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.
Photograph of an aquarium specimen of Apteronotus albifrons with the “double fin” phenotype. Note the elongate median fin along the dorsal body margin, a trait not observed in wild-caught specimens of this species or in any other gymnotiform species. Specimen not preserved. (Image copyright Robert Beke; reproduced with written permission from Robert Beke.)
Photograph of subadult Apteronotus albifrons “double fin” specimens. (A) LSUMZ 22459, 75 mm TL. (B) LSUMZ 22459, 80 mm TL. Note the different position of dorsal-fin origins (arrows) and fin-ray lengths between the specimens. Scale bar = 1 cm.
Diagram of the dorsal-median-fin-pterygiophore arrangement of Apteronotus albifrons “double fin” LSUMZ 22459, 75 mm TL. Abbreviations: DPC, distal pterygiophore cartilage; MP, medial pterygiophore; PP, proximal pterygiophore. Scale bar = 5 mm.
Arrangement of Apteronotus albifrons anal-fin pterygiophores. (A) Anterior portion of anal fin; (B) anal fin at midbody; (C) posterior portion of anal fin. Note changes in orientation of pterygiophores along the length of the fin and the shape and orientation of the distal pterygiophore cartilages (gray). Abbreviations as in Figure 3. Scale bar = 2 mm.
Dorsal margin of Apteronotus albifrons “double fin” LSUMZ 22459, 75 mm TL. Note the lateral line nerve bundles indicative of the apteronotid dorsal organ as dark gray lines. Scale bar = 1 mm.
Diagrams of (A) Gymnocorymbus sp. (Characiformes) dorsal-fin pterygiophores and (B) anal-fin pterygiophores. Scale bar = 1 mm. Abbreviations as in Figure 3, except DP, distal pterygiophore.
Hypothesis of median-fin evolution in gymnotiform fishes. In the plesiomorphic condition, the dorsal-fin analage (DFA) and anal-fin analage (AFA) generate a dorsal fin (DF) and anal fin (AF), respectively. In the Gymnotiformes, DF development is lost, and a derived ball-and-socket articulation of AF rays and pterygiophores is gained (BSAF). In the Apteronotidae, the DFA generates an electroreceptive dorsal organ (DO). Phylogeny from Tagliacollo et al. (2016).
Contributor Notes
Associate Editor: T. Grande