Meeting Abstract

32.5  Friday, Jan. 4  Morphological and functional investigation of the radialis muscle in shark tails FLAMMANG, B.E.; Harvard University bflammang@oeb.harvard.edu

Although the swimming kinematics and hydrodynamics of heterocercal tails in elasmobranchs have been studied previously, the locomotor functions of the internal morphological structures of the caudal fin remain largely unexplored. The morphology and function of the radial muscle, or radialis, during steady swimming at different speeds were investigated in the spiny dogfish Squalus acanthias, smooth hound shark Mustelus canis, and bonnethead shark Sphyrna tiburo. Comparative morphology of the caudal fin of these three species, as well as representatives of the families Squalidae, Alopiidae, Cetorhinidae, Lamnidae, Scyliorhinidae, Triakidae, Carcharhinidae, and Sphyrnidae, showed no variation in radial muscle morphology amongst species despite having vastly different external caudal fin morphologies and locomotory habitat usage. Using bilateral electromyography of the radialis and the red and white muscles of the caudal peduncle, synchronized with high-speed video, we determined that the point of activation of the radialis within the tailbeat cycle occurs after maximum lateral excursion of the caudal fin to the ipsilateral side. With increasing swimming speed, the radialis is active for a shorter duration of the tailbeat cycle, indicating a change in duty cycle of the muscle. Morphology and motor patterns of the radialis suggest that this muscle acts as postural reinforcement, and controls the orientation of the dorsal and ventral lobes of the caudal fin during steady swimming.