Sexual Dimorphism in the Electric Knifefish Sternarchogiton labiatus (Gymnotiformes: Apteronotidae) with a Hypermorphic Snout and Oral Jaws

Secondary sexual dimorphism is relatively common in the ghost knifefishes (Gymnotiformes: Apteronotidae), with males of several species growing greatly elongated snouts (e.g., Compsaraia samueli, Parapteronotus hasemani ), while others develop enlarged, protruding teeth on either the lower jaw ( Sternarchorhynchus spp.) or both jaws ( Sternarchogiton nattereri ). Of the four known species of Sternarchogiton , sexual dimorphism has so far only been reported in S. nattereri . Here we report that in an additional species, S. labiatus , mature males possess similar enlarged, external teeth on the dentary and premaxilla. We document this condition in three specimens collected during the high-water spawning season from the río Nanay near Iquitos, Peru. We analyze this morphology using high-resolution X-ray microcomputed tomography. Additionally, we use genetic sequence data to demonstrate that specimens bearing external teeth are genetically indistinguishable from those with the phenotype of S. labiatus . Finally, we review and summarize the current knowledge of sexual dimorphism within the Apteronotidae.