The present paper aims to summarize cases of sexual dimorphism in Siluriformes and Gymnotiformes. This focuses on Neotropical representatives, with emphasis on Amazonian fish. Fishes from this region exhibit high species-level diversity, although the intraspecific limits of many of these forms are unknown. In Loricariidae (Siluriformes), mapping of dimorphic traits on cladograms has helped to demonstrate that patterns of sex dimorphism are consistent with hypotheses of monophyly, thus reinforcing its role as an important evolutionary phenomenon. In Apteronotidae (Gymnotiformes), mapping of characters on phylogenetic trees suggests that sexual dimorphism (regarding snout size, form, and presence/absence of teeth) has evolved independently in multiple taxa. Recently in Gymnotiformes, detected taxonomic errors are attributed to extreme differences between males and females in their morphology.
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