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Throw Out All Your Textbooks, There's A Warm-Blooded Fish

And it's a doozy.

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This is an opah, and it's the first fish known to be fully warm-blooded.

NOAA / Reuters

As the image from the National Oceanic and Atmospheric Administration (NOAA) above shows, it's a whopper, weighing an average of 100 pounds. It's also known as a moonfish.

Warm-blooded animals, like mammals and our feathered friends, generate and conserve their own body heat.

The body temperature of a cold-blooded critter like a fish, though, relies on its surroundings. Some, like swordfish and marlins, are capable of selectively warming their eyes or brains for a vision boost while hunting, according to Ed Yong at National Geographic.

Tuna and a few shark species similarly heat up their swim muscles to amp up their speed, though they eventually cool off enough to have to backtrack closer to the surface to thaw.

But unlike its brethren, the opah is the only fish that distributes warmth throughout its entire body, including its brain and heart, giving it a metabolic advantage — and competitive edge — over colder-blooded species.

After tagging opah with temperature sensors, NOAA researchers discovered that their body temperature stays an average of 5 degrees Celsius warmer than the water surrounding them, even in the bitterly cold depths of 1,300 feet.

A sampling of gill tissue revealed a dense network of veins and arteries called rete mirabilia (Latin for "wonderful nets"), which uses a process called counter-current heat exchange.

In simplest terms, warm blood moving away from the core heats cooler water returning from the gills' surface. Fat also further insulates the gills and muscle tissues.

So while the opah appears to be a lumbering beast, it's actually an active predator.

NOAA Fisheries West Coast / Creative Commons / Via Flickr: nmfs_northwest

Deepwater fish tend to be a bit slow to conserve their energy. The opah, though, flaps its fins to generate heat, giving it the potential physiological perks of better aerobic performance and more muscle power.

Compared to its lethargic deep-sea friends that rely on ambushing, it has evolution in the bag.

Science Writer

Contact Kasia Galazka at kasia.galazka@buzzfeed.com.

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