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The Journal of Experimental Biology 204, 4043-4054 (2001)
© 2001 The Company of Biologists Limited

Water-tunnel studies of heat balance in swimming mako sharks

Diego Bernal*, Chugey Sepulveda and Jeffrey B. Graham

Center for Marine Biotechnology and Biomedicine, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0204, USA

*e-mail: dbernal{at}ucsd.edu

Accepted September 24, 2001

The mako shark (Isurus oxyrinchus) has specialized vascular networks (retia mirabilia) forming counter-current heat exchangers that allow metabolic heat retention in certain regions of the body, including the aerobic, locomotor red muscle and the viscera. Red muscle, white muscle and stomach temperatures were measured in juvenile (5–13.6 kg) makos swimming steadily in a water tunnel and exposed to stepwise square-wave changes in ambient temperature (Ta) to estimate the rates of heat transfer and to determine their capacity for the activity-independent control of heat balance. The rates of heat gain of red muscle during warming were significantly higher than the rates of heat loss during cooling, and neither the magnitude of the change in Ta nor the direction of change in Ta had a significant effect on red muscle latency time. Our findings for mako red muscle are similar to those recorded for tunas and suggest modulation of retial heat-exchange efficiency as the underlying mechanism controlling heat balance. However, the red muscle temperatures measured in swimming makos (0.3–3°C above Ta) are cooler than those measured previously in larger decked makos. Also, the finding of non-stable stomach temperatures contrasts with the predicted independence from Ta recorded in telemetry studies of mako and white sharks. Our studies on live makos provide new evidence that, in addition to the unique convergent morphological properties between makos and tunas, there is a strong functional similarity in the mechanisms used to regulate heat transfer.

Key words: lamnid, shark, Isurus oxyrinchus, tuna, thermoregulation, endothermy, red muscle, white muscle, stomach, heat balance, water tunnel.


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© The Company of Biologists Ltd 2001