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Xoconochcothelphusan to accommodate X. chiapensis and re-examined phylogenetic of Ehecatusa mixtepensis

Xoconochcothelphusan to accommodate X. chiapensis and re-examined phylogenetic of Ehecatusa mixtepensis

NEWS - The researchers established Xoconochcothelphusan gen. to accommodate X. chiapensis comb. and re-examined the phylogenetic relationships of X. chiapensis gen., n. comb. and Ehecatusa mixtepensis with other genera of the subfamily Pseudothelphusinae Ortmann 1893 in Mexico.

Species of the genera Ehecatusa Ng & Low (2010), E. chiapensis Rodríguez & Smalley (1972) and E. mixtepensis Rodríguez & Smalley (Smalley, 1970) were referred to as incertae sedis in the Pseudothelphusini classification system of Ortmann 1897 and as members of the subfamily Pseudothelphusinae based on morphology and molecular analysis.

The discovery of a new specimen of E. chiapensis from Chiapas, Mexico, again reveals the unresolved taxonomic situation in the genus Ehecatusa. New morphological evidence from the first male gonopod and phylogenetic analysis based on partial DNA sequences of mitochondrial and nuclear genes (COI, 16S and H3) support the placement of both species in different genera.

The subfamily Pseudothelphusinae is distributed exclusively in Mexico. The genus Pseudothelphusa occupies a large area in Mexico along the Pacific slope from southern Sonora which is the northernmost limit of the superfamily Pseudothelphusoidea, southward reaching the coastal rivers of Oaxaca, extending through the Transmexican Volcanic Belt from west in Nayarit to east in Puebla, with one additional record of P. parabelliana Alvarez 1989 in the Los Tuxtlas region of Veracruz.

The other eight genera in the subfamily occur in southern Mexico (Veracruz, Oaxaca, Tabasco and Chiapas) to which Chiapas was added a new region spanning the Pacific slope from the city of Tonalá to the border with Guatemala known as the Soconusco region which is the distribution area of X. chiapensis comb.

Ehecatusa, a replacement name for Epithelphusa Rodríguez & Smalley (Smalley, 1970), is included in the group of genera with one or two species each based on the complex morphology of G1. Morphologically, Ehecatus is included in the subfamily Pseudothelphusinae because G1 has a caudo-marginal projection that ends in a slender lobe distally, extending cephalad.

Ehecatusa can be distinguished from other genera of Pseudothelphusinae by the presence of the lateral process, which is formed by two curved, vertical horn-like spines and by the unique shape of the mesial process, which is a large, axe-shaped plate, directed cephalad, and extending proximally. Genetic analysis shows that this genus is a sister clade to all previous genera of Pseudothelphusini after Smalleyus Alvarez 1987.

The only extant specimen of E. mixtepensis is the holotype, since it was collected in 1964, there has been no new sampling at the type site, although in several nearby streams, the crab Pseudothelphusa sulcifrons Rathbun 1898 has been caught.

A concatenated tree obtained from multigene analysis based on partial DNA sequences of mitochondrial and nuclear genes (COI, 16S, and H3) finds X. chiapensis comb. and E. mixtepensis as independent branches within the subfamily Pseudothelphusinae. The new position of X. chiapensis is supported by the presence of caudo-marginal projections, a diagnostic character of the subfamily.

Based on the morphology of G1, the relationship of the new genus to other genera of Pseudothelphusinae is difficult to establish. However, it can be included in the subfamily through the distal fusion of the marginal plate with the caudal surface, to form a caudo-marginal projection that ends distally in a broad and transverse subrectangular plate, directed cephalad, with a straight superior border and a wavy inferior border.

The characteristics that distinguish Xoconochcothelphusan gen., from other genera in the subfamily Pseudothelphusinae are the form of the caudo-marginal projection and the fusion of the mesial and lateral processes that form the apical part of the main axis with three triangular, trident-like, prominent teeth, of similar size, directed cephalad and distally beyond the apical cavity.

The name of the new genus comes from the Nahuatl word “Xoconochco” (= Soconusco) which is the regional name for the Pacific slope of the state of Chiapas, the Xoconochtli region where the sour prickly pear grows. The distribution of the genus is so far known to be endemic to the state of Chiapas, Mexico.

X. chiapensis using specimen CNCR 34841, collected near the city of Escuintla, Chiapas, extends the distribution of X. chiapensis comb. about 20 km northwest, from Finca Victoria, near the Guatemalan border.

This specimen is slightly larger than the holotype and shows some differences in G1 morphology, especially in the size of the caudo-marginal plate, which is shorter, not as wide, and the inferior margin shows only a single notch. This rare species is known from only four males, therefore it is difficult to assess the degree of variation in G1 and other structures.

Original research

José Luis Villalobos-Hiriart, Eric G. Moreno-Juárez & Fernando Álvarez (2024). Xoconochcothelphusa, a new genus for Ehecatusa chiapensis (Rodríguez & Smalley, 1972), with notes on Spirothelphusa Pretzmann, 1965 (Crustacea: Decapoda: Pseudothelphusidae). Zootaxa 5523 (2): 171-191, DOI:10.11646/zootaxa.5523.2.2

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