According to conventional biology, only mammals and birds are supposed to be warm-blooded, or capable of fully maintaining an internal body temperature. But now scientists working with the National Oceanic and Atmospheric Administration (NOAA) have identified an unlikely new candidate: the moonfish. The discovery represents the first and only known case of a fully warm-blooded fish, reports USA Today.

Moonfish, or "opah" as they are often called, are large, oval-shaped deep sea predators. Their odd body shape might make them look like cumbersome swimmers, but they are actually extremely fast, able to hold their own against other speedsters in the ocean like tuna and swordfish. Unlike tuna and swordfish, though, which need to regulate their internal body temperature by swimming back and forth between cold and warm waters, moonfish have an advantage: They can stay down in the frigid deep waters.

How exactly the moonfish manages to remain so animated in such cold waters has long puzzled scientists. Usually, marine creatures living in colder waters are more sluggish in order to conserve heat. 

"Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments," said Nick Wegner, the lead author of the report. "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."

Researchers first suspected something fishy (or very un-fishy, depending on your outlook) was going on when they studied the gills of the moonfish. They found that some blood vessels there — incidentally the blood vessels carrying warm blood from the body — were wrapped around vessels carrying cold oxygenated blood from the gills. This system, which is basically a countercurrent heat exchange system, recycles heat generated from body movement. 

As a result of this remarkable system, moonfish can maintain their internal body temperature. This is quite different from the processes that allow some fish, such as sharks, to temporarily warm parts of their body as they move. The moonfish can maintain an elevated internal body temperature, even around its internal organs. 

Even though moonfish are frequently caught by longline fishermen and are increasingly becoming a popular fish served in restaurants — especially in Hawaii — very little is known about these swift wanderers of the deep. No official assessment of their population size has been conducted, though some researchers suspect that their numbers are actually on the rise, perhaps due to the overfishing of tuna and other fish that compete with them for prey. 

Those ordering moonfish or opah in restaurants should be warned, however. Their flesh is very high in mercury and the fish is a known carrier of the tropical marine toxin that causes ciguatera, a serious foodborne illness that has no cure.

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