South American gymnotiform electric fishes exhibit sexual dimorphism of shape within species, and divergence of shape among species. Recent collecting in floating vegetation mats near Manaus, Brazil, yielded a remarkable association of female and “normal” males of Apteronotus hasemani plus a series of sexually mature male specimens with greatly hypertrophied snouts and gapes. We argue that these fish represent a single species based on shared distinctive features of morphology and coloration, continuous variation of morphometric characters including allometric and dimorphic facial growth in males, ecological and possible reproductive association, and identity in 16S mt rDNA sequences. The degree of dimorphism shown by the large males greatly exceeds previously known limits of intraspecific variation for A. hasemani. The males with the most extreme snouts and gapes closely approach the holotype of Apteronotus anas that is also a mature male. We conclude that A. anas is based on a large male of A. hasemani; the older-named A. hasemani is the senior synonym.

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