The opah is the only fish currently known to exhibit whole body endothermy where all the internal organs are kept at a higher temperature than the surrounding water. This feature allows opahs to maintain an active lifestyle in the cold waters they inhabit. Unlike birds and mammals, the opah is not a homeotherm despite being an endotherm: while its body temperature is raised above the surrounding water temperature, it still varies with the external temperature and is not held constant. In addition to whole body endothermy, the opah also exhibits regional endothermy by raising the temperature of its brain and eyes above that of the rest of the body. Regional endothermy also arose by convergent evolution in tuna, lamnid sharks and billfishes where the swimming muscles and cranial organs are maintained at an elevated temperature compared to the surrounding water.