WILGA, C.D.*; LAUDER, G.V.: Function of the heterocercal tail in sharks: quantitative wake dynamics during steady horizontal swimming and vertical maneuvering.

The function of the heterocercal tail in sharks has long been debated in the literature. Previous kinematic data have supported the classical theory which proposes that the beating of the heterocercal caudal fin during steady horizontal locomotion pushes posteroventrally on the water which generates a reactive force directed anterodorsally. To quantify the function of the heterocercal shark tail during both horizontal locomotion and vertical maneuvering, we measured the orientation of tail vortices shed into the wake and the orientation of the central vortex jet through the core of these vortices. We used digital particle image velocimetry (DPIV) with both horizontal and vertical light sheet orientations to quantify patterns of wake velocity and vorticity behind the heterocercal tail of both leopard and bamboo sharks swimming freely at 1.0 L/s. Analysis of flow geometry indicates that both leopard and bamboo shark tails generate strongly tilted vortex rings with a mean jet angle of approximately 40^� below horizontal during steady horizontal swimming. The corresponding angle of the reaction force is much greater than either body angle (mean ~11^o) and the angle of the path of motion of the center of mass (mean ~0^o), thus strongly supporting the classical model for steady horizontal locomotion. Vortex jet orientation is significantly more inclined than the relatively horizontal jet generated by sturgeon tail vortex rings, further supporting the substantial differences in function in the heterocercal tails of sharks and sturgeon.