Meeting Abstract
How do new fins evolve? A comprehensive answer must integrate multiple kinds of explanations: how does development evolve to generate these new body parts, and what adaptive functions might mediate their evolution? We studied adipose fins, appendages that have evolved repeatedly in teleost fishes and that are positioned posterior to the dorsal fin, to inform how new median fins originate. We demonstrate that the adipose fin of the catfish Corydoras aeneus develops by the retention and elaboration of the larval fin fold and that this fin is mechanosensitive. Immunohistochemistry reveals extensive reorganization of nerves in a portion of the larval fin fold that will become the adipose fin membrane, and neurophysiological recordings of these nerves demonstrate that the magnitude to which the fin is displaced laterally is encoded by fin afferents. These data demonstrate that the larval fin fold can act not just as scaffolding for the development of differentiated median fins, but also as a substantive source of material contribution to a new fin, which then can be modified over ontogeny and phylogeny to generate a structurally and functionally complex appendage. This is consistent with outstanding hypotheses for how the earliest dorsal and anal fins originated, by the retention and transformation of a median fin fold. Mechanosensitivity of the adipose fin of C. aeneus is consistent with the hypothesis of their function as pre-caudal flow sensors, raising questions about the practice of adipose fin clipping as a means of tagging hatchery-raised fishes and of the ecological consequences of adipose fin diversity.
