A new species of sexually dimorphic and rheophilic ghost knifefish (Apteronotidae: Gymnotiformes) from the Amazon basin.

A new species of ghost knifefish, Apteronotus, is described from high-energy environments in the Rios Mapuera and Trombetas (at Cachoeira Porteira waterfalls), Brazil. X-ray microcomputed tomography (μCT scan) was used to access the internal anatomy of the type series. The new species is distinguished from all congeners by the anteriormost position of the anus, with its posterior margin extending less than one eye diameter beyond the vertical through the caudal limit of the posterior nostril, the low number of anal-fin rays (117-125) and the reduced number of branchiostegal rays (three). A series of modifications associated with secondary sexual dimorphism on the preorbital region of mature males are depicted and discussed. In addition, comments on homologies of the branchiostegal rays in Apteronotidae are provided.