The hammerhead shark's cephalofoil reduces fluid moments during turning motion

Abstract

Mechanical constraints are one factor that shapes organisms' behavior and morphology. Therefore, elucidating these biomechanics is essential for discovering unknown functions of organisms. The cephalofoil of hammerhead sharks (Sphyrnidae) has long been discussed as representative of the organism's unique morphology and function. While the physiological benefits of cephalofoil are becoming clearer, their kinematic function remains to be elucidated in some areas. Scalloped hammerheads (Sphyrna lewini) have been shown to receive significant hydrodynamic drag during straight swimming due to cephalofoil. In contrast, S. lewini is known to exhibit excellent turning ability, but the hydrodynamic effects of the cephalopod foil during turning have not been clarified. This study performed a numerical fluid dynamics analysis on a simplified 3D model of a cephalofoil to simulate its turning and swinging behavior. The results show that the cephalofoil shape reduces the moment from the fluid by reducing the separation of the flow during turning. The study also suggested that having a cephalofoil creates a trade-off between the efficiency of swimming straight and the ability to turn. This study contributes to our understanding of the function of the cephalofoil in hammerhead sharks and to a deeper understanding of their ecology.