Researchers have discovered the first fish that can keep its entire body warm, much like mammals and birds. The opah, or moonfish, lives in deep, cold water, but it generates heat from its massive pectoral muscles. And it conserves that warmth thanks to body fat and the special structure of blood vessels in its gills.

“It’s a remarkable adaptation for a fish,” says Diego Bernal, a fish physiologist at the University of Massachusetts, Dartmouth, who was not involved in the study. Having a warm heart and brain likely allows the little-known fish to be a vigorous predator, the researchers suspect.

Water will take the heat right out of most creatures. So fish typically remain the temperature of the water they swim in. And that, in turn, limits their biological functions in colder water, especially cardiovascular endurance. There are partial exceptions: Tuna, billfish, and some sharks can temporarily raise the temperature of their body muscles while they hunt, but they must return to warmer waters to bring their core temperature back to normal.

The opah (Lampris guttatus) doesn’t look like a fierce predator. This tubby fish, about a meter long, swims by flapping its pectoral fins. It lives in oceans around the world, but little is known about its biology. It hunts squid and fish, typically 50 m to 200 m below the surface, where the water is just 10°C and colder. In 2012, Owyn Snodgrass, a fisheries biologist with the National Oceanic and Atmospheric Administration in San Diego, California, caught some opah off the coast of California as part of a regular survey. He offered the gills to his colleague Nicholas Wegner, a fish physiologist. “I’m kind of known as the gill guy,” Wegner says.

The gills sat in a nearly 20-liter plastic bucket of preservative for a few months before Wegner pulled them out for a look. “I noticed right away that there was something unique,” he recalls. Fish have just a few large blood vessels that bring blood to and from the gills, where tiny vessels pick up oxygen from the water. But the opah has an elaborate network of tiny blood vessels, in which arteries lie next to veins in tightly packed arrays.

This arrangement of paired arteries and veins is known as a rete mirabile, or “wonderful net,” and often functions as a countercurrent heat exchanger in other species. Vessels carrying warm blood transfer its heat to the cold blood in vessels that comes back from the extremities. This anatomical trick helps aquatic birds minimize heat loss when their feet are in cold water; some whales have similar heat exchangers in their tongues. Tuna, billfish, and certain sharks use this rete mirabile to keep their muscles warm.

The opah is the first fish discovered with a rete mirabile around its gills. The gills’ heat exchanger is wrapped in a centimeter-thick layer of fat, which is unusual in fish. Presumably, it’s used for insulation.

Wegner, Snodgrass, and colleagues decided to measure the temperature of opah at sea. After hauling the fish aboard, they found that the average body temperature was roughly 5°C warmer than the water from which they were caught, as they report online today in Science.

The researchers also measured the muscle temperature in living fish as they swam. To do this, they caught opah using a hook and line, implanted a temperature monitor into the pectoral muscles, and let them swim for a few hours while still attached to the fishing gear. Even after the fish dove to 4°C water, the muscles remained about 13°C to 14°C.

The higher body temperature should provide several advantages, including more powerful swimming and better endurance, thanks to the warm heart. As other researchers reported in 2009, the brain and eyes of the opah are even warmer than the rest of the body, thanks to a small countercurrent heat exchanger at the base of its skull. The blood flowing to the eyes is warmed by specialized eye muscles that generate heat without contracting, a feature found only in fish.

Altogether, the evidence makes Wegner suspect that the opah is an active predator, unlike the other resident predators that tend to ambush prey. With acute vision, fast response times, and endurance, opah can pursue squid and fast fish called barracudinas.

Wegner and his colleagues have been gathering behavioral data on free-swimming opah by using sensors that pop off and float to the surface after 2 days. But much remains to be learned about this mysterious warm-blooded creature that has mastered the chilly depths. “It just goes to show how fish can adapt in amazing ways,” Wegner says.