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First published online March 16, 2007
Journal of Experimental Biology 210, 1194-1203 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02730

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Thermal dependence of contractile properties of the aerobic locomotor muscle in the leopard shark and shortfin mako shark

Jeanine M. Donley¹, Robert E. Shadwick^2,*, Chugey A. Sepulveda³ and Douglas A. Syme⁴

¹ Biology Department, MiraCosta College, Oceanside, CA 92056, USA
² Marine Biology Research Division, Scripps Institution of Oceanography University of California, San Diego, La Jolla, CA 92093-0202, USA
³ Pfleger Institute of Environmental Research, Oceanside, CA 92054, USA
⁴ Department of Biological Sciences, University of Calgary, Calgary, Alberta, T2N 1N4, Canada

^* Author for correspondence at present address: Department of Zoology, University of British Columbia, 6270 University Boulevard, Vancouver, BC, V6T 1Z4, Canada (e-mail: shadwick{at}zoology.ubc.ca)

Accepted 23 January 2007

The work loop technique was used to examine contractile properties of the red aerobic locomotor muscle (RM) in the ectothermic leopard shark Triakis semifasciata and endothermic shortfin mako shark Isurus oxyrinchus. The effects of axial position and temperature on the twitch kinetics, and the stimulus duration and phase producing maximum net positive work and power output were investigated. Contractile performance was measured over the temperature range of 15 to 25°C for Triakis and 15 to 28°C for Isurus at cycle frequencies (analogous to tailbeat frequencies) ranging from 0.25 to 3 Hz using muscle bundles isolated from anterior (0.4 L where L is total body length) and posterior (0.6–0.65 L) axial positions. Pairwise comparisons of twitch times for anterior and posterior muscle samples indicated that there were no significant differences related to body position, except in mako sharks at unphysiologically cool temperatures (<20°C). We found no significant differences in optimal stimulus duration, phase, net work or power output between anterior and posterior bundles in each species. With increasing cycle frequency the stimulus duration yielding maximum power decreased while optimal phase occurred earlier. The cycle frequency at which peak power was generated in leopard shark RM was only affected slightly by temperature, increasing from about 0.6 to 1.0 Hz between 15 and 25°C. In contrast, mako RM showed a much more dramatic temperature sensitivity, with the peak power frequency rising from <0.25 to 2.25 Hz between 15 and 28°C. These data support the hypothesis that the contractile properties of RM are functionally similar along the body in both species. In addition, our data identify a significant difference in the effect of temperature on net work and power output between these two shark species; at 15°C muscle from the ectothermic leopard shark performs relatively well in comparison with mako, while at higher temperatures, which reflect those normally experienced by the mako, the optimal cycle frequency for power is nearly double that of the leopard shark, suggesting that the mako may be able to maintain greater aerobic swimming speeds.

Key words: red muscle, temperature, contractile properties, work loop, lamnid, Isurus, Triakis