing either to the level of the antennules or to the frontal margin (see Phenotypic variations, under Gecarcinus ruricola ); leaving between them a narrow rhomboid gap, in which mandibles are exposed. Ischium and merus very unequal, their articulation clearly oblique; ischium smaller and merus elongated, strongly obliquely directed; with marked longitudinal groove. Merus triangular, narrowing anteriorly, with anterior margin

entire, not emarginate; meri of both sides more or less joining medially; palp inserted below merus: palp with first article fused to merus internal surface and with two mobile distal articles concealed, not visible. Exopod of mxp3 completely concealed, apex not reaching ischium-merus articulation, as well visible suboval plate, with lateral dense setae; without flagellum.

Chelipeds

Adult male chelipeds massive but not extraordinarily enlarged, 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 ( Fig. 1B View FIG ); 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 ( Fig. 1B View FIG ). 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.

Sterno-pleonal cavity and pleon

Sterno-pleonal cavity completely glabrous, long; its extremity ending either very close to suture 2/3 and even almost exceeding it, sometimes very slightly distant, with marked ridge around telson. Male pleon rather long, with all somites free plus telson; margins only with sparse setae; somite 6 with convex margins.

Thoracic sternum, locking pleonal structures and setal tufts Thoracic sternum wide (especially at level of somite 5); sternite 1 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 trace; 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.

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.

Male gonopore and penis

Male gonopore and penis at level of suture 7/8, emerging rather far from P5 coxo-sternal condyle.

G1 and G2

G1 tapering at its extremity, 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. G2 tiny, without flagellum.

Vulvae

Protruding, obliquely directed, normally occluded by a rigid calcified immobile operculum (see Hartnoll 1968).

REMARKS

In Le Règne Animal of Cuvier, H. Milne Edwards (1837b: pl. 24) illustrated Gecarcinus ruricola as his representation of Gecarcinus , and, on the basis of the title of this work, this can be considered a type designation. Some authors cite the year 1838, but, according to Cowan (1976), plate 24 was produced in March 1837 (see Ng et al. 2008: 215).

The genus Gecarcinus new status differs from Hartnollius n. gen. by many characters, which are listed in Table 1 View TABLE . The level of generality of the distinguishing characters between them is similar to those that differentiate Gecarcinus from Johngarthia , and their number is even greater.

No gecarcinid has such a globose carapace and small deeply embedded orbits as Gecarcinus ruricola . According to Britton et al. (1982), during growth an expansion in the lateral carapace dimension provides a small increase in branchial volume but a larger increase in surface area, which presumably facilitates gaseous exchanges with the atmosphere: small individuals ( 40 mm cw) of G. ruricola have approximately the same branchial chamber surface area than Hartnollius lateralis n. comb., whereas individuals of greater width have increasingly greater branchial area. If greater respiratory surface and respiratory capacity are desirable for G. ruricola , larger individuals will have a selective advantage. As it grows, the carapace swells more and more to accommodate the branchiostegal lung so that the proximal crest of the antero-lateral margin (see Fig. 4 View FIG A-C) becomes more and more attenuated until it disappears completely, and the orbits become enclosed in the carapace ( Figs 2A View FIG ; 17 View FIG ; 18 View FIG ).

Several species initially assigned to Gecarcinus were separated by Türkay (1970: 343), who created for them the subgenus Johngarthia , later elevated to generic rank ( Türkay 1987). Johngarthia Türkay, 1970 includes the type species Johngarthia planata (Stimpson, 1860) , J. lagostoma (H. Milne Edwards, 1837) , J. malpilenis (Faxon, 1893) , J. weileri (Sendler, 1912) , J. oceanica Perger, 2019 , and the enigmatic J. cocoensis Perger, Vargas & Wall, 2011 . The distinctive characters of Johngarthia mainly concerned the G1 (short, folded outwards, without a long, narrow horny tube), the merus of mxp3 with a slit-shaped fissure approximately on the inner margin, and the exopodite of mxp3 long, with setae extending beyond the ischium-merus articulation. Larval development also distinguishes Johngarthia planata from that of Hartnollius lateralis n. comb. ( Cabrera 1966; Cuesta et al. 2007). It is worth noting that Johngarthia lagostoma (see Tavares & Mendonça Jr 2022: fig. 47A, F) shows some resemblance to G. ruricola in the globular carapace, recessed antenna, long and obliquely directed merus of mxp3 that reaches almost the front. It is interesting to remind that Tavares (1991) found no synapomorphies to support the monophyly of Johngarthia . Colavite et al. (2021), studying the morphology of the first zoeal stage of J. lagostoma , endemic to the Atlantic oceanic islands of Ascencion, Trindade and Martin Vaz, Fernando de Noronha and the Rocas Atoll, suggested that the uncommon large eyes could be an autapomorphy of the species and again discussed the validity of this genus.