A warm-blooded fish known as an opah, or moonfish, is the first true fish ever shown to have a circulatory system similar to those seen in mammals and birds. The ability to pump warm blood throughout its body may provide the animal an evolutionary advantage living in the cold waters deep beneath the ocean surface.

Opah live in oceans around the world at depths of a few hundred feet beneath the surface of the waves. Most fish that live in the dark, chilly waters at these depths move slowly, to conserve energy. They typically wait for prey to come to them, rather than actively hunting.

Moonfish grow to be a few feet long, and propel themselves through the water using the sizable pectoral fins on the sides of their bodies to push themselves along. Researchers believe it is the movement of these fins that helps to warm the fish, speeding their metabolism, allowing them to move more quickly than other fish in the chilly depths of the ocean.

"Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments. 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," Nicholas Wegner of the Southwest Fisheries Science Center said.

The discovery was made as researchers examined gills from the fish, which exhibited an unusual structure known as a counter-current heat exchange. Working much like the radiator in an automobile, cold blood returning to the body from the gills is heated as it passes through a coil of vessels containing warm blood. Such a design allows the animal to conserve heat in the frigid waters.

"There has never been anything like this seen in a fish's gills before. This is a cool innovation by these animals that gives them a competitive edge. The concept of counter-current heat exchange was invented in fish long before we thought of it," Wegner said.

Temperature monitors attached to opah fish in the wild showed the creatures were able to maintain body temperatures around nine degrees warmer than the surrounding water. This makes the species the first true fish ever shown to maintain body temperatures above the environment immediately around them.

Muscles, gills, and the heart of the opah are covered in fatty tissue, further insulating them from the cold.

Certain sharks and tuna are able to warm specific parts of their bodies as they swim, but those systems are inadequate to warm internal organs, such as hearts. This forces these species to regularly return to warmer waters near the surface.

Analysis of the warm-blooded body structure of moonfish was profiled in the journal Science.

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