Genus Johngarthia Türkay, 1970

TYPE SPECIES. — Gecarcinus planatus Stimpson, 1860, by original designation.

OTHER SPECIES. — Johngarthia lagostoma (H. Milne Edwards, 1837); J. malpilenis (Faxon, 1893); J. weileri (Sendler, 1912); J. cocoensis Perger, Vargas & Wall, 2011 .

DIFFERENTIAL GENERIC DIAGNOSIS

Gecarcinus and Johngarthia are uniform with respect of selected characters: proepistome hardly discernible, inserted under the lower margin of narrow front (Fig. 7D, Gecarcinus ruricola; for Johngarthia lagostoma see Tavares 1989: fig. 9, as Gecarcinus; for Johngarthia weileri see N. K. Ng et al. 2007: fig. 6H, as Gecarcinus); male gonopore emerging far from P5 coxo-sternal condyle (Fig. 7B, C, G . ruricola); for J. planata (see Guinot 1979: fig. 54D; Ehrardt 1968, as Gecarcinus; Guinot & Bouchard 1998: fig. 25A, as Gecarcinus); for J. weileri (see N. K. Ng et al. 2007: fig. 4H, as Gecarcinus); sternite 1 as a small triangular tooth, not separated by suture from sternite 2; sternite 2 semi-ovate; suture 2/3 horizontal or V-shaped; no suture 3/4, without lateral trace; completely fused sternites 3 + 4 with straight, obliquely directed margins, thus not restricted at level of P1 (Fig. 7B, C, G . ruricola); suture 7/8 rather short; sternite 8 not developed medially, the posterior emargination reaching sternite 7 at level of very a narrow median bridge at level of suture 7/8; another weak median bridge at level of suture 6/7 (Fig. 7B, C, G . ruricola), or only some traces of such bridges ( G. lateralis, G. quadratus), or indistinct bridges ( Johngarthia); deep median line only sternite 7 (Fig. 7B, C, G . ruricola). No exposed portion of sternite 8 when pleon is folded.

Johngarthia differs from Gecarcinus by the already known traits (Türkay 1970; Cuesta et al. 2007; Perger et al. 2011) and by the locking structures. Instead of a button covered by setae in Gecarcinus ruricola (Fig. 7B) and G. quadratus (see Köhnk et al. 2017: fig. 19c, d), there is a large, oblique prominence covered by setae in Johngarthia (Guinot & Bouchard 1998: fig. 25A, as G. planatus); but in both genera the pleonal sockets are not delineated, so the locking is no longer functional.

A stridulatory apparatus characterises species of Johngarthia and Gecarcinus, but it is quite distinct from those of Leptograpsodes (Figs 1G; 2C, D), Discoplax (Figs 6A; 11A) and Epigrapsus, all three with a suborbital pars stridens and a plectrum on cheliped merus. Oblique rows of tubercles on the subhepatic region are rubbed by the tuberculated cheliped merus in G. quadratus and by the cheliped palm in G. lateralis (see Klaassen 1973: figs 5, 6; Abele et al. 1973: fig. 1; Davie et al. 2015a). It was shown that in G. lateralis stridulation was part of communication system transmitted by substrate vibration (Klaassen 1973).

In Gecarcinus and Johngarthia the pterygostomial region is glabrous. Setal tufts of dense setae are located along the first pleonal somites instead of between the pereiopods (Rathbun 1918: figs 163, 165).