Meeting Abstract
Phenotypes can evolve by leaps. One mode of saltation is the translocation of tissues or organs from one part of the body to another. Adipose fins (appendages found on many teleost fishes between the dorsal and caudal fin) have repeatedly evolved skeleton in this way—lepidotrichia having been coopted into this territory at least four times independently—providing a powerful system to explore this phenomenon. To understand the origin of this skeleton, I describe the morphology and development of lepidotrichia in the adipose fin of the redtail catfish, Phractocephalus hemioliopterus. Specimens (n=54) ranged in size from 20 to 90 cm standard length and came from wild populations and public aquariums. Adipose fins were x-rayed in lateral aspect. Radiographs were digitized, and adipose fin area and the amount of skeleton in the fin were quantified. The development of lepidotrichia in these fins differs in several ways from what has been described in other fin systems. These lepidotrichia begin to differentiate at the distal tip of the fin, while other lepidotrichia begin to differentiate proximally. These lepidotrichia appear to grow both proximally and distally, while other lepidotrichia are thought to grow only distally. Finally, these lepidotrichia can begin differentiating at multiple sites within the fin, while other fin skeleton begins differentiating at a single location. Adipose fin lepidotrichia are also uniquely variable: rays are wavy, irregular in their thickness, and exhibit no regular pattern of segmentation or branching. The skeleton of P. hemioliopterus adipose fins is, thus, among the most variable of all vertebrate appendages. These results imply a lack of canalization, highlight how diverse developmental programs can underlie similar morphologies, and suggest that the identity of serial homologs can be affected by developmental context.
