Tuna

A remarkable aspect of the physiology of tuna in the genus Thunnus is their ability to maintain a body temperature significantly warmer than that of the ambient seawater. For example, the bluefin tuna can maintain a core body temperature of 75-95°F (24-35°C), even in water as cold as 43°F (6°C). However, unlike typical endothermic creatures such as mammals and birds, the body temperatures of tuna are not held constant within a relatively narrow range.

The endothermy of tuna is achieved by conserving the heat generated through normal body metabolism. This is accomplished through the action of an intertwined meshwork of veins and arteries, known as the rete mirable (meaning "wonderful net"), located in the periphery of the body. The rete mirable reclaims much of the heat in the venous blood, and transfers it to arterial blood through the action of a counter-current exchange system. This heat transfer slows down the rate of cooling of the tuna at the body surface, and thereby allows the animal to maintain a warmer core temperature. Higher body temperatures allow tuna to use their muscles more efficiently, and therefore swim more quickly with relatively little expenditure of energy. Higher temperatures of the body core may also contribute to the rapid and efficient digestion and absorption of food.

Tuna are fast swimmers, reaching speeds of up to 56 mph (90 kph) in the case of the large bluefin tuna. Moreover, tuna are well adapted for cruising great distances at a relatively brisk speed. However, tuna can also accelerate quickly while predating on other fish, or to avoid their own predators. A tuna's body is relatively elongated and fusiform, that is, tapering at both ends. Their fins are of a size and position designed to minimize drag, so that maximum speeds can be achieved and maintained with relatively small expenditures of energy. The major source of forward thrust while swimming is side-to-side movements of the caudal, or tail fin. Tunas also have relatively large median fins (especially the dorsal, or top fin), which is an adaptation to minimizing the drag associated with sideways slippage of the body during swimming.