The car-tire-size opah is striking enough thanks to its rotund, silver body. But now, researchers have discovered something surprising about this deep-sea dweller: It’s got warm blood. That makes the opah (Lampris guttatus) the first warm-blooded fish every discovered. Most fish are exotherms, meaning they require heat from the environment to stay toasty. The opah, as an endotherm, keeps its own temperature elevated even as it dives to chilly depths of 1,300 feet (396 meters) in temperate and tropical oceans around the world.

“Increased temperature speeds up physiological processes within the body,” study leader Nicholas Wegner, a biologist at the National Oceanic and Atmospheric Administration (NOAA) Fisheries’ Southwest Fisheries Science Center in La Jolla, California, told Live Science. “As a result, the muscles can contract faster, the temporal resolution of the eye is increased, and neurological transmissions are sped up. This results in faster swimming speeds, better vision and faster response times.” The result, Wegner said, is a fast-swimming fish with an advantage for hunting slow, cold-blooded prey. MORE