In the cold waters off the California coast, researchers have discovered something no one ever knew existed: a warm-blooded fish. Not only can this large fish regulate its body temperature, but it does so hrough a truly unique mechanism.

The opah, or moonfish, inhabit oceans around the globe. This cartoonish, tire-sized fish, festooned with orangey fins and silver speckles, dwells in deep, frigid waters. And like most fish that inhabit such domains, it was assumed to be a slow, sluggish swimmer.

But the opah's reputation is being revamped by researchers from NOAA's Southwest Fisheries Science Center in La Jolla, California.

 "Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments," said Dr. Nicholas Wegner, lead author on the paper that appears in the journal Science. "But because it can warm its body, it turns out to be a very active predator that chases down agile prey like squid and can migrate long distances."

The NOAA team initially noticed that opah maintained body temperatures consistently warmer than the waters from which they were caught. They attached temperature monitors to the fish and tracked them as they dove to several hundred feet. And despite the colder, deeper waters, the fish were able to maintain body temperatures around 5 degrees C higher than their surroundings.

And how does this large fish achieve its unexpected endothermy? Apparently by flapping its fins.

The fins of the opah are insulated by a thick layer of fatty tissue, which not only propels the fish through the depths, but also traps warmth. But the flapping is just the beginning. As the fish moves through the water, blood is pumped into its gills to oxygenate its body. The opah possesses a specialized network of blood vessels within its gills, called rete mirabile, which acts as a radiator, collecting heat from the circulating blood and redistributing it throughout the body.

"There has never been anything like this seen in a fish's gills before," says Wegner. "This is a cool innovation by these animals that gives them a competitive edge."

Other fish, such as tuna a sharks, are able to warm parts of their bodies, which enables them to pursue faster, deeper prey. But as their organs become chilled, they are forced back to the surface. The opah is freed from such restrictions, which gives it an edge over its competitors.

"Nature has a way of surprising us with clever strategies where you least expect them. It's hard to stay warm when you're surrounded by cold water but the opah has figured it out."