Opah, the first warm-blooded fish

Deep in the water, where the light barely reaches, creatures tend to live life in the slow-lane.  Rather than chasing prey all about, they are usually ambush, opportunistic feeders with very little energy spent on actively swimming.  The opah (Lampris guttatus) is like finding a race car among roller skates.

The opah has an unusual blood vessel system in which vessels around the gills wrap around vessels which lead back to the core.  Blood is warmed in the core of the body and the heat is passed to blood coming back from the gills.  This means that the opah can maintain a temperature about 5C (41F) above the surrounding water from 150-1,000 ft deep.  Maintaining a warmer temperature not only helps them to resist the effects of cold water, but it can also boost their eye and brain function and muscle output and capacity.

Source Article: First fully warm-blooded fish: The opah or moonfish