Sylvathelphusa cavernicola Villalobos & Álvarez, 2013, n. sp.

Sylvathelphusa cavernicola n. sp.

(Figs. 4, 5)

Type material. Holotype: male, cb 32.8 mm, cl 20.3 mm; female allotype, cb 46.6 mm, cl 28.6 mm; Cueva de las Arañas, near the town of San Fernando (16°51'26.0" N, 93°16'01.5" W; 1140 m), Municipio de San Fernando, Chiapas, Mexico; 5 Sep 2008; colls: F. Álvarez, E. Torres, K. Zárate, G. Armendáriz, A. García and J.L. Villalobos; CNCR 25324. Paratypes: 3 males, cb 28.6- 24.1 mm, cl 17.1- 14.4 mm; 1 female, cb 17.0 mm, cl 11.0 mm; same locality, date and collectors as holotype, CNCR 27210.

Description. Crabs of moderate size (cb <47 mm). In dorsal view, carapace oval, widest in the middle (cb/cl average = 1.62); dorsal surface flat, punctated, regions faintly indicated, pair of gastric pits barely distinct. Cervical grooves shallow, straight, not reaching anterolateral margin (Fig. 4 a). Postfrontal lobes indistinct, separated by shallow median groove. Frontal region granulated, slightly curved towards superior frontal margin; inferior border faintly granulated, projected, divided into two convex lobes; superior border bilobed with shallow mesial notch. Superior orbital margin carinate, continuous with inferior frontal border; inferior orbital margin marked by row of granules; exorbital angle projected, narrow and acute (Fig. 4 d). Anterolateral margins serrated with sharp granules along anterior half, posterior half almost smooth. Posterior margin concave.

Eyes with cornea pigmented, faintly faceted, not filling orbits (Fig. 4 d). Epistome narrow, epistomial tooth low, triangular, slightly deflexed, borders smooth. Pterygostomial region covered by dense pubescence around mouthparts. Endopod of third maxilliped with outer margin of ischium sinuous, inner margin straight; external margin of merus widely curved; ratio exopod/ischium 0.52 (Fig. 4 c). Aperture of efferent branchial channel subrectangular.

Chelipeds similar in shape, subequal in length, same color as walking legs. Major cheliped with merus subtriangular in cross section; 3.3 times as long as wide; dorsal suface smooth, internal dorsal margin with row of conical, acute spines, increasing in size distally alternating with small spinules; external margin granulated. Carpus smooth, prominent and acute subdistal spine on inner surface, preceded by two acute spinules. Palm high, compressed, surface smooth, with minute squamous granules on dorsal and ventral margins. Fingers slender, shorter than palm, not gaping, tips crossing; both fingers with triangular sharp teeth on cutting edges, smaller distally; dactylus with smaller teeth sometimes interspersed with larger ones (Fig. 4 b). Thoracic sternites of third maxillipeds and first pereiopods completely fused, except for small notches at lateral edges of sternum; sternal sulci s4/s5, s5/s6 and s6/s7 distinct, just failing to reach midline of thoracic sternum. All abdominal segments free. Telson subtriangular, narrower than sixth abdominal segment; lateral margins straigth, tip acute.

Male gonopod latero-mesially compressed, slightly arched in caudal and cephalic views, distal half wider. In mesial view, marginal suture almost straight along most of gonopod, slightly concave in the middle (Fig. 5 a). Marginal plate between caudal and mesial surfaces, proximal half narrow, with few long marginal setae at base; distal half wider, forming rectangular projection, distal margin rounded with prominent spine (Fig. 5 a). Caudal portion of marginal plate limited by straight and incipient lateral suture. Mesial surface ending apically in tonguelike apical projection, disto-cephalically directed and longer than apical projection of marginal plate, distal margin with acute and robust spines. Mesial process as a triangular, stout, acute spine, laterad directed, forming an 85º angle with respect to principal axis of gonopod. Caudal surface with narrow, deep medial notch. Lateral suture visible through proximal two thirds; on distal third interrupted and reappearing separating marginal plate from caudal surface. In cephalic view, mesial process conical, stout, acute (Fig. 5 b). Lateral surface broad, semicircular, with one slender, acute distomedial tubercle. Caudal margin with acute spinules and distal triangular, acute spine. Apical projection of mesial surface tongue-like, with thin serrate margin. In lateral view, distal margin of mesial surface with 7 strong teeth, spine on lateral surface acute, constriction of gonopod at middle portion evident (Fig. 5 c). In caudal view, gonopod straight. Distal half with lateral margin expanded as rounded plate with spinular edge and distal, acute spine (Fig. 5 d). In apical view, apical projection of mesial surface with thin distal margin serrate (Fig. 4 e), caudal surface concave, with few short setae in the apical cavity. Mesial process conical, stout directed laterad. Apex cavity elongated, partially covered by hood formed by distal crest of lateral surface, with few setae. Caudal border of lateral plate ending distally in acute spine (Fig. 4 e).

Remarks. Sylvathelphusa cavernicola n. sp. represents the third stygobitic freshwater crab of the family Pseudothelphusidae recorded in Chiapas after Typhlopseudothelphusa mocinoi Rioja, 1952 and T. hyba Rodríguez & Hobbs, 1989. Interestingly, two different lineages represented by these two genera have colonized the karstic environment in a relatively reduced area in central Chiapas. Morphologically the gonopod of S. cavernicola n. sp. is different from those of the species of Typhlopseudothelphusa in that in the latter it is bent mesially, and there are no supra-apical processes, being more similar to Potamocarcinus where the gonopod is straight and stout. The most evident differences with Typhlopseudothelphusa are the stoutness of the gonopod, with the marginal plate and the distal crest of the mesial surface distally projected beyond the mesial process.