A new record of a hypogean ictalurid: cave population of Ictalurus sp. from the Candela River, Coahuila, México.

Authors: Fernando Alonzo-Rojo, Rodolfo Pérez-Rodríguez, Edith Avila-Treviño, Mariana Raya-Aguiar, María Solís-Guzmán, Gabriel Cardoza-Martínez, José Cardoza-Martínez

Year: 2025

Abstract

The Candela River is located within the Salado River, a tributary of the Río Grande basin, in Coahuila, Mexico. In addition to its surface aquatic ecosystems, the Candela River includes a cave system that represents a hypogean ecosystem, where three species of freshwater fish have been recorded, each belonging to the genera Ictalurus, Astyanax, and Herichthys. The present study focused on the taxonomic identification to species level of specimens belonging to Ictalurus. Within the native ichthyofauna of the Río Grande basin, the species Ictalurus punctatus, Ictalurus lupus, and Ictalurus furcatus are known to occur. Herein, taxonomic identification was performed using both morphological and molecular characters from seven sampled specimes, including mitocondrial and nuclear loci. The morphological assessment included linear measurenments (e.g. head length vs. anal fin base length), meristic counts (e.g. number of anal fin rays), and other distinctive traits such as the shape of the distal margin and tips of the caudal fin. For the molecular analysis, sequences from both loci were aligned with a broad dataset retrieved from GenBank. A Maximun Likelihood phylogenetic reconstruction, conducted separately for each locus, was then employed to assess the taxonomic assignment of the cave population. Results revealed a discrepancy between morphological, mitocondrial and nuclear data what suggests a complex evolutionary history for the Ictalurus population inhabiting the Candela River cave system. Morphological characters were ambiguos, exhibiting a combination of traits typical of both I. punctatus and I. lupus. Meanwhile, phylogentic reconstruction based on mitochondrial locus placed all cave specimens within I. punctatus, whereas the nuclear locus grouped them within I. lupus. Such observed discrepancy has important evolutionary implications. First, this pattern may indicate historical or recent hybridization between the two species, with the mitochondrial signal reflecting maternal ancestry from I. punctatus and the nuclear signal revealing biparental contributions from I. lupus. Second, the mitocondrial assignment to I. punctatus may suggest a case of complete mitochondrial introgression, meaning that sufficient time has passed for the mitochondrial genoma to become fixed within the population. Finally, the morphological ambiguity may be driven by local adaptation to the hypogean environment, leading to phenotypic convergence or reduction of diagnostic traits due to relaxed selective pressures. Nevertheless, further studies including genome-wide nuclear markers and broader sampling across both surface and cave populations are necessary to clarify the taxonomic status and evolutionary dynamics of this unique population.