Did you know that tuna fish are able to regulate their body temperature?
However, tunas use a thermoregulation system which is different from that of land-based homeotherms; they use a system called rete mirabile. This is a set of small arteries and veins arranged in parallel that act as a heat exchanger. The arterial blood, oxygenated and cooled in the gills, takes heat from the venous blood, heated by the metabolism of the fish. This avoids the loss of heat through the gills, leaving it trapped to warm the body.