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 ectotherms, 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.