We've been taught that, unlike warm-blooded mammals and birds, reptiles, amphibians, and fish are cold-blooded. However, there's one fish defying the odds — the opah.
Also known as the moonfish, the opah is the first warm-blooded fish discovered, according to an article in the journal Science. It lives hundreds of feet underwater, weighs more than 100 lbs, and is about the size of a car's tire.
Other fish, including tuna and certain sharks, are able to heat certain body parts, but the opah is the first one with the ability to keep its whole body warmer than its surroundings. This allows the opah to swim rapidly, unlike other fish in cold deep waters.
“Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments,” said National Oceanic and Atmospheric Administration Fisheries biologist 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.”
NOAA explains the opah has “Blood vessels that carry warm blood into the fish’s gills wind around those carrying cold blood back to the body core after absorbing oxygen from water.” This is the first time this has been seen in a fish’s gills.
“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.”
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