To come up with a definitive family tree, researchers looked at 107 eels captured in Brazil, Suriname, French Guiana, and Guyana. They analyzed the eel DNA, examined their body and bone structures, and mapped where they were caught. The data revealed three genetically separate groups with distinct geographical ranges, the researchers report today in Nature Communications. Electrophorus electricus lives the farthest to the north, mostly in Guyana and Suriname; E. varii is spread along the lowland Amazon Basin, mostly in northern Brazil; and E. voltai’s range dips even farther south into Brazil.

Although the species are nearly impossible to distinguish by sight— they all have brown wrinkly skin and frowny mouths—the team was able to find subtle differences in skull shape and body structure. E. electricus and E. voltai, for example, have depressed skulls that may have evolved as an adaptation for finding food on rocky river bottoms, or for efficient swimming in fast-flowing currents.