In a paper published today in Science, researchers from the National Oceanic and Atmospheric Administration (NOAA) describe the unique mechanism that enables the opah, a deepwater predatory fish, to keep its body warm. The secret lies in a specially designed set of blood vessels in the fish’s gills, which allows the fish to circulate warm blood throughout its entire body.

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Scientists already suspected the opah was special, says Heidi Dewar, a researcher at NOAA’s Southwest Fisheries Science Center and one of the paper’s authors. Most fish who live where the opah does — that is, hundreds of feet deep, in some of the ocean’s darkest and coldest places — are sluggish, thanks to the low temperatures. At these depths, even predatory fish tend to be slow-moving, waiting patiently for prey to come by rather than actively chasing it down. But the opah, which spends all its time in these deep places, has many features usually associated with a quick-moving, active predator, such as a large heart, lots of muscle and big eyes. These characteristics made the opah “a curiosity,” Dewar says.

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The opah’s secret first started to come out when NOAA researcher and lead author Nicholas Wegner looked at a gill sample and noticed something intriguing.

All fish have two kinds of blood vessels in their gills: vessels carrying blood in from the body to pick up oxygen, and other vessels carrying oxygenated blood back out again. In the opah, the incoming blood is warm after circulating through the fish’s body. This is because the opah swims by quickly flapping its pectoral fins, rather than undulating its body like many other fish do, to propel itself through the water — a process that generates high heat. But outgoing blood, which has just been in contact with water in the gills, is cold. Wegner noticed that in the opah’s gills, the two sets of vessels are tightly bundled against each other, so that the incoming blood vessels can warm up the outgoing blood before it goes anywhere else. This set-up, known as “counter-current heat exchange,” allows warm blood to be delivered throughout the body.

Some other types of fish, such as tuna, have similarly designed blood vessels in certain parts of their bodies, allowing for “regional endothermy” — warm-bloodedness that’s limited to certain organs or muscles, such as the eyes, liver or swimming muscles. But the opah is the only fish scientists know of that has this design in its gills, where most fish lose the majority of their body heat to the surrounding cold water. By warming up the blood in the gills before it goes anywhere else, the opah achieves not just regional endothermy, but whole-body endothermy, according to the paper’s authors. Testing showed that the opah is able to maintain a core body temperature about 5 degrees Celsius warmer than the surrounding water.

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Granted, not everyone agrees that “whole-body endothermy” is the best term for the opah’s adaptation. Diego Bernal, a professor of biology at the University of Massachusetts Dartmouth, says he believes the opah is still a regional endotherm — it’s just able to keep more of its body warm than other fish.

“It has a very warm core at the center of the body, but it gets cold as you go back into the body or back into the tail or as you go up or down,” Bernal says. A temperature profile of the opah included in the recent Science paper shows that the warmest areas in the opah’s body are the core, including the heart, and the area around the eyes and brain, while the edges of the body are a little cooler.

“I think that [Bernal’s] point is well taken in that ‘whole-body endothermy’ implies that all of the tissues of the fish are going to be elevated in temperature above the ambient temperature,” says Kathryn Dickson, chair of the department of biological science at California State University, Fullerton. “And the fins of the fish, the surface of the eye — a lot of parts are not going to be.” She adds that the term “warm-blooded” may be a bit archaic as well. Even if opah are able to keep large parts of their bodies warmer than the surrounding water, their blood certainly isn’t as warm as that of birds and mammals, she says.

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Even so, the opah’s ability to keep its heart warm is still unique, according to both Bernal and Dickson, who served as a reviewer on the new paper. And this feature gives the opah a major competitive advantage, says study author Dewar. Even fish with regional endothermy usually can’t keep their heart warm.

“If your heart’s cold, there’s only so much you can do,” Dewar says. “It doesn’t matter how much your muscles will perform, if your heart can’t deliver the oxygen and nutrients, your muscles can only do so much.” This means fish like tuna must constantly make trips back to the surface to warm up if they don’t want to slow down.

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The opah, on the other hand, can spend all its time in the deep waters without losing its edge. And being warm-blooded in such cold temperatures means its eyesight and muscle performance is sharper than its sluggish, cold-blooded cousins. “The prey’s at a complete disadvantage,” Dewar says.

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While only one species of opah is currently recognized — Lampris guttatus — scientists are starting to believe that they should actually divide the opah into several different species based on genetic variations in different populations around the world, according to Dewar. The opah in this study were found off the West Coast of North America, so the next step will be to start sampling opah in other parts of the world to see if they all have the same specialized gills, Dewar says.

The University of Massachusetts, Dartmouth’s Bernal believes more research should be done on the opah’s behavior and life history, which remain a little mysterious, if scientists want to learn more about the specific performance advantages the fish might enjoy with its higher body temperature. And, he says, the same goes for other deep-water species which remain poorly understood.

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Down the road, future studies could also examine other related types of fish to try and figure out how and when those special gills evolved. And since a variety of different fish already exhibit regional endothermy, including tuna and certain types of sharks, “all this information really shows you that this kind of physiological adaptation, has evolved numerous times in different lineages of fishes,” says Dickson.

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It’s possible that other deep-water species have similar adaptations as the opah, although Bernal says he believes it’s unlikely scientists will ever discover a fish that’s truly warm-blooded, in the way whales or other marine mammals are warm-blooded.

For now, though, the opah enjoys the spotlight as the world’s first, and so far only, warm-blooded fish — or at least the most warm-blooded that we know of. “I think that it’s really exciting that we spend so much time studying especially these larger fish to find something that’s completely unique and has never been seen before in any fish,” Dewar says. “We’ve been very excited about it.”

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Clarification: This story previously stated that Diego Bernal was a reviewer of the paper. He was not a formal reviewer, but was familiar with the work.