Warm blood makes unusual opah fish succeed in oceans

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 reported 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 U.S. west coast, researchers found that the fish had an average muscle temperature "about five degrees Celsius (nine degrees Fahrenheit) above the surrounding water while swimming about 150 to 1,000 feet (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.