Summary

The classical theory of locomotion in sharks proposes that shark pectoral fins are oriented to generate lift forces that balance the moment produced by the oscillating heterocercal tail. Accordingly, previous studies of shark locomotion have used fixed-wing aircraft as a model assuming that sharks have similar stability and control mechanisms. However, unlike airplanes, sharks are propelled by undulations of the body and tail and have considerable control of pectoral fin motion. In this paper, we use a new approach to examine the function of the pectoral fins of leopard sharks, Triakis semifasciata, during steady horizontal swimming at speeds of 0.5-2.0ls(−1), where l is total body length, and during vertical maneuvering (rising and sinking) in the water column. The planar orientation of the pectoral fin was measured using three-dimensional kinematics, while fluid flow in the wake of the pectoral fin and forces exerted on the water by the fin were quantified using digital particle image velocimetry (DPIV). Steady horizontal swimming in leopard sharks is characterized by continuous undulations of the body with a positive body tilt to the flow that decreases from a mean of 11 degrees to 0.6 degrees with increasing flow speeds from 0. 5 to 2.0ls(−1). Three-dimensional analysis showed that, during steady horizontal locomotion, the pectoral fins are cambered, concave downwards, at a negative angle of attack that we predict to generate no significant lift. Leopard shark pectoral fins are also oriented at a substantial negative dihedral angle that amplifies roll moments and hence promotes rapid changes in body position. Vortices shed from the trailing edge of the pectoral fin were detected only during vertical maneuvering. Starting vortices are produced when the posterior plane of the pectoral fin is actively flipped upwards or downwards to initiate rising or sinking, respectively, in the water column. The starting vortex produced by the pectoral fin induces a pitching moment that reorients the body relative to the flow. Body and pectoral fin surface angle are altered significantly when leopard sharks change vertical position in the water column. Thus, locomotion in leopard sharks is not analogous to flight in fixed-wing aircraft. Instead, a new force balance for swimming leopard sharks is proposed for steady swimming and maneuvering. Total force balance on the body is adjusted by altering the body angle during steady swimming as well as during vertical maneuvering, while the pectoral fins appear to be critical for initiating maneuvering behaviors, but not for lift production during steady horizontal locomotion.