It's cold at 1,300 feet deep in ocean water. But nature helps moonfish keep warm.
The moonfish, or opah, is the world's first fully warm-blooded fish scientists have ever discovered.
About the size of a car tire, the moonfish, a predator, can move more quickly in frigid waters because of a personal heating system. The fish's special circulation, helped by a large heart and flapping fins, is more similar to mammals than to other fish.