Revisão taxonômica do complexo de espécies Apteronotus albifrons (Gymnotiformes: Apteronotidae)

Abstract

The Neotropical electric fishes Apteronotus albifrons, Apteronotus caudimaculosus and Apteronotus camposdapazi constitutes the Apteronotus albifrons species complex, whose synapomorphy is the presence of two clear bands encircling the caudal region throughout their development. These species are morphologically very similar, differing mainly in subtle variations in body pigmentation, which has led to taxonomic problems. Furthermore, there is no designated type material for A. albifrons sensu stricto, nor a proper diagnosis, which raises questions about the validity of these three species. The main aims of the present study are to conduct a taxonomic revision of A. albifrons, A. caudimaculosus, and A. camposdapazi, to provide a detailed description of their osteology, test their taxonomic validity and, if applicable, propose amended diagnoses for them, as well as to unterstand the phylogenetic relationships of Apteronotus based on molecular data. To this end, the external morphology of specimens of A. albifrons, A. caudimaculosus, and A. camposdapazi from various ichthyological collections was examined. The osteology of some specimens was analyzed using destructive techniques for skeleton preparation (clearing and staining, and maceration by dermestid beetles) and through imaging methods such as X-rays (CT scans and radiographs) for the osteological descriptions.A phylogenetic analysis was also performed using sequences of the mitochondrial gene COI from species of Apteronotus obtained from GenBank. The specimens were re-identified based on scientific literature and new characters observed in this study. The osteological data allowed us to provide a detailed description of the skeleton of those species, with comments on the variability observed in their skulls. This included differences in the positioning of the hyoid arch, where the condyle of articulation between the ventral hypohyals could be observed posteroventral to the retroarticular in some specimens previously identified as A. albifrons or A. caudimaculosus. This feature was perceived in external morphology as a medial process at the isthmus region. Based on this characteristic, two new species were discovered within this complex. The molecular phylogeny resulted in a tree with several polytomies and some clades with low support, but overall, the COI gene proved sufficient to group haplotypes of the same species within Apteronotus. Despite the poor resolution of the tree, it highlights taxonomic issues within the genus and raises questions about whether the sequences of the Apteronotus albifrons species complex were correctly identified in GenBank. The results confirm that some previously considered diagnostic characters are not as consistent as expected. However, by combining them with new characters observed here, such as the extension of the anal fin relative to the first clear caudal band, it was possible to confirm the taxonomic validity of A. albifrons, A. caudimaculosus, and A. camposdapazi, as well as the discovery of two new species within this complex and three morphotypes whose taxonomic placement remains uncertain.