Scleroductus (Platyhelminthes: Monogenea) infecting Rhamdia spp. (Siluriformes: Heptapteridae) in Mexico, and their phylogenetic position within Gyrodactylidea.

Gyrodactylidae Cobbold, 1864 is a monophyletic family of hyperviviparous species, with 36 genera and approximately 700 species. Here, we focused on one of these genera - namely Scleroductus Jara & Cone, 1989 - characterizing its morphological variation systematically and testing its phylogenetic position in relation to other genera in the family. We collected and describe two new species of monogenean parasites of Scleroductus infecting Neotropical freshwater catfishes in Mexico. Scleroductus veracruzano n. sp. infects Guatemalan chulín, Rhamdia guatemalensis in Veracruz, while Scleroductus oaxaqueno n. sp. infects filespine chulín, Rhamdia laticauda in Oaxaca, Mexico. Sequences of the internal transcribed spacer (ITS rDNA) and 18S rDNA were generated for both species. Phylogenetic hypotheses based on maximum likelihood and Bayesian inference analyses of 18S rDNA indicate that the two new Mexican taxa are closely related to each other, and both appear as sister species to an undescribed but molecularly characterized Scleroductus sp. infecting South American catfish, Rhamdia quelen in Brazil. Our analyses confirm the placement of Scleroductus spp. within the monophyletic family Gyrodactylidae, whose sister clade is the Oogyrodactylidae. Within Gyrodactylidae, Scleroductus spp. form a clade sister to a well-supported clade composed by members of the genera Ieredactylus, Gyrodactyloides and Laminiscus.