DONLEY, Jeanine/M; SHADWICK, Robert/E: Steady swimming muscle dynamics in the leopard shark (Triakis semifasciata)

We investigated the muscle dynamics of the aerobic red myotomal muscle during steady swimming in the leopard shark, Triakis semifasciata. Electromyographic and sonometric techniques were used to measure red muscle activation patterns and instantaneous muscle length. The timing of muscle activation and shortening and relationships between muscle strain and local body curvature were assessed at two axial positions (0.44L and 0.63L). In this preliminary study, it was observed that the onset of muscle activation occurred just before peak muscle length and offset occurred just before the end of the muscle shortening cycle. Based on in vitro work loop studies, this type of activation pattern has been shown to produce net positive work and provide maximal contractile force and power output. Activation occurred at the same phase of the strain cycle at both axial positions, and duty cycles were similar at the two positions. Red muscle strain and thus the degree of muscle shortening increased caudally. Strain was synchronous with local body bending, a character that reflects the superficial red muscle position in the leopard shark. These preliminary data contribute to a larger study whose goal is to characterize the locomotion and muscle dynamics of the shortfin mako shark (Isurus oxyrhincus; family Lamnidae) and to determine the degree of similarity between the locomotion and muscle dynamics in tunas and lamnids. Because little is known about the muscle dynamics of elasmobranchs, the first step towards this goal was to characterize the muscle dynamics of a typical ectothermic elasmobranch with subcutaneous red muscle position to establish a baseline to which the muscle dynamics of the mako shark, an endothermic species with internalized red muscle, will be compared.