Introducing the opah, the first known warm-blooded fish

This photo courtesy of the National Oceanic and Atmospheric Administration Fisheries/Southwest Fisheries Science Center shows an opah fish being released with sensors to track temperatures as it dives. (NOAA/AFP/File)

MIAMI - The silvery, round-bodied opah fish is the first warm-blooded fish known to science and this unusual ability gives it a competitive edge in the cold ocean depths, researchers said Thursday.

Also known as a moonfish, the creature is about the size of a tire and can warm itself much the same way as a car radiator, researchers said in the journal Science.

The fish has blood vessels in its gills that carry warm blood from the body's core.

These vessels wrap around other vessels near the gills, where the fish breathes, bringing in oxygenated, cold blood.

The result is a self-made heating system that keeps the fish's brain sharp and its muscles active so it can swim fast and grab prey.

By attaching temperature monitors to opah fish off the US west coast, researchers found that the fish had an average muscle temperature "about five degrees Celsius above the surrounding water while swimming about 45-300 meters below the surface," the study said.

Most fish are cold-blooded, so the discovery of a fish that can warm its body much the same way as mammals and birds was a surprise to scientists.

"Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments," said lead author Nicholas Wegner of the National Oceanic and Atmospheric Administration's (NOAA) Southwest Fisheries Science Center in La Jolla, California.

"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."

Opah fish live in the deep ocean where predators tend to ambush prey rather than chase it.

Some other fish, like tuna and some sharks, can warm up some certain parts of their bodies and muscles to boost swimming performance in the cold depths, but their internal organs quickly get cold, forcing them to rise to shallow waters in order to warm up.

With red fins that are constantly flapping, the opah fish stays warm even when the water gets colder, speeding its metabolism and maintaining a quick reaction reflex.

In addition to the network of warming blood vessels, the opah fish have fatty tissue around the gills, heart and muscle tissue to insulate itself and stay warm.

"There has never been anything like this seen in a fish's gills before," Wegner said.

"This is a cool innovation by these animals that gives them a competitive edge."