Redescription and phylogenetic position of the enigmatic neotropical electric fish Iracema caiana Triques (Gymnotiformes: Rhamphichthyidae) using X-ray computed tomography

We redescribe Iracema caiana, a monotypic genus of the gymnotiform electric fish family Rhamphichthyidae. Iracema is known only from the type series (holotype and three paratypes) collected from the rio Jauaperi, affluent to the rio Negro in the Amazon basin, and was never collected again. Previous morphological studies were limited to features of external morphology. To study the osteology of Iracema we examined two specimens of different sizes using high-resolution x-ray computed tomography, a non-invasive and non-destructive technique to visualize internal anatomical structures. We describe and illustrate the osteology of Iracema caiana, and present data on morphometrics and external morphology. Contrary to previous hypotheses we propose that Iracema is the sister group to Rhamphichthys based on four synapomorphies: intermuscular bones present in the adductor mandibulae, reticulated texture of opercles, fully ossified Baudelot's ligaments, and elongate scales above the lateral line in the posterior portion of the body.