P1.115 Wednesday, Jan. 4 Adipose fin function in Horabagrus brachysoma: first identification of a muscular control mechanism for the adipose fin STEWART, TA*; HALE, ME; Univ. of Chicago; Univ. of Chicago email@example.com
The adipose fin lies between the dorsal and caudal fins of some actinopterygian fishes. Among fish fins it is unique in its lack of intrinsic musculature, and is further distinguished by the frequent absence of endoskeletal support. Unlike other actinopterygian fins, it has been presumed to be a passive structure, with no muscular control of its movement. Adipose fins have not been studied in depth and represent perhaps the least well understood of vertebrate appendages. To understand the function of adipose fins in Siluriformes (catfishes), we surveyed the gross anatomy of the Asian sun catfish, Horabagrus brachysoma. We identify a muscular linkage associated with the adipose fin, the first putative active control mechanism for this fin system. Tendons extending anteriorly from the postero-lateral sides of the adipose fin connect with paired muscles at the dorsal midline. We propose that this linkage may help control adipose fin position, and call these muscles ‘extrinsic adipose fin muscles’ (AFCMs). Biomechanical modeling of AFCMs and associated tendons and adipose fin skeletal elements is used to predict fin movement ability and to generate hypotheses that may be tested functionally with muscle stimulation approaches. Currently, hypotheses of adipose fin function are limited to the passive influence of flow around the caudal fin or its serving as a pre-caudal flow sensor. The linkage identified here allows for new hypotheses for adipose fin function. For example, some adipose fins may actively modulate pre-caudal flow or control fin position for sensory functions. This work suggests unrecognized diversity in the adipose fin system, further studies of which may improve our understanding of the evolution and development of novel appendages in vertebrates.