What is the only warm blooded fish in the world?

Opahs (also commonly known as moonfish, sunfish (not to be confused with Molidae), kingfish, redfin ocean pan, and Jerusalem haddock) are large, colorful, deep-bodied pelagic lampriform fishes comprising the small family Lampridae (also spelled Lamprididae).

The opah is the first fish discovered to have a warm heart, in May 2015. The opah can keep most of its body consistently at a temperature above the water temperature.

In May 2015, the opah was shown to maintain its entire body core above ambient temperature, becoming the first known fish with this trait ('whole-body endothermy'). The fish generates heat as well as propulsion with continuous movements of its pectoral fins (the musculature of which is insulated by a one-cm-thick layer of fat), and the vasculature of its gill tissue is arranged to conserve heat by a process of countercurrent heat exchange, a structure known as a rete mirabile (plural, retia). It can consistently keep its body core approximately 5 °C warmer than its environment. Elevated body core temperature should improve aerobic performance and physiologic function.