sciencehabit writes: The opah lives in the dark, chilly depths of the world's oceans, using heated blood to keep warm. It's the first fish found to be fully warm-blooded. Certain sharks and tuna can warm regions of their body such as swimming muscles and the brain but must return to the surface to protect vital organs from the effects of the cold. The opah on the other hand, generates heat from its pectoral muscles, and conserves that warmth thanks to body fat and the special structure of its gills. “It’s a remarkable adaptation for a fish,” says Diego Bernal, a fish physiologist at the University of Massachusetts, Dartmouth.