First Warm-Blooded Fish Found Off California

The first fully warm-blooded fish has been discovered in the frigid ocean depths off the California coast, researchers say.

The opah fish generates heat throughout its entire body by the constant flapping of its wing-like pectoral fins, according to a report in the journal Science.

The deepwater predator maintains an average muscle temperature of about 7-9F (4-5C) above the surrounding water temperature.

Its unique structure prevents the heat from being lost, said researchers from the US National Oceanic and Atmospheric Administration's (NOAA) National Marine Fisheries Service.

Other fish, including tuna and some sharks, are capable of warming specific parts of their bodies which allows them to forage for food in the chilly depths, but they must return to the surface to protect vital organs from the effects of the cold.

"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 NOAA fisheries biologist Nicholas Wegner.

Endotherms are warm-blooded animals such as birds and mammals that generate their own heat and maintain a body temperature independent of their environment.

Most fish are categorised as ecotherms along with other cold-blooded species such as amphibians, reptiles and invertebrates.

The opah is a rusty reddish colour, has white spots and bright red fins. It weighs up to 200lb (90kg) and is about the size of a car tyre, with an oval body shape.

Found in oceans worldwide, the opah spends most of its time at depths of 165 to 1,300ft (50 to 400 metres), hunting fish and squid.

Most fish at such depths are slow and sluggish and rely on ambushing rather than pursuing food.

The warm-blooded opah, however, has the advantage of being able to swim faster and react more quickly as it hunts in the frigid deep.