All fish are cold-blooded, right? Not anymore, according to the National Oceanic and Atmospheric Administration (NOAA) Fisheries’ Southwest Fisheries Science Center. NOAA researchers published a paper in Science detailing how the opah or moonfish circulates warm-blood through its body, giving the predatory fish an advantage at 150-1,300 feet below the surface of the water. Most fish living in those dark and chilly depths rely on ambush to catch their prey, but the agile opah—which NOAA says is about the same size as a large automobile tire—flaps its bright red pectoral fins to race through the water.



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The opah is equipped with blood vessels that carry warm blood to the fish’s gills. These wrap around other blood vessels that carry cold blood back to the body after absorbing oxygen from water, ensuring a core body temperature that is consistently about 5 degrees Celsius higher than the surrounding water. NOAA says this is known in engineering as “counter-current heat exchange.”

Lead author of the new paper and fisheries biologist Nicholas Wegner said this unique circulation system allows the fish to move faster, see better, and react more quickly than its cold-blooded counterparts.

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

Wegner added that the higher body temperature should increase the opah’s muscle output and capacity, while also boosting eye and brain function. The fish also is able to stay in deep water longer without risking damage to their heart and other organs. Fatty tissue surrounding the gills, heart and muscle tissue are said to act as an insulator that protects the fish against icy waters.

“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.”

+ NOAA Fisheries’ Southwest Fisheries Science Center

Via Washington Post

Images via NOAA Fisheries’ Flickr page