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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We thank the staff at the University of Alaska Seward Marine Center for the use of laboratory facilities, the captain and crew of the F/V Legend for their assistance in the fishing operations, and J. Valdez for logistical support. This work was supported by funding from the NSF and the University of California San Diego Academic Senate (R.E.S. and D.B.) and the NSERC (D.A.S.). Author Contributions All authors contributed equally to project planning, experimental work and writing of this Letter.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
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