Scientists announced a startling discovery on Thursday: the existence of the only known truly warm-blooded fish—the opah, or moonfish. Researchers from the National Oceanic and Atmospheric Administration (NOAA) outlined their discovery in the journal Science, upending the general classification that mammals and birds are warm-blooded and fish cold-blooded.
Here’s more on the opah from National Geographic:
Most fish have body temperatures that match the surrounding water. A small number of them can warm specific parts of their bodies… But none of these animals can heat their entire bodies. Their hearts and other vital organs stay at ambient temperature, so while they can hunt in deep, cold waters, they must regularly return to the surface to warm their innards. The opah has no such problem. It can consistently keep its entire body around 5 degrees Celsius warmer than its environment. It doesn’t burn as hot as a bird or mammal, but it certainly outperforms its other relatives… The opah’s brain is warm. Its muscles are warm. And perhaps most importantly, its heart is warm—a first for a fish. Not even a great white shark has a warm heart.
“The opah roams the depths of the ocean, where most of its neighbors are slow and sluggish,” according to Discovery. “Unlike its friends, the opah constantly flaps its fins, which heats up its body and fuels its metabolism. In turn, it also has a distinct predatory advantage, as the opah can move more quickly than other marine predators.”