Neotropical Monogenoidea. 57. Revision and phylogenetic position of Scleroductus Jara & Cone, 1989 (Gyrodactylidae), with descriptions of new species from the Guatemalan chulin Rhamdia guatemalensis (Günther) (Siluriformes: Heptapteridae) in Mexico and the barred sorubim Pseudoplatystoma fasciatum (Linnaeus) (Siluriformes: Pimelodidae) in Brazil.

The diagnosis of Scleroductus Jara & Cone, 1989 (Gyrodactylidae) is amended to include viviparous species having a large spine associated with the ejaculatory duct within the male copulatory organ, two band-like projections originating from the posterior margin of the superficial bar and hooks evenly distributed along the haptoral margins. Two new species of Scleroductus are described from the external surfaces of siluriform fishes of Mexico and Brazil: Scleroductus lyrocleithrum n. sp. from the Guatemalan chulin Rhamdia guatemalensis (Günther) (Heptapteridae) in a cenote on the Yucatan Peninsula, Mexico; and Scleroductus angularis n. sp. from the barred sorubim Pseudoplatystoma fasciatum (Linnaeus) (Pimelodidae) in the Rio Tocantins, Tocantins, Brazil. Scleroductus sp. is recorded from the jundiá Rhamdia quelen (Quoy & Gaimard) (Heptapteridae) in the Iguaçu drainage of southern Brazil. Phylogenetic analyses of 18S rDNA suggest that Scleroductus (represented in the analyses by Scleroductus sp. from R. quelen), Ieredactylus Schelkle, Paladini, Shinn, King, Johnson, van Oosterhout, Mohammed & Cable, 2011 (represented by its type-species) and Gyrodactyloides Bychowsky, 1947 (represented by G. bychowskii Al'bova, 1948) comprise the sister taxon to the remaining viviparous gyrodactylids, with Scleroductus serving as the sister group to Ieredactylus + Gyrodactyloides. The clade containing the three taxa is supported by two putative synapomorphic features: two band-like projections (R1 sclerites) originating from the posterior margin of the superficial bar and a large spine associated with the ejaculatory duct within the male copulatory organ.