Scientists Discover the First Fully Warm-Blooded Fish

Scientists have discovered another apparent first, according to new research published in Science: a fully warm-blooded fish.

The opah, which researchers say dwells in the cold, dark depths of the ocean, is able to produce heat by constantly flapping its fins like wings as it moves about, keeping its blood warm as it circulates throughout its body. The opah’s warm-bloodedness is advantageous for the fish, as it’s able to keep itself at least 5 degrees Celsius warmer than its surrounding water and move about quickly to prey on other fish.

The researchers from the National Oceanic and Atmospheric Association said the fish is the first known one to be identified as fully warm-blooded, a characteristic typical to mammals and birds; tuna and shark are only partially endothermic, meaning warm blood pumps to only select organs.

Researchers told the Washington Post on Thursday they were curious about the fish given its large size, big eyes, and agility in cold water.