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.
Lateral view of a terminal male Sternarchogiton labiatus, MUSM 59458–IQ17036, 198 mm TL. Specimen collected from the río Nanay at Iquitos, PE. Scale bar equals 1 cm.
Detail of the head of living monomorphic male Sternarchogiton labiatus (A) and terminal male S. labiatus with external dentition (B), both from MUSM 59458.
Detail of the tri-lobed structure on the lower jaw of terminal male Sternarchogiton labiatus IQ17042 (A) and the lower jaw of a female S. labiatus (B), both from MUSM 59458. Arrows indicate lateral lobes on lower jaw.
Micro-CT scans of the skull in lateral view of male Sternarchogiton labiatus, MUSM 59458–IQ17036 (A); female S. labiatus, MUSM 59458 (B); male S. nattereri, MUSM 54682 (C); and female S. nattereri, MUSM 54644 (D). Abbreviations: d, dentary; end, endopterygoid; fr, frontal; let, lateral ethmoid; met, mesethmoid; mx, maxilla; pmx, premaxilla.
Dentary and premaxillary dentition of Sternarchogiton labiatus and S. nattereri. Lower jaw in medial view of male S. labiatus, MUSM 59458–IQ17036 (A); female S. labiatus, MUSM 59458 (B); male S. nattereri, MUSM 54682 (C); female S. nattereri, MUSM 54644 (D). Paired premaxillae in ventral view of male S. labiatus, MUSM 59458–IQ17036 (E); female S. labiatus, MUSM 59458 (F); male S. nattereri, MUSM 54682 (G); female S. nattereri, MUSM 54644 (H).
Neurocrania of Sternarchogiton in dorsal and lateral view, showing sexually dimorphic facial elongation in male S. labiatus, MUSM 59458–IQ17036 (A); female S. labiatus, MUSM 59458 (B); toothed male S. nattereri (C); and female S. nattereri (D). Abbreviations: fr, frontal; met, mesethmoid; pas, parasphenoid; v, vomer.
Maximum likelihood phylogeny of Sternarchogiton based on mitochondrial cytochrome b sequence data. The two toothed male Sternarchogiton IQ17036 and IQ17042 are shaded in gray. Numbered nodes show bootstrap support based on 500 replicates. See Data Accessibility for tree file.
Contributor Notes
Associate Editor: M. P. Davis.