Meet the World's First Warm-Blooded Fish

Your old science teacher was wrong: It turns out that not all fish are cold-blooded. Scientists have discovered that the opah, a deep-sea dweller also known as the moonfish, is, in fact, a warm-blooded creature and the first such fish ever found, reports LiveScience. Thanks to a unique set of blood vessels in its gills, the opah stays about 9 degrees warmer than the water around it. That may not sound like much, but the Washington Post explains that it gives the fish a huge advantage: Most of the other creatures who live in the deepest and thus coldest parts of the ocean move at a snail's pace because of those cold temperatures. The opah, however, is a relative speedster, which comes in handy when hunting prey or eluding predators.

"There has never been anything like this seen in a fish's gills before," says Nicholas Wegner of the National Oceanic and Atmospheric Administration, lead scientist of a study in Science. "This is a cool innovation by these animals that gives them a competitive edge." A handful of fish can warm up parts of their bodies, but the opah warms up the entire thing. National Geographic provides a sense of how it's done:

• "The blood vessels carrying warm blood from heart to gills flows next to those carrying cold blood from the gills to the rest of the body, warming them up. So, while a tuna or shark might isolate its warm muscles from the rest of its cold body, the opah flips this arrangement. It isolates the cold bits—the gills—from everything else."

The opah generates the heat needed to stay warm by flapping its large pectoral fins like wings while swimming, notes a post by the NOAA at Science Daily. (This is so much less creepy than the fish with human-looking teeth.)