Scientists have discovered the world's first fully warm-blooded fish - a moonfish about the size of a car tyre that flaps its fins to generate heat.
Opah, or moonfish, circulates heated blood throughout its body much like mammals and birds, giving it a competitive advantage in the cold ocean depths.
The silvery fish, roughly the size of a large automobile tyre, is known from oceans around the world and dwells hundreds of feet beneath the surface in chilly, dimly lit waters, researchers said.
It swims by rapidly flapping its large, red pectoral fins like wings through the water.
Fish that typically inhabit such cold depths tend to be slow and sluggish, conserving energy by ambushing prey instead of chasing it.
But the opah's constant flapping of its fins heats its body, speeding its metabolism, movement and reaction times, scientists at the US National Oceanic and Atmospheric Administration (NOAA) have found.
That warm-blooded advantage turns the opah into a high-performance predator that swims faster, reacts more quickly and sees more sharply, said Nicholas Wegner of NOAA Fisheries' Southwest Fisheries Science Centre in La Jolla, California, lead author of the paper in the journal Science.
"Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments," Wegner said.
"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," Wegner said.
Wegner recognised an unusual design: Blood vessels that carry warm blood into the fish's gills wind around those carrying cold blood back to the body core after absorbing oxygen from water.
The design is known in engineering as "counter-current heat exchange." In opah it means that warm blood leaving the body core helps heat up cold blood returning from the respiratory surface of the gills where it absorbs oxygen.
Resembling a car radiator, it's a natural adaptation that conserves heat. The unique location of the heat exchange within the gills allows nearly the fish's entire body to maintain an elevated temperature, known as endothermy, even in the chilly depths.
"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," said Wegner.
While mammals and birds typically maintain much warmer body temperatures, the opah is the first fish found to keep its whole body warmer than the environment.
A few other fish such as tuna and some sharks warm certain parts of their bodies such as muscles, boosting their swimming performance.
But internal organs including their hearts cool off quickly and begin to slow down when they dive into cold depths, forcing them to return to shallower depths to warm up.
