HETEROCERCAL TAIL FUNCTION IN LEOPARD SHARKS - A 3-DIMENSIONAL KINEMATIC ANALYSIS OF 2 MODELS

Two different models have been proposed to explain the function of the heterocercal tail in shark locomotion, The classical model proposes t hat, as a result of lift generated by the tail as it beats, the net fo rce acting on the tail is directed dorsally and anteriorly, In contras t, Thomson's model suggests that the tail generates a net force direct ed through the shark's center of gravity, i.e. ventrally and anteriorl y. In this study, we evaluate these two models by describing the three -dimensional kinematics of the heterocercal tail in the leopard shark Triakis semifasciata during swimming, Lateral and posterior views of t he tail were examined from four individuals swimming in a flow tank at 1.2Ls(-1) (where L is total length) using two high-speed speed video cameras filming simultaneously at 250 fields s(-1). These two simultan eous views allowed eight landmarks on the tail to be followed in three dimensions through time. These landmarks allowed the tail to be divid ed into separate surfaces whose orientation over time was calculated, Points located anteriorly on the tail go through significantly smaller excursions and reach their maximum lateral excursion significantly ea rlier in the beat cycle than points on the trailing edge of the tail, Three-dimensional angle calculations show that the terminal lobe leads the ventral lobe through a beat, as predicted by the classical model, Dye-stream visualizations confirmed that this pattern of movement def lects water ventrally and posteriorly to the moving tail, providing st rong support for the classical model, Additionally, our results show t hat a three-dimensional analysis is critical to understanding the func tion of the heterocercal tail.