Letter

Nature 437, 1349-1352 (27 October 2005) | doi:10.1038/nature04007; Received 15 April 2005; Accepted 11 July 2005

Mammal-like muscles power swimming in a cold-water shark

Diego Bernal1,2, Jeanine M. Donley3, Robert E. Shadwick2,5 & Douglas A. Syme4

  1. Department of Biology, University of Massachusetts, Dartmouth, North Dartmouth, Massachusetts 02747, USA
  2. Marine Biology Research Division, Scripps Institution of Oceanography, La Jolla, California 92093-0202, USA
  3. Department of Biological Sciences, Miracosta College, Oceanside, California 92056, USA
  4. Department of Biological Sciences, University of Calgary, Alberta, T2N 1N4 Canada
  5. †Present address: Department of Zoology, University of British Columbia, Vancouver, British Columbia, V6T 1Z4, Canada

Correspondence to: Diego Bernal1,2Robert E. Shadwick2,5 Correspondence and requests for materials should be addressed to R.E.S. (Email: shadwick@zoology.ubc.ca) or D.B. (Email: dbernal@umassd.edu).

Effects of temperature on muscle contraction and powering movement are profound, outwardly obvious, and of great consequence to survival1, 2. To cope with the effects of environmental temperature fluctuations, endothermic birds and mammals maintain a relatively warm and constant body temperature, whereas most fishes and other vertebrates are ectothermic and conform to their thermal niche, compromising performance at colder temperatures2, 3. However, within the fishes the tunas and lamnid sharks deviate from the ectothermic strategy, maintaining elevated core body temperatures4, 5 that presumably confer physiological advantages for their roles as fast and continuously swimming pelagic predators. Here we show that the salmon shark, a lamnid inhabiting cold, north Pacific waters, has become so specialized for endothermy that its red, aerobic, locomotor muscles, which power continuous swimming, seem mammal-like, functioning only within a markedly elevated temperature range (20–30 °C). These muscles are ineffectual if exposed to the cool water temperatures, and when warmed even 10 °C above ambient they still produce only 25–50% of the power produced at 26 °C. In contrast, the white muscles, powering burst swimming, do not show such a marked thermal dependence and work well across a wide range of temperatures.

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