FLAMMANG, B.E.*; AZIZI, E.; Moss Landing Marine Laboratories; University of Massachusetts, Amherst: The radial muscle: A new chapter in shark tails

The swimming kinematics and hydrodynamics of heterocercal tails in elasmobranchs have been the focus of a number of recent studies. However, the locomotor functions of the internal morphological structures of the heterocercal tail remain unexplored. In this study we examine the morphology and function of the radial muscle, or radialis, during swimming in the spiny dogfish, Squalus acanthias. The radialis consists entirely of red muscle fibers and is located ventral to the segmented axial myomeres in the most distal region of the caudal fin, originating on the ventral processes of the vertebral column and inserting along the horizontal septum. Individual muscle fibers are arranged dorsoposteriorly at angles ranging from 14 to 88 degrees relative to the vertebral column. The muscle fibers of the radialis share a similar fiber orientation and lie in close association with the deepest layer of the subdermal connective tissue sheets. Electrical stimulation of the radialis in anesthetized sharks results in visible movement of the ceratotrichia of the dorsal lobe of the caudal fin.. Using sonomicrometry, we found that muscle fibers shortened by <1% of their resting length during stimulated contractions. Finally, we combined bilateral electromyography of the radialis with simultaneous video to determine the point of activation of the radialis within the tailbeat cycle. Our results indicate that the radialis is active immediately after maximum lateral excursion of the caudal fin to the ipsilateral side. We also find that the activity patterns of the radialis on the right and left side of the body are approximately 180 degrees out of phase. Morphology and motor patterns of the radialis suggest that this muscle is acting as postural reinforcement, and controlling the orientation of the dorsal lobe of the caudal fin during steady swimming.