A new study has turned one of the most elementary biology facts taught in schools on its head as it makes a pitch for a fish to be included in the warm-blooded club. The study, conducted by researchers from the National Oceanic and Atmospheric Administration (NOAA), has revealed that a fish called “Opah” circulates heated blood throughout its body much like mammals and birds.

Opah, a silvery fish, about the size of a large automobile tire, dwells hundreds of feet beneath the ocean surface in cold and dimly lit waters. While other fish that inhabit such depths are slow and sluggish, Opah’s constant flapping of its fins heats its body and speeds up its metabolism, movement and reaction times, according to the study, published in the journal Science on Thursday.

Before the discovery, Opah was thought to be “a slow-moving fish, like most other fish in cold environments,” Nicholas Wegner of NOAA Fisheries’ Southwest Fisheries Science Center in La Jolla, California, and the study’s lead author, said in a statement. “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 researchers examined a sample of the fish’s gill tissue and recognized unusual blood vessels that allow it to circulate warm blood throughout its body. As part of the study, the researchers collected temperature data from the fish caught during surveys off the West Coast and found that its body temperature was regularly warmer than the surrounding water.

The researchers discovered that Opah, also called the moonfish, had an average muscle temperature of about 7 degrees to 9 degrees Fahrenheit (4-5 degrees Celsius) above the surrounding water while swimming at about 150 feet to 1,000 feet below the surface.

“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.”

Scientists said that based on satellite tracking the Opah spends most of its time at depths of 150 feet to 1,300 feet, while its body temperature likely increases its muscle output and capacity, and boosts its eye and brain function.

“With a more whole-body form of endothermy, opah don't need to return to surface waters to warm and can thus stay deep near their food source continually,” Reuters quoted Wegner as saying.

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