New research by NOAA Fisheries has revealed the opah, or moonfish, as the first fully warm-blooded fish that circulates heated blood throughout its body much like mammals and birds, giving it a competitive advantage in the cold ocean depths. 
 
 The silvery fish, roughly the size of a large automobile tire, is known from oceans around the world and dwells hundreds of feet beneath the surface in chilly, dimly lit waters. It swims by rapidly flapping its large, red pectoral fins like wings through the water. 
 
 Fish that typically inhabit such cold depths tend to be slow and sluggish, conserving energy by ambushing prey instead of chasing it. But the opah's constant flapping of its fins heats its body, speeding its metabolism, movement and reaction times, scientists report in the journal Science. 
 
 That warm-blooded advantage turns the opah into a high-performance predator that swims faster, reacts more quickly and sees more sharply, said fisheries biologist Nicholas Wegner of NOAA Fisheries' Southwest Fisheries Science Center in La Jolla, Calif., lead author of the new paper. 
 
 "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." 
 
 Gills show unusual design 
 
 Wegner realized the opah was unusual when a coauthor of the study, biologist Owyn Snodgrass, collected a sample of its gill tissue. Wegner recognized an unusual design: 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. 
 
 The design is known in engineering as "counter-current heat exchange." In opah it means that warm blood leaving the body core helps heat up cold blood returning from the respiratory surface of the gills where it absorbs oxygen. Resembling a car radiator, it's a natural adaptation that conserves heat. The unique location of the heat exchange within the gills allows nearly the fish's entire body to maintain an elevated temperature, known as endothermy, even in the chilly depths. 
 
 "There has never been anything like this seen in a fish's gills before," Wegner said. "This is a cool innovation by these animals that gives them a competitive edge. The concept of counter-current heat exchange was invented in fish long before we thought of it." 
 
 The researchers collected temperature data from opah caught during surveys off the West Coast, finding that their body temperatures were regularly warmer than the surrounding water. They also attached temperature monitors to opah as they tracked the fish on dives to several hundred feet and found that their body temperatures remained steady even as the water temperature dropped sharply. The fish had an average muscle temperature about 5 degrees C above the surrounding water while swimming about 150 to 1,000 feet below the surface, the researchers found. 
 
 While mammals and birds typically maintain much warmer body temperatures, the opah is the first fish found to keep its whole body warmer than the environment. 
 
 A few other fish such as tuna and some sharks warm certain parts of their bodies such as muscles, boosting their swimming performance. But internal organs including their hearts cool off quickly and begin to slow down when they dive into cold depths, forcing them to return to shallower depths to warm up. 
 
 Warmth provides competitive edge 
 
 Satellite tracking showed opah spend most of their time at depths of 150 to 1,300 feet, without regularly surfacing. Their higher body temperature should increase their muscle output and capacity, boost their eye and brain function and help them resist the effects of cold on the heart and other organs, Wegner said. 
 
 Fatty tissue surrounds the gills, heart and muscle tissue where the opah generates much of its internal heat, insulating them from the frigid water. 
 
 Other fish have developed limited warm-bloodedness (known as regional endothermy) to help expand their reach from shallower waters into the colder depths. But the opah's evolutionary lineage suggests that it evolved its warming mechanisms in the cold depths, where the fish can remain with a consistent edge over other competitors and prey. Recent research has found distinctive differences among opah from different parts of the world, and Wegner said scientists are now interested in comparing warm-blooded features among them. 
 
 "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 research surveys off California have caught more opah in recent years, but biologists are not sure why. Current conditions may be favoring the fish, or their population may be growing. Opah are not usually targeted by fishermen off California but local recreational anglers and commercial fisheries occasionally catch the species. The opah's rich meat has become increasingly popular in seafood markets. 
 
 "Discoveries like this help us understand the role species play in the marine ecosystem, and why we find them where we do," said Francisco Werner, director of the Southwest Fisheries Science Center. "It really demonstrates how much we learn from basic research out on the water, thanks to curious scientists asking good questions about why this fish appeared to be different." ||||| Thank you for your interest in spreading the word about Science. 
 
 NOTE: We only request your email address so that the person you are recommending the page to knows that you wanted them to see it, and that it is not junk mail. We do not capture any email address. ||||| Researchers from the National Oceanic and Atmospheric Administration discovered that the opah, a deepwater predatory fish, is able to keep its body warm through a specially designed set of blood vessels in the fish's gills, which allows the fish to circulate warm blood throughout its entire body. (Science Magazine) 
 
 This story has been updated. 
 
 It’s one of the most basic biology facts we’re taught in school growing up: Birds and mammals are warm-blooded, while reptiles, amphibians and fish are cold-blooded. But new research is turning this well-known knowledge on its head with the discovery of the world’s first warm-blooded fish — the opah. 
 
 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. 
 
 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. 
 
 [No, genetically modified pet fish are not going to wreak ecological havoc] 
 
 
 
 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. 
 
 
 
 Image downloaded from flickr NOAA Fisheries West Coast, Opah captured during longline surveys by the Southwest Fisheries Science Center. 
 
 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. 
 
 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. 
 
 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. 
 
 [Why a smallmouth bass with a rare, cancerous tumor has officials worried] 
 
 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. 
 
 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. 
 
 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. 
 
 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.” 
 
 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. 
 
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 The car-tire-size opah is striking enough thanks to its rotund, silver body. But now, researchers have discovered something surprising about this deep-sea dweller: It's got warm blood. 
 
 That makes the opah (Lampris guttatus) the first warm-blooded fish ever discovered. Most fish are ectotherms, meaning they require heat from the environment to stay toasty. The opah, as an endotherm, keeps its own temperature elevated even as it dives to chilly depths of 1,300 feet (396 meters) in temperate and tropical oceans around the world. 
 
 "Increased temperature speeds up physiological processes within the body," study leader Nicholas Wegner, a biologist at the National Oceanic and Atmospheric Administration (NOAA) Fisheries' Southwest Fisheries Science Center in La Jolla, California, told Live Science. "As a result, the muscles can contract faster, the temporal resolution of the eye is increased, and neurological transmissions are sped up. This results in faster swimming speeds, better vision and faster response times." 
 
 The result, Wegner said, is a fast-swimming fish with an advantage for hunting slow, cold-blooded prey. [See Photos of the Gigantic Warm-Blooded Opah (Moonfish)] 
 
 Undersea moon 
 
 The opah, also known as the moonfish, has relatively small red fins decorating its large, round body, which can grow up to 6 feet (1.8 meters) long. These fins, which flap rapidly as the fish swims, turn out to be important in generating body heat for the opah. 
 
 "The opah appears to produce the majority of its heat by constantly flapping its pectoral fins which are used in continuous swimming," Wegner said. 
 
 Researchers first suspected that something might be strange about the opah after analyzing a sample of the fish's gill tissue. According to the new study, published today (May 14) in the journal Science, the blood vessels in the tissue are set up so that the vessels carrying cool, oxygenated blood from the gills to the body are in contact with the vessels carrying warm, deoxygenated blood from the body to the gills. As a result, the outgoing blood warms up the incoming blood, a process called counter-current heat exchange. 
 
 "There has never been anything like this seen in a fish's gills before," Wegner said in a statement. "This is a cool innovation by these animals that gives them a competitive edge. The concept of counter-current heat exchange was invented in fish long before [humans] thought of it." 
 
 To confirm that these special gills helped the opah stay toasty, the researchers tagged a number of moonfish with temperature monitors and tracked the fish as they dove. The fish spend most of their time at least 150 feet (45 m) below the ocean surface. No matter how deep they dive, however, their body temperature stays about 9 degrees Fahrenheit (5 degrees Celsius) warmer than the surrounding water. Fat deposits around the gills and muscles help insulate the fish, the researchers found. 
 
 Built for speed 
 
 Warm blood gives deep-sea fish a boost, according to Wegner. The opah's muscles and nervous system likely function faster than an equivalent fish with cold blood. Other deep-diving fish, such as tuna and some sharks can shunt blood to certain body parts to keep them warm during deep dives. But these fish have to swim up out of the depths frequently to prevent their organs from shutting down. 
 
 In contrast, the opah can stay deep for long periods of time. 
 
 "Nature has a way of surprising us with clever strategies where you least expect them," Wegner said in a statement. "It's hard to stay warm when you're surrounded by cold water, but the opah has figured it out." 
 
 Next, Wegner told Live Science, he and his colleagues want to study Lampris immaculatus, the opah's southern cousin. This fish, the southern opah, lives in colder waters than the northern opah, so it would be harder to keep warm, Wegner said — but even more beneficial. 
 
 Editor's Note: This story was updated to state that most fish are "ectotherms" and not "exotherms" as it was written. 
 
 Follow Stephanie Pappas on Twitter and Google+. Follow us @livescience, Facebook & Google+. Original article on Live Science. ||||| There’s nothing about the opah that says “fast-moving predator”. Tuna, sharks, and swordfish are fast-moving predators and accordingly, their bodies look like streamlined torpedoes. By contrast, the opah looks like a big startled frisbee, with thin red fins stuck on as an afterthought. 
 
 It’s pretty (silver body and red fins) and big (up to two metres long), but fast? Nicholas Wegner from the National Oceanic and Atmospheric Administration certainly didn’t think so when he first started studying it. Since then, he has discovered that the opah is an active predator, which has a trait that no other fish possesses. 
 
 It is warm-blooded. 
 
 Most fish have body temperatures that match the surrounding water. A small number of them can warm specific parts of their bodies. Swordfish, marlins, and sailfish, can temporarily heat their eyes and brains, sharpening their vision when pursuing prey. Tuna and some sharks, including the mako and great white, can do the same with their swimming muscles, going into turbo mode when they need to. But none of these animals can heat their entire bodies. Their hearts and other vital organs stay at ambient temperature, so while they can hunt in deep, cold waters, they must regularly return to the surface to warm their innards. 
 
 The opah has no such problem. It can consistently keep its entire body around 5 degrees Celsius warmer than its environment. It doesn’t burn as hot as a bird or mammal, but it certainly outperforms its other relatives. 
 
 Wegner discovered its ability by accident. His team just happened to catch more opah during one of their research trips, and they used the opportunity to learn more about this little-known species. As they dissected the animals, Wegner immediately noticed that its gills contain a beautiful and intricate tangle of red and blue blood vessels. “That was when we realised what it was capable of,” he says. Wegner had seen blood vessels like those before. They’re called retia mirabilia—Latin for “wonderful nets”—and they’re the secret behind the heating systems of tuna and sharks. 
 
 All animal muscles produce heat when they contract, but in most fish, that heat is almost immediately lost to the environment through the skin or the gills. The gills are especially problematic. No matter how much insulation a fish has, the blood that runs through the gills has to make close contact with the seawater. A tuna can produce as much heat as it likes in its swimming muscles, but as soon as the blood from those muscles reaches the gills, as it must do to be reloaded with oxygen, it ought to quickly cool. But it doesn’t, because of the wonderful nets. 
 
 In those nets, the veins that carry warm blood away from the hot muscles are interwoven with the arteries that carry cold blood in from the gills. They run so close that the veins offload their heat onto the arteries, before it can reach the gills and disappear. Through these “countercurrent exchangers”, the tuna can retain whatever heat it generates. But since its retia mirabilia are located in its swimming muscles, those are the only body parts that stay warm. That’s why its heart still runs cold. 
 
 The opah’s wonderful nets are in its gills, and that makes all the difference. The blood vessels carrying warm blood from heart to gills flows next to those carrying cold blood from the gills to the rest of the body, warming them up. So, while a tuna or shark might isolate its warm muscles from the rest of its cold body, the opah flips this arrangement. It isolates the cold bits—the gills—from everything else. 
 
 This allows its huge pectoral muscles, which generate most of its heat, to continuously warm the rest of its body. It also keeps that heat with the help of thick layers of fat, which insulate the heart from the gills, and the pectoral muscles (which produce most of the animal’s heat) from the surrounding water. 
 
 Wegner’s team confirmed this by catching opah, implanting them with small thermometers, and then releasing them. The instruments inside the fish recorded consistently higher temperatures than those dropped into the surrounding water. The opah’s brain is warm. Its muscles are warm. And perhaps most importantly, its heart is warm—a first for a fish. Not even a great white shark has a warm heart. “That’s why opah can stay at depth,” says Wegner. “These guys are specialised for living deeper than those other predators.” 
 
 So, it’s fast, then? Despite the somewhat comical physique? “That’s what’s really blew my mind about this discovery,” says Wegner. “Just from looking at it, I really thought it was a slow, sluggish, deep-water fish that doesn’t do very much. But all indications are that this is a very fast fish and an active predator. We’ve put some tags on them to show that they migrate thousands of kilometres.” 
 
 Reference: Wegner, Snodgrass, Dewar & Hyde. 2015. Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus. Science http://dx.doi.org/10.1126/science.aaa8902
– Your old science teacher was wrong: It turns out that not all fish are cold-blooded. Scientists have discovered that the opah, a deep-sea dweller also known as the moonfish, is, in fact, a warm-blooded creature and the first such fish ever found, reports LiveScience. Thanks to a unique set of blood vessels in its gills, the opah stays about 9 degrees warmer than the water around it. That may not sound like much, but the Washington Post explains that it gives the fish a huge advantage: Most of the other creatures who live in the deepest and thus coldest parts of the ocean move at a snail's pace because of those cold temperatures. The opah, however, is a relative speedster, which comes in handy when hunting prey or eluding predators. "There has never been anything like this seen in a fish's gills before," says Nicholas Wegner of the National Oceanic and Atmospheric Administration, lead scientist of a study in Science. "This is a cool innovation by these animals that gives them a competitive edge." A handful of fish can warm up parts of their bodies, but the opah warms up the entire thing. National Geographic provides a sense of how it's done: "The blood vessels carrying warm blood from heart to gills flows next to those carrying cold blood from the gills to the rest of the body, warming them up. So, while a tuna or shark might isolate its warm muscles from the rest of its cold body, the opah flips this arrangement. It isolates the cold bits—the gills—from everything else." The opah generates the heat needed to stay warm by flapping its large pectoral fins like wings while swimming, notes a post by the NOAA at Science Daily. (This is so much less creepy than the fish with human-looking teeth.)