Receive emails about upcoming NOVA programs and related content, as well as featured reporting about current events through a science lens. Email Address Zip Code Subscribe

Scientists have discovered a fish that can keep its entire body several degrees warmer than the waters it lives in, suggesting that endothermy evolved on yet another occasion.

The fish, known as the opah or

Support Provided By Learn More

Opah are large, deep-water predators that are endothermic.

, is a large species with a conspicuously round profile. It can weigh almost 600 pounds and swims not by flipping its tail but by flapping its pectoral fins. The opah lives in the mesopelagic zone, which is about 660 to 3300 feet deep where the water is particularly cold.

Ichthyologists had long thought that such deep-water predators hunted more like alligators—they conserved their energy, preferring to ambush their prey rather than actively hunt it. Since the discovery by Nicholas Wegner and his colleagues from the National Oceanic and Atmospheric Administration, they’ve revised their view of opah, at least. The fish is actually a swift and agile predator, similar to the tuna.

But unlike tuna, which are partially endothermic—meaning they can warm parts of their bodies like muscles—all of an opah’s tissues are kept several degrees above the temperature of the surrounding water. They accomplish that through a variety of adaptations. One is a thick layer of fat around the muscles that control its propulsive pectoral fins, which traps the warmth generated by the contractions. Opah also have another unique approach to conserving body heat. Jonathan Webb, reporting for BBC News, has more:

Crucially, when blood is pumped into the animal’s gills to collect oxygen, heat loss is minimised by a dense, intertwined network of blood vessels called a rete mirabile . At the surface of the gills, blood picks up oxygen and loses warmth. But as it passes back through the sponge-like rete mirabile (Latin for “wonderful net”), regains some heat from the still-warm blood arriving from inside the fish’s body. The design was a bit like a car’s radiator, Dr Wegner said. “There has never been anything like this seen in a fish’s gills before. This is a cool innovation by these animals that gives them a competitive edge.”

Wegner and his colleagues note that L. guttatus isn’t the only species of opah that’s extant—several others have recently been discovered, including one in the southern oceans. They don’t know whether those opah are similarly endothermic, but by comparing species, they hope to unravel the mystery of how this fish evolved its own way to stay warm