The description of a new species of the Neotropical land crab genus Gecarcinus Leach, 1814 (Crustacea, Decapoda, Brachyura, Gecarcinidae)

Abstract In this contribution a new species of the land crab genus Gecarcinus Leach, 1814, from the Neotropical Pacific coast of South America is described and illustrated. In addition to its unique body color, Gecarcinus nobilii sp. n. is distinguished from congeners by a distinctly wider carapace front and differences in the shape of the infraorbital margin. The new species is not isolated from Gecarcinus populations from the Pacific coast of Central America by an insurmountable geographic barrier. Considering the closure of the Panamanian Isthmus as a calibration point for morphological divergence between the trans-isthmian mainland populations of Gecarcinus, the virtual lack of morphological differentiation (other than color) between them and the distinctness of G. nobilii sp. n. suggests that G. nobilii sp. n. evolved from a common ancestor before the Isthmus closed.


Introduction
Land crabs of the genera Gecarcinus Leach, 1814, Johngarthia Türkay, 1970, andGecarcoidea H. Milne Edwards, 1837, play an important ecological role on many tropical islands (see Lindquist et al. 2009 for a review). On some islands their biomass may exceed the total mass of animals reported in tropical rain forests in Costa Rica and the central Amazon (Lindquist et al. 2009) and they may occupy the top of the energy pyramid (Burggren and McMahon 1988). Nevertheless, despite their success on islands and the possibility of dispersal of their marine larvae via ocean currents, most species of these genera are absent from continental mainland habitats (Türkay 1987; see also Perger 2014 for a short review). Only Johngarthia planata (Stimpson, 1860) (see Perger 2014) and two populations of the Neotropical genus Gecarcinus are successfully established on the continental mainland. The Atlantic population, known as Gecarcinus lateralis (Fréminville, 1835), occurs along mainland beaches from Florida to Venezuela (Türkay 1970) and is partially sympatric with G. ruricola (Linnaeus, 1758), which is restricted to Western Atlantic islands. The distribution of the Pacific population, originally described as G. quadratus Saussure, 1853, extends along Pacific shorelines from Mexico to Peru (Türkay 1970).
Although both mainland populations of Gecarcinus have been separated by the definite closure of the Isthmus of Panama some three million years ago, the morphology of Pacific and Atlantic forms is so similar that Türkay (1970) treated G. quadratus as a subspecies of G. lateralis. As Türkay could not find any differences in the supplemental material he examined, he synonymized G. lateralis quadratus with G. lateralis (see Türkay 1973). The taxonomy of Gecarcinus has not been revised since Türkay's (1970Türkay's ( , 1973 works: subsequent contributions on the ecology and general aspects of both trans-isthmian populations have followed Türkay's classification (e.g. Hartnoll 1988;Boyko 2000;Capistran-Barrados et al. 2003), while others maintained G. quadratus as a valid species without further justification (e.g. Abele and Kim 1986;Tavares 1991;Ng et al. 2008).
As in trans-isthmian Gecarcinus, the taxonomical status of the South American Pacific population has been uncertain. Whereas Nobili (1901) described specimens from the Ecuadorian coast as G. ruricola, following works do not distinguish between Gecarcinus from the Pacific coast of Central and South America and specimens from the latter have been referred to as G. quadratus (see Rathbun 1918), G. lateralis quadratus (see Türkay 1970;Prahl and Manjarres 1984) and G. lateralis (see Türkay 1987). To add further complexity, Prahl and Manjarres (1984) and Türkay (1987: Fig. 7) reported white forms of Gecarcinus from the Pacific coast of Colombia. However, such color forms have not been observed in other areas (see Bright 1966;Chace and Hobbs 1969;Martinez and Bliss 1989), nor were such color forms observed in a field sample of 678 individuals (carapace width 20-66 mm) from the Pacific and Atlantic coastline of Central America (Table 1; Fig. 1) that was examined by one of us (RP).
An Internet search revealed additional photographs of Gecarcinus individuals from the Pacific coast of South America with a body color different from the specimens that were collected on the Central American coasts. Further examination of museum specimens and re-examination of the freshly collected specimens from Central America has revealed that the color differences are accompanied by differences in morphological structures as well. In accordance with these differences, a new species of Gecarcinus is herein proposed. Specimens from the following institutions were examined: Academy of Natural Sciences of Drexel University, Philadelphia, USA (ANSP); Natural History Museum of Los Angeles County, Los Angeles, USA (LACM); Muséum National d'Histoire Naturelle, Paris, France (MNHN); Museo de Zoología, Universidad de Costa Rica, San José, Costa Rica (MZUCR); Naturhistorisches Museum, Basel, Switzerland (NHMB) and National Museum of Natural History, Smithsonian Institution, Washington, D.C., USA (USNM).

Type species. Cancer ruricola
Diagnosis. Exopod not projecting beyond third maxilliped ischium-merus articulation, without flagellum; palp concealed by third maxilliped merus margin. Mesial segment of first male gonopod distally reduced and terminal segment basally exposed, terminal segment projecting beyond apical setae.
Remarks. Because further evidence challenging Türkay's (1973) synonymization of G. quadratus with G. lateralis has not been presented to date, we follow his taxonomy and treat G. quadratus, described from the Pacific mainland, as a junior synonym of G. lateralis.  3D-F) of G. nobilii sp. n. was labeled as "Gecarcinus festae Nobili/(co-type)/St. Helena/Festa/Museum Paris/Ecuador/Nobili 1901" without information about the label author. Nobili never published a description of a species by this name. In 1901 he described Sesarma festae and Uca festae and in the same contribution referred to specimens of Gecarcinus collected in Ecuador as Gecarcinus ruricola, which is restricted to West Atlantic Islands (Türkay 1970;Bright and Hogue 1972).

Key to the species of Gecarcinus
Additionally, high resolution photographs of 14 captive individuals with unknown origin and 17 individuals taken at the following locations (confirmed by the photographers): Gorgona Island (Colombia) (  -190711). Additional comparative material is cited in Table 1.
Derivation of specific epithet. The species is named in honor of Giuseppe Nobili, who provided important contributions on the knowledge of crustaceans and built the crustacean collection in the Turin Museum (Italy). The species name is a noun in the genitive case.
Diagnosis. Frontal width distinctly wider than the distance between the mesial ends of the suborbital cristae (Figs 2C; 3B, E). Width of mesial lobe of infraorbital margin at point of contact with carapace front longer than shortest distance between carapace front and mesial end of suborbital crista (Figs 2C; 3B, E). Light lateral margin on dorsal carapace without lighter anterolateral and posterior patches (Figs 4C; 5D); cheliped carpus and palm homogeneously red or white (Fig. 4).
Chelipeds subequal; in large males larger with respect to the carapace width, surfaces relatively smooth, weakly granulate. Merus with transversal rows of small tubercles; dorsal margin rugose or with moderately developed, obtuse granules; ventral margin lined with weakly developed granules, otherwise smooth. Carpus with 2-5 well developed triangular inner subdistal tooth (Fig. 3A, B, E). Merus and carpus margins smooth in large individuals. Chela large, length not exceeding carapace width, surfaces smooth; lower margin gently sinuous. Fingers slightly shorter or as long as palm, tapering, gently curved, drop-shaped in cross-section, proximal half with irregular arranged pores and low, pectinated teeth; teeth on distal portion of finger arranged on well defined, subparallel longitudinal ridges, longitudinally separated by grooves with pores. Cutting margins with distinct triangular teeth along length; fingers without or with small gap between them when closed, ending in sharp, pectinated tips.
Male abdomen relatively broad (Fig. 3C), all abdominal somites and telson distinct, freely articulating. Somite 1 filling space between last pair of ambulatory legs, longitudinally very narrow. Shape of somite 2 similar to somite 1 but narrower. Somites 3-5 increasingly trapezoidal in shape, lateral margins relatively straight. Somite 6 longest, longer than telson, distinctly wider than long, with lateral margin strongly convex. Telson sub-triangular, narrowest abdominal segment; as long as wide, lateral margins gently concave to almost straight, tip rounded (Fig. 3C).
First male gonopod with basal and terminal segment (Fig. 2E). Basal segment stout, straight, with digiform projection on distomesial end, projection directed in same manner with distal segment, fringed with long setae. Terminal segment about one-third of first gonopod (when seen from lateral view, Fig. 2E), folded longitudinally, compressed dorsoventrally, tapering and curved distally, slightly projecting over distal setae, laterally with narrow, longitudinal furrow.
Sex independent color dimorphism: red and white males and females (Fig. 4). Both forms with dark median pattern and contrasting light lateral margin on dorsal carapace without lighter anterior and posterior patches (Fig. 5D), margin of same color as lateral carapace; dark dorsal carapace pattern with pointed anterolateral edges anteriorly reaching the orbits (Fig. 5D). Mesial lobe of infraorbital margin mostly grey (Fig. 4). Red form with red lateral margin on dorsal carapace. Carapace pits white to orange. Legs and chelipeds uniformly red, inner sides of fingers cream to white, margin of the third maxilliped merus, coxa, basis and ischium of chelipeds and ambulatory legs whitish. In white form, lateral margin on dorsal carapace orange/white or completely white. Ventral carapace and chelipeds white, legs and carapace pits light orange to white (Fig. 4). Geographic distribution. G. nobilii sp. n. is currently known to occur from Punta Galera and St. Elena (Ecuador). It is also documented in photographs taken at Gorgona Island (Colombia) (Fig. 4A-C), Chucheros Beach (Buenaventura, Colombia) (Fig. 4D), Canangucho Forest Reserve (Nuquí, Chocó, Colombia) (Fig. 4E), Ayampe (Manabí, Ecuador) (Fig. 4F), and Isla de la Plata (Ecuador). Individuals of Gecarcinus previously reported from Peru (Türkay 1970) may also refer to G. nobilii sp. n.
Available data and the photographs found during the Internet search suggest that G. nobilii sp. n. replaces Pacific G. lateralis between the Darien province (Panama) and the Choco dept. (Colombia). In addition to the individuals of Pacific G. lateralis found during fieldwork (Table 1), the Internet search revealed numerous photographs of Pacific G. lateralis from Central America. However, there are no photographs of Pacific G. lateralis from within the distributional area of G. nobilii sp. n.
Remarks. The resemblance of the general habitus, the shape and the surface structure of carapace, chelipeds, ambulatory legs (Figs 3; 6-8) and the first male gonopod (Fig. 2E, F) indicate a close relationship between all mainland Gecarcinus populations. However, the trans-isthmian populations of G. lateralis differ from G. nobilii sp. n. by having a carapace front approximately as wide as the distance between the mesial ends of the suborbital cristae, and the width of the mesial lobe of the infraorbital margin at the point of contact with the carapace front is shorter than the shortest distance between the carapace front and the mesial end of the suborbital crista (Figs 2B; 6B, E; 7A, B).
A character commonly used to distinguish between species of the Gecarcinidae is the shape of the third maxilliped merus margin (e.g. Rathbun 1918;Türkay 1970;Perger et al. 2011). According to Rathbun (1918), the Atlantic population of G. lateralis has an emarginated margin and the Pacific population a continuous margin. However, Bott (1955) and Türkay (1970Türkay ( , 1973 recognized the presence of both character states in trans-isthmian populations from Central America, South America and the Antilles, which we also observed in our sample from Central America. In contrast, all examined specimens of G. nobilii sp. n. have a relatively continuous third maxilliped margin. In addition to observed differences in morphological structures, the body color of Pacific G. lateralis (sensu Türkay 1973) differs from G. nobilii sp. n. in the following manner: Dorsal carapace without light lateral margin (Fig. 5A), dark dorsal carapace pattern extending laterally over anterolateral carapace border (Fig. 7E, F); orange patch at anterolateral and posterior carapace border (Figs 5A; 8G, H). Cheliped carpus and palm violet, rarely purplish (Fig. 8G, H). Atlantic G. lateralis are distinguished from G. nobilii sp. n. by following color differences: Light lateral margin on dorsal carapace with lighter (yellow to orange) patch at anterolateral and posterior carapace border (Figs 5B; 8A, B) or margin and anterolateral patch absent (Figs 5C; 8D-F). Ventrolateral carapace color heterogeneous (Fig. 7A-C). Cheliped carpus and palm heterogeneously purple, red, orange and/or whitish (Fig. 8A-F). Apart from the consistent color differences between G. nobilii sp. n. and the other mainland populations of Gecarcinus, which provided the starting point for this contribution, the color differences between the trans-isthmian populations of Gecarcinus (Table 2; Figs 5A-C; 7; 8) also drew our attention. According to Bright (1966), Gecarcinus from the Pacific coast of Central America has a brownish-red carapace and chelipeds with a purple tinge, while G. lateralis from the Atlantic coast of Central America has a dark red carapace pattern and reddish-gray chelipeds. Chace and Hobbs (1969) provided the first color description of G. lateralis from the West Indies, which widely agreed with the brief description of the Gecarcinus specimens from the Pacific coast of Central America by Bright (1966). Martinez and Bliss (1989) later described and illustrated the color of G. lateralis from Bermuda and Bimini Islands in detail. The authors observed a remarkable variability in color and even color changes in single individuals, leaving open the possibility that Pacific G. lateralis might also approach the range of variability observed in Atlantic G. lateralis. While the color of examined G. lateralis from the Atlantic coast (n= 362) varied within the range described by Martinez and Bliss (1989) (see Fig. 8A-F), the color of individuals of Pacific G. lateralis examined in this study (n= 316) showed little variation (Fig. 8G, H) and did not approach the range of variability found in G. lateralis from the Atlantic coast of Central America (Table 2) and Bermuda and Bimini Islands (see Martinez and Bliss 1989). In a sample totaling 678 individuals, only a single female from the Atlantic coast (Fig. 7D) did not clearly match with each of the color characters attributed to either the Atlantic and Pacific population (Table 2). This individual had a dark dorsal carapace color expanding laterally over the dorsolateral carapace border and sharply contrasting with the ventrolateral carapace color. However, the remaining characters (Table 2) agreed with the other individuals of Atlantic G. lateralis. Within several groups of decapod crustaceans, color and color pattern reli- ably distinguish a number of species that differ little in morphology (e.g. Bruce 1975;Knowlton 1986). Color pattern-level and genetic differentiation between cryptic species has been observed in hermit crabs (e.g. Malay and Paulay 2009), spiny lobsters (Ravago and Juinio-Menez 2003), porcelain crabs (Hiller et al. 2006) and in the Gecarcinidae genus Discoplax A. Milne-Edwards, 1867 (Ng and Davie 2012). Studies of genetic divergence and reproductive isolation are needed to evaluate whether G. quadratus should be retained as a synonym of G. lateralis, or alternatively, the trans-isthmian populations of G. lateralis represent allopatric sister species.
Evolutionary relationships. When we consider the closure of the Panamanian Isthmus as a calibration point for morphological divergence between the trans-isthmian populations of G. lateralis, the virtual lack of morphological differentiation (other than color) between them and the distinctness of G. nobilii sp. n. suggests that G. nobilii sp. n. evolved from a common ancestor before the Isthmus closed. The common ancestor of the trans-isthmian G. lateralis may have been restricted to North America and/or the emerging Isthmus, which is assumed to have been a peninsula of North America (Kirby et al. 2008), and the ancestor of G. nobilii sp. n. to South America. Nevertheless, the distribution of the gecarcinid Johngarthia planata Stimpson, 1860, from Gorgona Island to Mexico (reviewed by Perger et al. 2013) and Cardisoma crassum Smith, 1870, from Peru to Mexico (Türkay 1970) as well as the absence of G. nobilii sp. n. from the Atlantic coast of South America suggests a more complex pattern. A promising approach to further investigation of the speciation processes in Neotropical Gecarcinidae may be the evaluation of how the connection between the habitats of the adults via sea currents may have changed during the formation of the Isthmus (see Schneider and Schmittner 2006;Molnar 2008). Further studies should also take into account that even today, as indicated by the actual distribution, there appear to be mechanisms separating the Gecarcinus populations from the Central and South American Pacific coast.
(Museum Victoria, Melbourne, Australia) and the anonymous reviewers for providing comments and suggestions on the manuscript.