A new study by scientists at NOAA Fisheries has revealed the world’s first fully warm-blooded fish, but it is not a newly discovered species. In fact, the fish is one that is already well-known to anglers—the opah, or moonfish as it is commonly called.

In a press release on Tuesday, NOAA Fisheries confirmed that the opah circulates heated blood throughout its body much like birds or mammals, albeit at a much lower temperature.

“Fish that typically inhabit such cold depths tend to be slow and sluggish, conserving energy by ambushing prey instead of chasing it,” NOAA stated. “But the opah’s constant flapping of its fins heats its body, speeding its metabolism, movement and reaction times, scientists report today in the journal Science.”

According to researchers, warm blood gives the fish a distinctive advantage in cold waters, making the opah a “high-performance predator” with faster reaction time, better vision, and increased speed. Compared to other fish in a cold environment, the opah is a supercharged speedster.

“Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments,” said the study’s lead author, Nicholas Wegner. “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.”

The opah may not look like much of a predator, but its unique circulatory system gives it a significant edge. Other fish, such as certain species of sharks or tuna, can mimic warm blood by regulating the temperature of their bodies to swim faster or resist the cold depths, but they eventually have to swim back to the surface to warm up. Opahs do not have to, thanks to what experts call “counter-current heat exchange.” Warm blood leaving the fish’s core heats up cold blood returning from the gills where the opah absorbs oxygen, thus keeping the fish’s entire body at a set temperature.

“Nature has a way of surprising us with clever strategies where you least expect them,” Wegner said. “It’s hard to stay warm when you’re surrounded by cold water but the opah has figured it out.”

So how does a warm-blooded fish taste? Pretty good, apparently, since opahs are popular in places where they can be freshly harvested, like Hawaii. Those who have tried this exotic fish say that the flesh has a mild flavor and can be be very versatile in a number of dishes. Actually landing one can be difficult. Although opahs are popular among deep-sea anglers, they are only rarely caught.

Images courtesy NOAA Fisheries

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