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.

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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.”