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

Abstract

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.Iracema caiana, um genêro monotípico de peixes elétricos Gymnotiformes da família Rhamphichthyidae, é aqui redescrito. Iracema é conhecido apenas da série-tipo (holótipo e três parátipos) coletado no rio Jauaperi, afluente do rio Negro na bacia amazônica, nunca sendo coletado novamente. Estudos morfológicos prévios de Iracema foram limitados somente à morfologia externa. Neste trabalho, a osteologia do gênero foi examinada com base em dois espécimes de diferentes tamanhos usandose tomografia computadorizada de alta resolução, uma técnica não invasiva para a visualização de estruturas anatômicas internas. Caracteres externos e morfométricos são apresentados e a osteologia é ilustrada e descrita. Contrariamente a hipóteses prévias, Iracema é proposto como grupo irmão de Rhamphichthys com base em quatro sinapomorfias: presença de ossos intermusculares no adductor mandibulae, textura reticulada do opérculo, ligamento de Baudelot completamente ossificado e escamas alongadas na região posterior do corpo acima da linha lateral.

The adductor mandibulae muscle complex in lower teleostean fishes (Osteichthyes: Actinopterygii): comparative anatomy, synonymy, and phylogenetic implications

“…A curious feature of the adductor mandibulae of some gymnotiforms is the presence of filamentous intermuscular bones within the segmentum facialis. Such ossifications have been reported for the gymnotid Gymnotus carapo , the apteronotid Orthosternarchus tamandua and the rhamphichthyids Iracema caiana , Rhamphichthys marmoratus , and Rhamphichthys rostratus (Aguilera, ; Albert, ; Hilton et al ., ; Carvalho & Albert, ). These structures probably represent ossifications of some internal tendons of the adductor mandibulae of these fishes.…”

The pseudotympanum in the Gymnotiformes (Teleostei, Ostariophysi, Otophysi): homology and evolution of a previously unexplored system in Neotropical electric fishes

“…A resolution of that question (or whether some other scheme of relationships best reflects the available evidence) necessitates not only the inclusion of the evidence detailed above, but also incorporation of the multiple proposed synapomorphies documented in the last decade in publications (e.g. Albert et al ., ; Hulen, Crampton & Albert, ; de Santana & Vari, ; Carvalho & Albert, ; de Santana & Crampton, ; Vari et al ., ; de Santana et al ., ) and unpublished results (e.g. Carvalho, ).…”

New species of the miniature genus Gelanoglanis (Siluriformes: Auchenipteridae) from the Tocantins River basin (Brazil) and osteological description of G. nanonocticolus

“…In order to resolve this matter, a specimen was analysed by high-resolution X-ray computed tomography (HRXCT), to tentatively access some level of osteological information. Recent papers (Carvalho & Albert, 2011;Maxime & Albert, 2014) positively demonstrated that HRXCT images are helpful in osteological descriptions of rare species when they are unavailable to undergo chemical preparation. Based on HRXCT images, a brief osteological description of G. nanonocticolus is presented here, with a discussion of its relationships and miniature characteristics.…”