Colorful and mysterious opah found to be ‘first fully warm-blooded fish’

Opah image captured by ©Ralph Pace during a PIER research cruise.

The opah, or moonfish, is an oval-shaped denizen of the deep that would appear to be sluggish, given its cold-water habitat and ungainly body design.

But new research by NOAA Fisheries reveals this beautifully colored creature to be “the first fully warm-blooded fish that circulates heated blood throughout its body much like mammals and birds, giving it a competitive advantage in the cold ocean depths.”

The research, published Thursday in the journal Science, explains that the opah’s constant fin flapping heats its blood and increases its metabolism, transforming the fish into a swift predator.

Opah swims off Southern California. Photo: ©Ralph Pace Photography

“Before this discovery I was under the impression that this was a slow-moving fish, like most other in the cold environments,” Nicholas Wegner, of NOAA Fisheries’ Southwest Science Center in San Diego, said in a news release. “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.”

Then opah’s unique gill design allows for a phenomenon known as “counter-current heat exchange,” which essentially involves warm blood leaving the body core to warm cold blood returning from the respiratory surface of the gills, where oxygen is absorbed.

Great white sharks also are considered warm blooded, to an extent, but the mechanisms are different.

Rare opah trifecta scored last summer off Baja California. Photo: Excel Sportfishing

“There has never been anything like this seen in a fish’s gills before,” Wegner said, comparing the process to the workings of a car radiator. “This is a cool innovation by these animals that gives them a competitive edge.”

Their elevated body temperature is believed to enhance muscle output, and boost brain and eye function, Wegner added.

The opah is unique in other ways, too. It’s largely a solitary swimmer that gathers in groups only during the spawning season.

These moonfish typically roam depths between 150 and 1,000 feet, and are sometimes found in the vicinity of feeding tuna.

They’re caught sporadically by commercial longline fishermen, but there’s no direct commercial fishery for opah, because of their solitary nature.

Their flesh is considered delectable, however, and is sometimes sold in markets and restaurants.

Understandably, the fish are considered a prize when the rare catch of an opah is made by anglers on sport-fishing boats.

Last summer off California and Baja California, several opah catches were logged, including the extraordinary catch of three large opah, one after another, on the San Diego-based Excel.

One of the fish weighed 180 pounds, 12 ounces, and was recently approved as the world record.

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