BASIC CONCEPTS RELEVANT TO HEAT-TRANSFER IN FISHES, AND THEIR USE IN MEASURING THE PHYSIOLOGICAL THERMOREGULATORY ABILITIES OF TUNAS

Aerobic heat production and heat loss via the gills are inexorably lin ked in all water breathing teleosts except tunas. These processes are decoupled in tunas by the presence of vascular counter-current heat ex changers, and sustained (i.e., steady state) muscle temperatures may e xceed water temperature by 10-degrees-C or more in larger individuals. The presence of vascular counter-current heat exchangers is not clear ly advantageous in all situations, however. Mathematical models predic t that tunas could overheat during strenuous activity unless the effic acy of vascular heat exchangers can be reduced, and that they may be a ctivity limited in warmer waters. Tunas may likewise be forced out of potentially usable habitats as they grow because they have to occupy c ooler waters. Vascular counter-current heat exchangers also slow rates of heating and cooling. A reduced rate of muscle temperature decrease is clearly advantageous when diving into colder water to chase prey o r avoid predators. A reduced rate of heat gain from the environment wo uld be disadvantageous, however, when fish return to the warmer surfac e waters. When subjected to changes in ambient temperature, tunas cann ot defend a specific body temperature and do not thermoregulate in the mammalian sense. Yet when appropriately analyzed, data taken under st eady state and non-steady state conditions indicate that tunas are not strictly prisoners of their own thermoconserving mechanisms. They app arently can modify overall efficiency of their vascular counter-curren t heat exchangers and thus avoid overheating during bouts of strenuous activity, retard cooling after diving into colder water, and rapidly warm their muscles after voluntarily entering warmer water. The exact physiological mechanisms employed remain to be elucidated.