DONLEY, Jeanine, M; SHADWICK, Robert, E; SYME, Doug; SEPULVEDA, Chugey, A.; Scripps Institution of Oceanography, UCSD; Scripps Institution of Oceanography, UCSD; Department of Biological Sciences, University of Calgary; Scripps Institution of Oceanography, UCSD: Contractile properties of aerobic locomotor muscle in the leopard shark and shortfin mako shark
The work loop technique was used to examine the contractile properties of the red aerobic locomotor muscle (RM) in the leopard shark (Triakis semifasciata) and shortfin mako shark (Isurus oxyrinchus). The effects of axial position and temperature on the stimulus duration and phase producing maximum net positive work and power output were investigated. Contractile performance was measured at multiple temperatures from 10 to 26�C at cycle frequencies (analogous to tailbeat frequencies) ranging from 0.25 to 3 Hz using muscle bundles isolated from anterior (A) (0.4L where L is total body length) and posterior (P)(0.6-0.65L) axial positions. There was no significant difference in optimal stimulus duration, phase, net work, power output or isometric twitch kinetics in A and P bundles. Stimulus duration yielding maximum power decreased and optimal phase occurred earlier with increasing cycle frequency. Values for duration and phase, as well as the time course for isometric twitches were statistically similar between the two species. Temperature had little effect on optimal cycle frequencies for power output in the leopard shark, but frequencies increased dramatically (0.25 to 2 Hz) from 10 to 26�C in the mako. The data presented here support the hypothesis that the contractile properties of RM are constant along the body in both species. In addition, these data identify a significant difference in the effect of temperature on net work and power output in these two shark species, such that at temperatures which reflect those normally experienced in the mako the optimal cycle frequency is nearly double that of the leopard shark, suggesting that the mako may be able to maintain greater overall aerobic swimming speeds.
