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San Diego, CA All too often the term "cold-blooded" is taken too literally, with members of Osteichthyes (fishes) assumed to have their body automatically change to the surrounding temperature. This assumption misses an important finding that nature knows how to ensure that certain fish species have physiological qualities that allow them to regulate body temperatures to survive and thrive in different environments.

One such example of this phenomenon is the tuna fish. Tunas are regional endotherms, maintaining elevated temperatures in deep red swimming muscles by way of vascular counter-current heat exchangers (retia mirabilia) that trap heat produced as a by-product of tissue metabolism. The close contact of blood vessels in the retia allows heat in the venous blood to be transferred to arterial blood that returns to the tissues, preventing loss of heat to the surrounding water as the venous blood passes through the gills. Some tuna species also warm the viscera (internal organs) and brain through associated vascular networks or retia. These retia keep tissues at temperatures higher than the surrounding water, enhance metabolic capacity, and act as effective insulators, which slow heat loss during the fish's journeys into cooler waters.

One species of tuna that makes such as journey is the yellowfin Thunnus albacares, a tropical-subtropical inhabitant of surface waters, that periodically makes rapid dives into deeper waters in search of prey, often encountering temperature gradients as much as 10C cooler. Previous studies have found that the Thunnus tuna possess a carotid rete, or vascular network, in the blood supply to the eye and brain that acts as a thermal barrier.

The Study

The objective of this study was to measure the ability of the carotid rete to insulate the brain of yellowfin tuna from rapid changes in environmental temperatures. The authors of "The Regulation of Brain Temperature in Yellowfin Tuna:
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Contact: Donna Krupa
djkrupa1@aol.com
703-967-2751
American Physiological Society
26-Aug-2002

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