The opah has the highest known body temperature among fish because it can keep its entire body warmer than the surrounding water. This unusual ability is called whole-body endothermy.
Most fish are ectothermic, meaning their body temperature largely follows the water around them. The opah generates heat through powerful pectoral-muscle activity and conserves it with specialized heat exchangers in its blood vessels. These adaptations help it remain active in cold, deep waters.
The opah is sometimes confused with tuna, sharks, and billfishes, which can also warm specific regions such as their swimming muscles, brains, or eyes. Those fish have regional endothermy; the opah is exceptional because its internal organs are also kept warmer. It is not perfectly warm-blooded like a mammal, however: its temperature still changes with the surrounding environment.
Opahs are broad, disk-shaped marine fish found in oceans worldwide. Their ability to stay warm improves swimming performance and helps them hunt efficiently at depths where the water is cold.