Two synchronized high-speed video cameras allowed simultaneous measurement of shark body position and wake structure. We used DPIV with both horizontal and vertical light sheet orientations to quantify patterns of wake velocity and vorticity behind the heterocercal tail of leopard sharks ( Triakis semifasciata) and bamboo sharks ( Chiloscyllium punctatum) swimming at 1.0 Ls -1, where L is total body length. Both steady horizontal locomotion and vertical maneuvering were analyzed. To address the two models of shark heterocercal tail function, we applied the technique of digital particle image velocimetry (DPIV) to quantify the wake of two species of shark swimming in a flow tank. In this paper,we quantify the function of the tail in two species of shark and compare shark tail function with previous hydrodynamic data on the heterocercal tail of sturgeon Acipenser transmontanus. An alternative model suggests that the heterocercal shark tail functions to direct reaction forces through the center of mass. 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, generating a reactive force directed anterodorsally and causing rotation around the center of mass. The function of the heterocercal tail in sharks has long been debated in the literature.
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