SHADWICK, R E; BERNAL, D; SYME, D A ; DONLEY, J M; UC San Diego; Weber State Univ.; Univ. of Calgary; UC San Diego: Hot shark in cold water: contractile properties of muscle from the endothermic alaskan salmon shark, Lamna ditropis.

We studied properties of red (RM) and white (WM) muscle from 3 Lamna ditropis (2.2m long, aprox. 160kg), captured in Prince William Sound, Alaska. Muscle temperatures measured immediately after capture showed a large thermal gradient, from about 26�C in core RM to 10�C in superficial WM, with ambient sea surface temperatures of 6 to 8�C. Muscle twitch times measured in situ by a portable stimulator/isotonic transducer showed no longitudinal variation in twitch time of superficial WM, with an average time to peak force of 140ms at 10�C. Shorter twitch times occurred in deep WM, corresponding to an increasing temperature gradient toward the RM. Contractile tests were conducted on isolated bundles of muscle fibers. The average time to peak force for WM ranged from 130ms at 10�C to 43ms at 26�C, yielding a Q10 of 2.0. In contrast, RM was very temperature sensitive below 20�C and incapable of contracting below 15�C. RM twitch times had a Q10 of 4.8 below 20�C and 2.4 above 20�C; times to peak force were about 1500ms at 15�C, 700ms at 20�C and 260ms at 31�C. Work loops using optimized stimulus parameters generated temperature dependent power spectra with WM cycle frequency maximum power ranging from 4Hz at 10�C to 6Hz at 26�C, and positive power being maintained up to 9Hz at 26�C. RM positive power was restricted to low frequencies, <2Hz at 31�C, <1Hz at 20�C, and 0.5Hz at 15�C. Salmon shark WM can power fast swimming across a wide range of temperatures, but RM is specialized for use at warm temperatures only. Thus, a large body size and continuous swimming allow salmon sharks to maintain optimal muscle working conditions by elevating RM temperatures, permitting them to be very active and exploit energy-rich cold water habitats.