Mechanism describes an organism’s structures and how they function in regards to its proximate factors. It specifically refers to underlying internal factors such as nervous, genetic, hormonal stimuli and fundamental biological functions of the behavior.
Endothermy is the ability to maintain body temperatures that are higher than environmental temperatures. Tuna are endothermic and therefore are able to migrate over huge distances and make deep vertical dives in order to catch prey and avoid predators while maintaining a high over-all body temperature. (Kubo et al)
Atlantic bluefin tuna subject themselves to intense vertical descents and extensive migrations across many varying ambient temperatures. With such temperature changes that can be as extreme as 15°C in 10 to 15 minutes, bluefin tuna must thermoregulate in order to avoid hypothermia and shock in general, keeping in mind that theses changes also occur on a more prolonged scale for long migrations. (Brill and Bushnell)
Endothermy, or warm-bloodedness, is the phenomenon that enables bluefin tuna to make their way across vast oceans and dive to such severe depths. Endothermy is the maintenance of high body temperatures that typically result from frequent resting, aerobic heat production rates in soft tissues and thick body insulation. (Ruben) Endothermic fishes use blood vessels arranged as counter current heat exchangers retia mirbilia between their endothermic tissue and gills where heat transferred from tissue would be lost in water. Some endothermic fish such as shortfin mako, white sharks, and northern bluefin tunas evolved to be able to elevate temperature of core body tissues in order to maintain stable body temperatures even as ambient temperatures fluctuate. (Dickson and Graham)
Thermal difference is highest between ambient water temperatures and red muscle. Because tuna maintain constant temperatures during aerobic swimming, it suggests that swimming generates heat internally and this heat gained is balanced out by heat lost to gill ventilation of cooler waters. The source of the internal body heat is gained trough the metabolic heat energy gain by aerobic and anaerobic swimming motions, while a small portion of heat is lost though the body surface contact and gill exposure to low ambient temperatures. The elevated body temperature is then maintained by using a vascular heat exchanger system which creates a countercurrent heat exchanger system recycling though warm venous blood and cool arterial blood. (Kubo et al)
They have vascular counter-current heat exchangers that enable them to maintain elevated temperatures in deep red-muscle fibers and their critical organs well above ambient water temperature. These heat exchangers enable the bluefin tuna’s blood to be subjected to “closed-system” temperature changes. Closed-system temperatures changes are fluctuations of blood temperature when the blood is not able to exchange gases, such as O2, as it passes through the tunas’ gills. Therefore the O2 levels are able to stay constant even in extremely cold ambient temperatures. (Kitagawa et al) In order to retain such high body temperatures, there must be a source of heat as well as a mechanism to retain that metabolic heat. This phenomenon is described as the tuna’s thermoconservation ability, which is their primary mechanism for being able to migrate long distances and make deep dives. (Kitagawa et al)