Apteronotus albifrons (Otophysi: Gymnotiformes): limitations of pigmentation patterns for the taxonomy of a species complex

The Neotropical electric fish Apteronotus albifrons is part of a species complex whose synapomorphy is the presence of two clear bands encircling the caudal region throughout their development. These species are very similar, differing mainly in subtle variations in body pigmentation. A. albifrons can be recognized by the absence of dark spots on the first clear band and an evenly dark body color. Although its taxonomic validity had not been questioned since its description until recently, there is currently no designated type material for A. albifrons sensu stricto, nor a proper diagnosis. We examined specimens previously identified as A. albifrons from various ichthyological collections from Brazilian universities. Each specimen was measured, photographed, and assessed with a focus on pigmentation patterns for comparison with other species, such as Apteronotus caudimaculosus. Morphological differences were observed among the specimens. However, what drew particular attention was the presence of dark spots on the first clear band in some individuals. Variation in the number and size of the spots was observed among these specimens, even among those collected from the same locality. This variability appears to be unrelated to either body size or geographic distribution. These results suggest that the main characters currently used to distinguish species within this complex may not be as discontinuous as previously thought. Since other studies have identified genetic differences between A. albifrons sensu stricto and A. caudimaculosus, they are probably distinct taxa whose morphology remains incompletely detailed. Further data are needed beyond pigmentation patterns—such as osteological data—to better understand these issues involving A. albifrons sensu stricto and its species complex.