Morphological and molecular evidence for a new rhabdochonid species, Rhabdochona (Nematoda: Rhabdochonidae), parasitizing Eugerres mexicanus (Perciformes: Gerreidae), from the Lacantún River in the Biosphere Reserve of Montes Azules, Chiapas, Mexico

Abstract A new nematode species, Rhabdochona juliacarabiasae sp. n. is described from specimens recovered from the intestine of the freshwater Mexican mojarra, Eugerres mexicanus (Steindachner) (Gerreidae) captured in the Lacantún River, Chiapas, Mexico. Rhabdochona juliacarabiasae sp. n. is characterized by possessing a prostome armed with 12 anterior teeth (excepcionally 11), small bifurcated deirids with a markedly long base, an unusual tetrafurcate tip of the larger (left) spicule, a smaller (right) spicule without dorsal barb on its distal tip, and eggs with 1 short filament on each pole. In addition, in order to assess the phylogenetic relationships of the new species, and further demonstrate its distinctiveness against closely related congeners, specimens of R. juliacarabiasae sp. n, R. xiphophori, R. salgadoi, R. kidderi, R. canadensis, R. guerreroensis, and R. mexicana were sequenced for the cox1 gene, and the sequences were aligned with other nematode sequences available in GenBank (R. adentata, R. ahuhuellensis, R. ictaluri, and R. osorioi). Maximum likelihood and Bayesian inference analyses indicated that this new nested in a well-supported clade with high posterior probability. The molecular analyses support the validity of the new species. R. juliacarabiasae sp. n. was recovered as sister to the clade formed by R. kidderi and R. ictaluri, and shows genetic divergence that range between 11.18 and 14.89% (P- distances) compared with other congeners.