Three-dimensional kinematics and wake structure of the pectoral fins during locomotion in leopard sharks Triakis semifasciata

The classical theory of locomotion in sharks proposes that shark pectoral f ins are oriented to generate lift forces that balance the moment produced b y the oscillating heterocercal tail, Accordingly, previous studies of shark locomotion have used lived-wing aircraft as a model assuming that sharks h ave similar stability and control mechanisms. However, unlike airplanes, sh arks are propelled by undulations of the body and tail and have considerabl e control of pectoral fin motion. In this paper, we use a new approach to e xamine the function of the pectoral fins of leopard sharks, Triakis semifas ciata, during steady horizontal swimming at speeds of 0.5-2.0 l s(-1), wher e l is total body length, and during vertical maneuvering (rising and sinki ng) in the water column. The planar orientation of the pectoral fin was mea sured using three-dimensional kinematics, while fluid how in the wake of th e pectoral fin and forces exerted on the water by the fin were quantified u sing digital particle image velocimetry (DPIV). Steady horizontal swimming in leopard sharks is characterized by continuous undulations of the body wi th a positive body tilt to the flow that decreases from a mean of 11 degree s to 0.6 degrees with increasing how speeds from 0.5 to 2.0 l s(-1). Three- dimensional analysis showed that, during steady horizontal locomotion, the pectoral fins are cambered, concave downwards, at a negative angle of attac k that we predict to generate no significant lift. Leopard shark pectoral f ins are also oriented at a substantial negative dihedral angle that amplifi es 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 pla ne 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 b ody relative to the flow. Body and pectoral fin surface angle are altered s ignificantly when leopard sharks change vertical position in the water colu mn, 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 bo dy 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 s teady horizontal locomotion.