GOLDBOGEN, J.A.; SHADWICK, K.E.; SHADWICK, R.E.; Scripps Institution of Oceanography, University of California, San Diego; Scripps Institution of Oceanography, University of California, San Diego; Scripps Institution of Oceanography, University of California, San Diego: Muscle activation during sprint swimming and fast starts in the leopard shark Triakis semifasciata

The initial stage of a fast start in teleosts involves rapid contraction of white glycolytic muscle fibers with near-synchronous activation along the body mediated by Mauthner neurons. Variations in the activation patterns have been described in different fish species, with some exhibiting muscle activity only on the side of the body where muscle shortens while others also exhibit synchronous activation, although of lower intensity, of muscle on the side that lengthens. Mauthner cells have been described in some larval elasmobranches such as dogfish, but not in adults of any species, so it is unclear if sharks have a rapid-conduction neuronal system to control their escape maneuvers. To investigate this problem we measured muscle activation by electromyography at 3 axial positions (0.3, 0.5, 0.7L, where L is total body length) in leopard sharks ranging from 70-120cm L, during escape responses induced by either a visual or electrical stimulus. The general form of the escape response is a tight �C� bend of the body, and in strong responses the curvature is great enough that the head touches the tail. In these cases we typically observed near synchronous activation on both sides of the body, with the time delay from 0.3-0.7L typically <10ms. In contrast, activation of white fibers during fast swimming is accompanied by a delay of 200-300 ms from 0.3-0.7L, indicating that the activation travels at 1-2L/s along the body. These results suggest that escape responses are controlled by a faster muscle activation system than is used for fast undulatory swimming.