Epigrapsus HelIer, 1862

Guinot, Danièle, Ng, Ngan Kee & Rodríguez Moreno, Paula A., 2018, Review of grapsoid families for the establishment of a new family for Leptograpsodes Montgomery, 1931, and a new genus of Gecarcinidae H. Milne Edwards, 1837 (Crustacea, Decapoda, Brachyura, Grapsoidea MacLeay, 1838), Zoosystema 40 (26), pp. 547-604 : 570-572

Genus Epigrapsus HelIer, 1862

TYPE SPECIES. — Epigrapsus politus Heller, 1862 by monotypy.

OTHER SPECIES. — Epigrapsus notatus (Heller, 1865) .

DIAGNOSIS

Carapace with lateral border defined in third part, and with weak subproximal indentation; no posterolateral striae. Proepistome as small plate inserted under lower frontal margin ( Tavares 1989: figs 5, 6). Orbit not closed laterally ( Tavares 1989: fig. 6), suborbital area granulated; suborbital crest very long, descending obliquely (instead of joining external angle of orbit), consisting of minute, close striae (stridulatory pars stridens), rubbing on ridge lined by short setae (plectrum) located on inner margin of P1 merus, present in both sexes. Male chelae with only minor heterochely or with clear heterochely (palm strongly inflated and with large gap between fingers). Thoracic sternum widened; sternite 1 small, triangular, seemingly fused with semi-ovate sternite 2 that bears medially setose transversal depression; suture 2/3 straight; sternites 3 + 4 not restricted at level of P1, thus with rather straight, obliquely directed margins; suture 7/8 short; sternite 7 wide; sternite 8 very wide, due to posterior emargination that does not reach sternite 7; only a minute portion of sternite 8 is dorsally exposed (below setae in E. notatus ) when pleon is folded; no median bridge. Median line on sternite 8 and extending on whole sternite 7; its distal portion bumping at level of suture 6/7 into a median, weakly raised area that superficially units both sides of sternal plate and becomes the bottom of sterno-pleonal cavity. Pleon triangular, with short somite 6; long telson. Male gonopore emerging rather far from P5 coxo-sternal condyle and suture 7/8, sternite 7 joining sternite 8 for a rather long distance. Locking button remarkably large and inflated, occupying more than the middle of sternite 5, with strongly granulated prominence; pleonal socket with markedly calcified margin. Presence of conspicuous setal coxal pouches between P3/P4.

REMARKS

Epigrapsus , unusual among gecarcinids with regards to the relatively small size at <40 mm cw and the flatter carapaces of the two known species, E. politus and E. notatus (see Türkay 1974a; Ng et al. 1998; Ng 2003; Liu & Jeng 2005; Fujita 2017), was always considered apart from other land crabs. E. villosus Ng, 2003 is actually a junior subjective synonym of E. notatus (see Naruse et al. 2018b).

The disposition of the genital male region of Epigrapsus , with the gonopore remote from the P5 coxa, differs from those of Cardisoma , Discoplax and Tuerkayana n. gen. and is closer to those of other gecarcinids Gecarcinus , Johngarthia and Gecarcoidea studied below.The larval morphology ( Cuesta et al. 2002, 2007) shows close relationships of Epigrapsus with this second subclade of gecarcinids. In contrast, the opened orbit with a long suborbital crest (see Ng et al. 1998 b: fig. 2c), forming a pars stridens, looks more like the disposition of Discoplax , but the direction of suborbital crest is opposite: joining the orbit via a short granulous line in Discoplax , vs longer, obliquely descending, not connected to orbit in Epigrapsus . The presence in Epigrapsus of a thick setal coxa pouch between P3/ P4 is unique to Gecarcinidae . Species of the first gecarcinid subclade, i.e., Cardisoma , Discoplax and Tuerkayana n. gen., lack setal coxal pouches and do not show distinct external terrestrial adaptations, except the inflated lateral regions of carapace to accommodate the the highly modified respiratory structures. The second gecarcinid subclade including Gecarcinus ( Fig. 7 A-C), Johngarthia and Gecarcoidea ( Fig. 7 E-G) have dense tufts of hydrophilic setae that are located along the margins of the pleon and at its junction with the carapace (see Terrestrial adaptations of grapsoids).

Two types of male chelae have been found in the two species of Epigrapsus by Hartnoll et al. (2017: figs 3-5, tables 2-4). In both E. notatus and E. politus , males either have morphologically similar chelae with minimal heterochely, or show a clear heterochely with different morphologies in the major and minor chelae; the proportion of heterochelous males increases in the larger mature size classes. Progressive polymorphism (i.e., when different chelar morphologies succeed each other within the mature phase, as recorded in Johngarthia lagostoma ) needs to be confirmed.

Some aspects of the reproduction of both Epigrapsus species are also unusual: the fast and vigorous larval release behaviour (ovigerous females shake their whole body laterally to release the larvae instead of using fanning motions of the pleon) and the choice of surge channels as release sites ( Liu & Jeng 2005). The affinities of Epigrapsus are so ambiguous to the point of questioning its status.

LARVAL CHARACTERS AND MOLECULAR PHYLOGENY

OF THE TWO GECARCINID SUBCLADES

Morphological features of Gecarcinus , Gecarcoidea and Epigrapsus allow differentiation of two distinct gecarcinid subclades. In the cladistic analyses of Tavares (1989), which showed a sistergroup relationship of Gecarcinidae-Grapsidae, Gecarcinidae was separated in at least two subclades: Cardisoma + Discoplax and Gecarcinus (including Johngarthia ) + Gecarcoidea , with Epigrapsus seemingly closer to the second group.

The recent data from larval morphology or genetic analyses must take into account that Cardisoma hirtipes is hereby considered to be not a Cardisoma or a Discoplax and is assigned, instead, to another genus that we introduce, Tuerkayana n. gen. The results of our proposals for the new generic names of gecarcinids are provided in Table 1.

Larval characters emphasize the need to split the family Gecarcinidae ( Willems 1982; Shokita & Shikatani 1990). According to Cuesta et al. (2002) zoeal features supported the recognition of two major groups: Epigrapsus , Gecarcinus and Gecarcoidea on one hand, and Cardisoma ( C. guanhumi , C. carnifex ) and Tuerkayana hirtipes n. comb. (as Discoplax ) on the other hand, the setation of the maxillar endopodite separating the latter group from the rest of the Gecarcinidae . Based on overall similarities of zoeal morphology, Cuesta et al. (2002) and Cuesta & Anger (2005) suggested affinities of the group Gecarcinus + Gecarcoidea + Epigrapsus with Varunidae , whereas Cardisoma (our Cardisoma sensu stricto) and Tuerkayana hirtipes n. comb. (as Discoplax ) shared with Sesarmidae antennal and pleonal morphologies, as well as the setation pattern of the maxillar endopodite. A new review of zoeal features of gecarcinids, with more complete data ( Cuesta et al. 2007: tables 5, 6) distinguished the same two groups: one with Cardisoma sensu stricto ( C. guanhumi , C. carnifex , C. armatum ) and Tuerkayana hirtipes n. comb. (as Discoplax ), and a second group with Epigrapsus , Gecarcinus , Gecarcoidea , and Johngarthia ; interestingly, this paper shows that the megalopa of Tuerkayana hirtipes n. comb. (as Discoplax ) differs from that of Cardisoma sensu stricto, which supports the distinction of Tuerkayana n. gen.

Genetic studies by Schubart et al. (2006) recovered Cardisoma as basal, followed by Gecarcoidea - Gecarcinus . N. K. Ng et al. (2007), noting that the orbital structure and the gonopore position were extremely variable within gecarcinids, recovered two groups: Cardisoma sensu stricto ( C. carnifex , C. crassum ) + “ Discoplax ” (i.e., Tuerkayana hirtipes n. comb. and T. rotundum n. comb.), and Gecarcinus + Gecarcoidea .

The molecular and larval analysis of Schubart & Cuesta (2010) demonstrated that the status of the apparently paraphyletic Gecarcinidae was still unresolved, with Cardisoma and Discoplax forming a sister-group to Gecarcinus and Gecarcoidea . Van der Meij & Schubart (2014: fig. 2) concluded that Gecarcinidae did not cluster together and also recovered two groups: Cardisoma carnifex + Tuerkayana hirtipes n. comb. (as Discoplax ) and Gecarcinus lateralis + Gecarcoidea lalandii . Using molecular data from three markers (mitochondrial 12S and 16S rRNAs, and nuclear histone H3) and covering a total of 15 thoracotreme families, Tsang et al. (2018), as Chu et al. (2015), confirmed the polyphyly of Grapsoidea and Ocypodoidea , as well as that of Pinnotheroidea De Haan, 1833. Tsang et al. (2014: fig. 1) recognised a Gecarcoidea clade ( G. natalis + G. lalandii ) and a sister clade Cardisoma - Tuerkayana n. gen. (as Discoplax ). Tsang et al. (2018: fig. 2) recognised two gecarcinid clades: a Gecarcoidea clade ( G. lalandii and G. natalis ) and a Cardisoma clade, actually including two genera: Cardisoma for C. crassum and Tuerkayana n. gen. for their “ Discoplax hirtipes ”.

BIOLOGY OF GECARCINIDAE

According to Liu & Jeng (2007) biological aspects showed the same divergent tendency, the length of the breeding season being longer in species of Cardisoma (including Cardisoma sensu stricto and Tuerkayana n. gen.) compared to other gecarcinids. In terms of overall pattern of gecarcinid breeding migrations, opposing ends of the spectrum are presented by cavernicolous Discoplax longipes (eggs are laid and incubated in the caves where the adults live, and the females with ripe eggs migrate directly to the sea; no males migrate; see Ng & Guinot 2001) and by Gecarcoidea natalis , the iconic “ Christmas Island red crab”(males and females migrate to the sea in similar numbers; only after both dip in the sea, does courtship and mating occur; see Hicks 1985; Hartnoll et al. 2010). Johngarthia lagostoma (mating and laying occur at all phases of the migration, and males migrate towards the sea in decreasing proportion with distance) and Gecarcinus ruricola (migrating crabs are mostly female with predominance of ovigerous females, so some females mate and lay eggs on the landward side, others on the seaward side; migrating females on average larger than migrating males) have a median position ( Hartnoll et al. 2006a, b, 2007).

The relationships within Gecarcinidae can also be examined in relation to their landward migration stages, their degree of terrestrialisation. Gecarcinus , Gecarcoidea and Johngarthia form a group with markedly terrestrial habit: the megalopa is the landward migration stage in Gecarcinus and Johngarthia (the megalopa of Gecarcinus ruricola being the most terrestrially adapted megalopa described to date for locomotion on land: it is the returning stage from the sea into fully terrestrial habitats before moulting to the first crab instar (see Hartnoll & Clark 2006: 162; Rodríguez-Rey et al. 2016). On the contrary, the first crab in Cardisoma sensu stricto and Tuerkayana n. gen. (as “ Discoplax hirtipes ”) form a second group, with less terrestrial habit. Tuerkayana celeste n. comb. migrates up streams as the megalopa and only emerges onto land as the first crab ( Hartnoll et al. 2014, as Discoplax ).

Figures

FIG. 7. —Thoracic sternum and frontal view: A-D, Gecarcinus ruricola (Linnaeus, 1758), ♂ 55.0× 76.0 mm, Cuba, MNHN-IU-2017-8392 (= MNHN-B13155); E-H, Gecarcoidea lalandii H. Milne Edwards, 1837, ♂ 52.0 × 73.3 mm, Papua New Guinea, MNHN-IU-2013-13254: A, E: thoracic sternum (brushed) with pleon; B, F, thoracic sternum (brushed) without pleon; C, G: genital region with gonopore and penis, D, H, front, proepistome, cephalic appendages, orbit and mxp3. Scale bars: 10 mm.