Scientists Discover The World's First Warm-Blooded Fish

The large, round fish—about the size of a manhole cover—uses its warm blood as an advantage in the ocean's freezing depths.

 Southwest Fisheries Science Center Biologist Nick Wegner holds captured opah.


Deep-water fish called opah appear to be the first fully warm-blooded fish species ever discovered, according to a new paper published in Science. Researchers say the unique biology behind opah, also known as moonfish, allow the species to operate at peak performance even within frigid ocean depths.

Being warm-blooded has its perks. Birds and mammals (or endotherms) conserve their internal heat to maintain high body temperatures, which helps them flee predators, chase prey and thrive in sub-zero climates. But fish—and other cold-blooded animals, like reptiles and amphibians—aren't so lucky. Most deep-sea fish move slowly, preferring to ambush prey rather than give chase, as their low body temperatures (and reaction speeds) mirror the cool ocean water. But now, scientists say they have discovered one exception to this rule: opah.