Move over, mammals and birds, and make room for a fish called the opah in the warm-blooded club.

Researchers said in the journal Science on Thursday that this deepwater denizen is the first fish known to be fully warm-blooded, circulating heated blood throughout its body, enabling it to be a vigorous predator in frigid ocean depths.

Tuna and certain sharks can warm specific regions of their body such as swimming muscles, brain and eyes in order to forage in chilly depths but must return to the surface to protect vital organs such as the heart from the effects of the cold.

NOAA Fisheries biologist Nick Wegner holds an opah caught during a research survey off the California Coast. (NOAA Fisheries/Southwest Fisheries Science Center)

The opah, also called the moonfish, internally generates heat through constant flapping of wing-like pectoral fins, with an average muscle temperature about 4 to 5 degrees Celsius (7 degrees to 9 degrees Fahrenheit) above the surrounding water temperature at the time.

The opah boasts a unique structure that prevents this heat from being lost to the environment.

Warm-blooded animals, such as birds and mammals, known as endotherms, generate their own heat and maintain a body temperature independent of the environment. Cold-blooded animals, known as ectotherms, include amphibians, reptiles, invertebrates and most fish.

No need to return to warm surface

"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," said fisheries biologist Nicholas Wegner of the U.S. National Oceanic and Atmospheric Administration's National Marine Fisheries Service.

The higher temperature of the opah's eye should allow it to see better. (NOAA Fisheries/Southwest Fisheries Science Center)

The opah is a rusty reddish color, has white spots and bright red fins. It weighs up to 90 kg (200 pounds) and is about the size of a car tire, with an oval body shape. Found in oceans worldwide, it spends most of its time at depths of 50 to 400 metres (165 to 1,300 feet), hunting fish and squid.

A unique structure within its gills lets warm blood that leaves the body core help heat up cold blood returning from the gills' respiratory surface, said fisheries biologist Owyn Snodgrass of NOAA and Ocean Associates Inc.

Being warm-blooded gives it distinct advantages over its cold-bodied prey and competitors including faster swimming speeds and reaction times, better eye and brain function and the ability to withstand the effects of cold on vital organs.

Fish dwelling at such depths typically are slow and sluggish, ambushing rather than pursuing prey.

The researchers documented that opah are warm-blooded by tagging and tracking them off California's coast, measuring their body temperature, water temperature and the depths at which they swam.