Gecarcinus Leach 1814

Toledano-Carrasco, Ia Atzimba, Villalobos, José Luis & Álvarez, Fernando, 2021, A morphological, phylogenetic and phylogeographic reappraisal of the land crabs Gecarcinus quadratus De Saussure, 1853, and G. lateralis Fréminville in Guérin 1832 (Decapoda: Gecarcinidae). Are they different species?, Zootaxa 5048 (2), pp. 215-236 : 225-226

Genus Gecarcinus Leach 1814

Diagnosis. Carapace transversely oval with branchial regions inflated. Pterygostomian regions glabrous. Anterior part of lateral border marked by marginal line. Fronto-orbital border 1/2 or less than 1/2 greatest breadth of carapace. Front strongly deflexed, frontal edge horizontal, from 1/5 to 1/8 the greatest breadth of carapace. Orbits deep, not much wider than high, outer angle obtuse, not prominent, inner angle as stout angular tooth touching inferior angle of front; a deep U-shaped sinus next to it. Eyes nearly filling orbits ( Rathbun 1918). Proepistome hardly discernible, inserted under lower margin of narrow front ( Guinot et al. 2018). Epistomal edge sometimes covered by third maxillipeds. Oral cavity subcircular or rhomboid, wider in the middle, margins with dense strip of bristles; third maxillipeds separated, central opening rhomboid, exposing mandibles; ischium and merus broad, subequal in length, merus suboval, hiding short and robust palp, the latter articulated in the middle of anterior margin of merus. Exognath hidden behind ischium, without flagellum, shorter than ischio-meral articulation. Chelipeds sturdy, almost smooth, in adults can be slightly or evidently different in size and robustness (smaller and weaker in females); internal surface of palm of major chela with or without tuberculated longitudinal ridge. Robust ambulatory appendages, second pair longest, last 3 joints armed with spines, dactyli with 4-6 longitudinal rows. First male pleopod (G1) semi-cylindrical, terminal apical element more than 1/3 of G1 length, extending distally well beyond apex, as narrow, chitinous, semi-cylindrical structure, open with longitudinal furrow in lateral view, distal end widening in caudal and cephalic views. Main axis of G1 with suture mostly visible in mesial view, distal portion of cephalic surface, ending in finger-like projection, pointed and elongated or rounded and relatively short, directed apically or cephalically, ornamented with slender bristles extending distally ( Fig. 3A, B ) ( Toledano-Carrasco 2019). In lateral view, distal portion of main axis excavated, ending in two projections, a corneous process and finger-like projection of cephalic surface; the former extending distally, rectangular, smoothly ribbed, ending obliquely with cephalic margin reaching beyond caudal one, the latter sharply triangular, about 1/2 as long as the former ( Fig. 3C ). In caudal view, excavation of G1 main axis evident, corneous process inclined mesially. In cephalic view, apical plate directed distolaterally, fused to distal end of lateral surface, apically concave; marginal suture along cephalic surface of main axis ( Fig. 3A, B ) ( Toledano-Carrasco 2019).

Sternal surface of carapace with bands of short setae between sternites 4–5. Sternite 1 as small triangular tooth, not separated by suture from sternite 2; sternite 2 semi-ovate or trapezoidal; suture 2/3 straight, weakly marked, or V-shaped and well-developed; sternites 3–4 completely fused with straight or convex, obliquely directed lateral margins, thus not restricted at level of first pereopods (P1). Sternites 5–7 with similar shape, sutures well defined. Sternite 8 totally hidden when pleon is folded ( Figs. 2B, F ; 3D, E, F ), posterior emargination reaching sternite 7 at level of narrow median bridge situated at level of suture 7/8. Another weak median bridge present at level of suture 6/7 ( G. ruricola ) or traces of it ( G. lateralis , G. quadratus ). Male genital orifice completely sternal ( Guinot, 1979). Sternites 3–4 with sterno-pleonal rim well-marked or rounded and slightly marked. Locking structure formed by button covered with setae in G. ruricola and G. quadratus ( Köhnk et al. 2017; Guinot et al. 2018). Abdomen subtriangular in males, semi-circular in females, with 7 articulated somites in both sexes ( Figs. 2B, F ; 3D, E, F ) ( Toledano-Carrasco 2019). Margins of abdomen with dense tufts of hydrophilic setae, facing row of setae on adjacent portions of last sternites ( Bliss 1963; Wolcott 1984), Müller’s channel absent ( De Oliveira 2014).

Remarks. Guinot et al. (2018) reexamined all gecarcinid morphological characters, and divided the family Gecarcinidae into two subclades. The first clade included: Cardisoma Latreille, 1828 , Discoplax A. Milne-Edwards, 1867 , and Tuerkayana Guinot et al., 2018 ; the second subclade, with a higher degree of terrestriality, included: Gecarcinus Leach, 1814 , Gecarcoidea H. Milne-Edwards, 1837 , and Johngarthia Türkay, 1970 . Among the important characters shared by Gecarcinus and Johngarthia is the stridulatory apparatus, which is used to produce sounds when these crabs are disturbed in their retreats ( Abele et al. 1973). The sound is produced when oblique rows of rugosities on the subhepatic region are rubbed against the tuberculated internal margin of the merus of the cheliped in G. quadratus , or against a longitudinal tuberculated ridge on the internal surface of the palm of the major cheliped in G. lateralis ( Klaassen 1973; Abele et al. 1973).

The ability to absorb water from the substrate is well developed in Gecarcoidea natalis ( Pocock, 1889) and Gecarcinus lateralis , in which setal tufts extend along the first three pleonal somites and on the coxa of P5 to establish a connection with the pericardial sacs ( Bliss 1963, 1968; Greenaway 1988). In Gecarcinus ruricola and Gecarcoidea lalandii H. Milne Edwards, 1837 , the posterior margin of sternite 7 is lined with dense hydrophilic setae in contact with tufts of setae on the coxa of P5 and pleonal somites 1-3 ( Guinot et al. 2018).

Figures

FIGURE 2. Gecarcinus lateralis from Río Prieto, Veracruz (CNCR 33942): A) frontal view, B) ventral vie, C) rows of spines in the dactyli of the pereiopods, D) major chela. Gecarcinus quadratus from Boca del Cielo, Chiapas (CNCR 34630): E) frontal view, F) ventral view, G) rows of spines in the dactyli of the pereiopods, H) major chela.

FIGURE 3. Left male first gonopod (G1): Cephalic view, A) G. lateralis from Río Prieto, Veracruz (CNCR 33942); B) G. quadratus from Boca del Cielo, Chiapas (CNCR 34630). Lateral view, C) G. quadratus from Boca del Cielo, Chiapas (CNCR 34630). Ventral surface, D) G. lateralis female from Barra de Sontecomapan, Veracruz (CNCR 33943); E) G. quadratus female from Playa Higuera Blanca, Nayarit (CNCR 34012); F) G. ruricola male from Cuba (MNHN-IU-2017-8392). Frontal view, G) G. ruricola from Cuba (MNHN-IU-2017-8392) (G. ruricola photos modified from Guinot et al., 2018).