Rodriguezia adani Alvarez & Villalobos 2018, n. sp.

Rodriguezia adani n. sp.

(Figs. 2–4)

Type material. Male holotype, cl 9.5 mm, cb 9.5 mm; Grutas de Agua Blanca (17°37’16.4”N, 92°28’21.6”W; 133 m asl), municipality of Macuspana, Tabasco, Mexico, 16 July 2016, coll. A. Gómez & E. Moreno, CNCR 33912. Female allotype (carrying 4 crabs in abdomen, measuring cb 1.7 mm), cl 9.9 mm, cb 10.4 mm; same locality, date and collector as holotype, CNCR 33913.

Paratypes. 2 males, cl 5.5, 6.4 mm, cb 5.3, 6.7 mm; 4 females, cl 4.6, 5.0, 6.2, 7.9 mm, cb 4.8, 5.2, 6.5, 8.5 mm, same locality, date and collectors as for holotype, CNCR 33914. 1 male, cl 8.7 mm, cb 9.1 mm, same locality as holotype; July 2015; coll. A. Gómez & C.N. Rico, CNCR 33915.

Description. Dorsal surface of carapace smooth; in dorsal view suborbicular, slightly wider than long; branchial, gastric, cardiac regions faintly marked; lateral margin complete, devoid of teeth; posterior margin slightly arched, almost straight; posterolateral and anterolateral angles rounded; anterior margin bilobed, central portion slightly depressed, laterally with small rounded projections that limit orbits (Fig. 2A, B). In frontal view, superior border of carapace formed by minute tubercles; median portion elevated; laterally, portion between insertion of antennule and antenna slightly depressed, portion around orbits elevated. Internal orbital angle weakly marked, occlusive tooth rounded, buccal angle oval-shaped (Fig. 3A, B).

Abdomen of male and female with all segments distinct. Male abdomen triangular, last segment with posterior margin rounded, twice as wide as long (Fig. 3C). Female abdomen broadly rounded, partially covering thoracic sternites, last segment 4 times as wide as long (Fig. 3D).

Antennule and antenna, typical of genus (Fig. 3E, F). Eyes absent, ocular peduncle strongly reduced, not movable, appearing as blunt plug occupying most of orbit (Fig. 3A, B). Third maxilliped approximately rectangular, ischium slightly wider than merus; exopod shorter than ischium and merus combined, flagellum with eight articles (Fig. 3G).

Chelipeds subequal in size, carpi with tooth on internal dorsal margin, chelae with dorsal tooth on proximal third, fingers not gaping. Major chela with swollen palm, twice as long as high; fingers about one third length of chela, cutting edges with low irregular teeth, tips acute; in lateral view, movable finger approximately triangular (Fig. 3H). Minor chela 2.4 times as long as high, fingers about half length of chela (Fig. 3I).

Pereiopods 2–5, elongate, representing 1.84, 2.26, 2.48, 2.21 times carapace breadth (Fig. 2A). Propodi and dactyli with fine pubescence, more dense on the latter.

First gonopod straight, slender, proximal third wider (Fig. 4A, E). Middle constriction, typical of genus, faintly visible in lateral and mesial views (Fig. 4C, E). Field of spines covering distal third except for lateral surface (Fig. 4C); all spines similar in size and shape. Apical pore suboval, drop-shaped. Second pleopod less than half length of first, distal portion slender, acute (Fig. 4B).

Distribution. The new species is only known from Grutas de Agua Blanca, Macuspana, Tabasco, Mexico.

Etymology. We name the new species after Adán Gómez-González, explorer, biologist and friend, who found these crabs while exploring caves in Tabasco and Chiapas, Mexico.

Remarks. Rodriguezia adani n. sp. and R. mensabak are more similar to each other than either is to R. villalobosi. While R. mensabak and R. adani are stygobitic species with elongated legs, lacking eyes, with chelipeds subequal, and a male gonopod without an obvious middle constriction, R. villalobosi is an epigean species with normal eyes, short pereiopods, stongly unequal chelipeds, and a male gonopod with a strong middle constriction. Rodriguezia adani differs from R. mensabak in: the length of the pereiopods relative to carapace breadth, the new species has shorter legs; and in the extremely reduced ocular peduncle, which appears as a plug inside the orbit. Regarding other characters, R. adani and R. mensabak are similar in the first gonopod and carapace morphology, and the shape of the male abdomen.

Two interesting differences between R. adani. and R. mensabak are the following. Grutas de Agua Blanca where the new species was collected is at an altitude of 136 m, while Nicolás Bravo Cave near the town of Tila, Chiapas, the type locality and only known occurrence of R. mensabak, is at an altitude of 1,160 m (Cottarelli & Argano 1977). Although the two caves are only 30 km apart, the former lies at the northern edge of the Sierra Norte de Chiapas, while the latter is well within the Altos de Chiapas region. The second difference is their body size, the largest R. adani, a female carrying four young crabs, has a carapace breadth of 10.4 mm, whereas the holotype of R. mensabak has a carapace breadth of 16.2 mm (Cottarelli & Argano 1977). The largest known adults of the new species are 2/3 the size of the largest adults of R. mensabak.

Regarding the habitat of Rodriguezia adani n. sp. in Grutas de Agua Blanca, the crabs were collected about 150 m from the cave entrance in complete darkness. There is a strong water flow near the entrance, which diminishes to an almost imperceptible flow in ponds that collect in the deeper part of this section that goes for 200 m. The crabs were found along the edge of ponds under rocks in sandy substrate with clear water. Grutas de Agua Blanca is also the type locality of the blind catfish Rhamdia macuspanensis Weber & Wilkens, 1998. Apparently the new species described herein was collected in the same section of the cave as R. macuspanensis (Weber & Wilkens 1998). Other organisms collected in the cave are the pseudothelphusid crab Odontothelphusa palenquensis Alvarez & Villalobos, 1998, an undescribed species of freshwater prawn of the genus Macrobrachium, a juvenile crayfish of the genus Procambarus, and asellid isopods.

It remains a relevant question for future studies to determine if the two stygobitic species originated from the epigean R. villalobosi, or if they represent two separate lineages. In case they do, a complex geological setting will have to be invoked since R. villalobosi is the species occurring farther inland, and would have had to reach its present geographic position very early in the formation of this part of southern Mexico, during early Cretaceous times (Ecosur 2001).