Mechanosensation in an adipose fin

Adipose fins are found on approximately 20% of ray-finned fish species. The apparently rudimentary anatomy of adipose fins inspired a longstanding hypothesis that these fins are vestigial and lack function. However, adipose fins have evolved repeatedly within Teleostei, suggesting adaptive function. Recently, adipose fins were proposed to function as mechanosensors, detecting fluid flowanterior to the caudal fin. Herewe test the hypothesis that adipose fins are mechanosensitive in the catfish Corydoras aeneus. Neural activity, recorded from nerves that innervate the fin, was shown to encode information on both movement and position of the fin membrane, including the magnitude of fin membrane displacement. Thus, the adipose fin of C. aeneus ismechanosensitive and has the capacity to function as a ‘precaudal flowsensor’. These data force reevaluation of adipose fin clipping, a common strategy for tagging fishes, and inform hypotheses of how function evolves in novel vertebrate appendages.