Taxonomic resurrection of the amphidromous shrimp Macrobrachium mexicanum (De Saussure, 1857) (Decapoda: Caridea: Palaemonidae) from the Mexican Atlantic slope

This study, the first extensive study on a species from the Mexican Atlantic slope, is part of a national project focused on the systematics of the amphidromous species of Macrobrachium Spence Bate, 1868, which occur in coastal drainage basins of Mexico. Five amphidromous species of Macrobrachium have been found to occur in this region: M. acanthurus (Wiegmann, 1836), M. carcinus (Linnaeus, 1758), M. heterochirus (Wiegmann, 1836), M. hobbsi Nates & Villalobos in Villalobos Hiriart & Nates Rodríguez, 1990, and M. olfersii (Wiegmann, 1836). We studied specimens of the putative M. acanthurus (type locality: coast of Brazil) from basins of the states of Tamaulipas, San Luis Potosí, Veracruz, Tabasco, Chiapas, and Campeche. Comparison of their COI haplotypes with M. acanthurus sequences from Costa Rica, Panama, and Brazil deposited in the GenBank revealed high genetic distance (9.3 to 10.4%) between them. Maximum likelihood and Bayesian inference analyses performed by using concatenated 16S-COI sequences to investigate the evolutionary relationship of our haplotypes and other Macrobrachium species in the GenBank showed the Mexican haplotypes in a separate clade from its closest entity the M. acanthurus clade from regions south of Mexico. The wide COI genetic distance and the phylogenetic inference of the 16S-COI haplotypes show that our specimens correspond to M. mexicanum (De Saussure, 1857), a cryptic species previously considered a junior synonym of M. acanthurus. We studied photographs of four syntypes of M. mexicanum and conclude that their morphological characteristics fit well with our studied individuals.