A Dorsally Expressed Anal Fin in the Black Ghost Knifefish Apteronotus albifrons (Gymnotiformes: Apteronotidae)

The Gymnotiformes, or electric knifefishes, is a clade of freshwater ostariophysan fishes native to tropical South and Central America that are easily recognized by the presence of an elongate body and elongate undulating anal fin, and lack of dorsal and pelvic fins. In this study, we report on unusual specimens of the Black Ghost Knifefish Apteronotus albifrons (Apteronotidae) exported from Asian aquaculture facilities, which exhibit a fin located on the median dorsal surface. This fin exhibits the distinctive ball-and-socket articulating joint between the fin rays and pterygiophores otherwise only observed in gymnotiform anal fins and differs from the laterally restricted dorsal-fin-pterygiophore articulation of other ostariophysan taxa. We interpret the dorsally located fin of these specimens of Apteronotus albifrons as an ectopic expression of a median fin with an anal-fin phenotype and do not interpret the dorsally located fin as representing the reappearance of the plesiomorphic ostariophysan dorsal fin. The anterior–posterior position of the dorsally expressed fin corresponds closely with that of the dorsal organ, a fleshy electroreceptive structure that lies along the dorsal body margin of other apteronotids. This topological concordance implies the presence of a conserved embryonic tissue underlying development of both the dorsal organ and the dorsally expressed median fin.