The opah is the fish known to warm its entire body, including its heart and brain.
In 2015, scientists reported that the opah, also called the moonfish, maintains elevated temperatures throughout its body. This is unusual because most fish are ectothermic: their internal temperature largely follows the surrounding water. The opah achieves this partly through specialized blood vessels in its gills, where warm blood leaving the muscles transfers heat to cooler blood returning from the gills.
Unlike tuna and lamnid sharks, which can warm selected swimming muscles, the opah distributes heat more broadly. Its warm heart and brain help it function efficiently in the cold, dark waters where it hunts. The opah is not a mammal and does not maintain a mammal-like constant temperature; its body temperature still varies with the environment.
Opahs are deep-bodied, brightly colored fish found in several oceans. Their thermal adaptation is especially valuable during active pursuit of squid and small fish at depth.