You probably learned in elementary school that mammals and birds are warm-blooded whereas other animals such as fish and reptiles are cold-blooded. A recent discovery, however, suggests that this rule is not so hard-and-fast as was previously thought.

Flapping their way through deep ocean waters worldwide are huge fish called opahs. Despite the frigid temperatures hundreds of feet beneath the water's surface where these fish live, they are able to keep their bodies relatively warm, according to a paper published today in the journal Science Advances.

"This changes the way we think about endothermy, the warming of the entire body," lead study author Nicholas Wegner of the National Oceanic and Atmospheric Administration said in an interview. "We thought this was unique to mammals and birds, but now we're seeing it in this fish."

Adult opah typically weigh about 100 pounds, but they can weigh as much as double that.

"They swim by flapping their large pectoral fins, kind of like the wings of a bird - they almost look like penguins swimming," Wegner says. "Their pectoral muscles are disproportionally large compared to that of other fish."

As it swims, the opal's pectoral muscles generate a lot of heat. For most fish, holding onto heat in cold water is not an easy task.

"Fish have a real difficulty maintaining warmth because their blood comes in close proximity to the water when it moves through the gills," Wegner says.

But opah have found a way to get around this problem. They minimize this heat loss by positioning the blood vessels that carry warm blood from the muscles alongside blood vessels that carry cold blood away from the gills. So the warmth of the blood coming from the muscles gets transferred to the cooler blood coming back from the gills instead of getting lost to the surrounding water.

"That allows them to increase the temperature of the entire body, including vital organs such as the heart, which are not warmed by any other fish," says Wegner

While opah are much better at keeping their bodies warm than other known species of fish, Wegner emphasizes that they are not able to do it as effectively as other endothermic animals can.

"Opah do not warm their body temperatures as high as mammals and birds do, but the fact that they do warm their bodies and can warm the entire body is quite an exciting discovery," he says.

The Science Advances article can be found here.

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