(Science Magazine) 
 
 This story has been updated. 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. 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. 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. No matter how much insulation a fish has, the blood that runs through the gills has to make close contact with the seawater. 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. 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. 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. 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. 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. 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. 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. “These guys are specialised for living deeper than those other predators.” 
 
 So, it’s fast, then? “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. 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. "Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments," Wegner said.
– 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.)