Though rare, some fish are able to internally regulate temperature.
Unlike ectotherms, which rely upon environmental temperatures, endotherms are able to metabolically control their body temperature. This thermoregulatory strategy is rare among fish but is present in tunas and some sharks, including the Great White Shark (Carcharodon carcharias) and Shortfin Mako Shark (Isurus oxyrinchus). Endothermic tunas and sharks use a network of capillaries in their swimming muscles, the Rete mirabile, as a heat exchanger. Through counter-current exchange, the heat produced through muscle activity is transported by the blood. Through this metabolic process, sharks, for example, can maintain a body temperature of 5 – 14°C above ambient water temperature. This process is an evolutionary advantage for these long distance, migratory fish, allowing them to travel extensive distances and dive deep while maintaining body temperature, conserving energy, and avoiding thermal shock from changes in water temperature.
Tunas are one example of endothermic fish.