The opah is the only known fish capable of heating its entire body above the surrounding water.
In 2015, researchers described how the opah, also called moonfish, retains heat produced by its muscles. Its blood vessels in the gills are arranged as a countercurrent heat exchanger: warm blood leaving the body transfers heat to cooler blood returning from the gills. Fat in the gill tissue and around the heart provides additional insulation.
This system gives the opah an advantage in cold, deep water. Warm muscles improve swimming performance, while a warmer heart and brain can support sustained activity. Most fish lose heat rapidly across the gills because the gills are directly exposed to the environment.
Some other fish, including tunas and lamnid sharks, can warm particular muscles or organs. They are therefore regionally endothermic rather than fully warm-bodied in the same way. The opah’s circular, flattened body and colorful appearance also make it distinctive among pelagic fish.