Have you taken a family trip where you finally make it to a coast and immediately run towards the ocean, only to realize too late that the water is shockingly cold? Despite taking most of Earth’s solar radiation, the temperatures of Earth’s oceans don’t change very much. They stay perpetually brisk because it takes a lot of energy to raise the temperature of water. Evolved to deal with this relative stability, most ocean creatures can be cold-blooded — their internal temperatures match the oceans’ — and not waste energy trying to heat themselves.

Large marine predators like great white sharks, tuna, and swordfish can warm parts of their bodies like brains or eyes for a boost in hunting ability, but so far no fish has been found that warms all the way through like a bird does or like we do. Enter the fish that looks like a giant reflector disk with fins: the opah, or moonfish.

Wegner holds an opah.

Reported yesterday in the journal Science, Nicholas Wegner and his colleagues at the National Oceanic and Atmospheric Administration have shown with implanted thermometers that the opah keeps its whole body — the heart, the brain, swimming muscles, etc. — about five degrees Celsius warmer than the surrounding water. Wegner speculates that this gives the opah a competitive edge in searching out food. While other fish have to periodically return to the surface to warm up, the opah can hunt 1,000 feet below the surface without slowing down.

“Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments,” said Wegner. “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.”

So what’s the opah’s secret? Radiator-like gills.

When Wegner opened up a captured opah, he and his team found blood vessel structures in its gills that looked like heat exchangers — a way to conserve heat inside a system or dump it. Your car does this with heat imparted to your engine coolant. Inside the radiator, heated coolant is exposed to wind blowing through the car and enough surface area to release all that heat quickly to the environment.

The marlins, tuna, and sharks that warm parts of their bodies have a specific kind of heat exchanger in their muscles and organs: rete mirabile, a complex of veins and arteries that are close enough together to exchange heat to itself. That is to say, warm blood leaving the muscles passes right by cold blood coming in from the gills and exchanges heat. The result is a massive reduction in total heat lost to the environment in what is called a countercurrent exchange.

Warm-blooded animals have these rete mirabile too, typically those animals that need to thrive in cold environments. For example, both ducks and arctic foxes have countercurrent exchange systems in their legs. The foxes can tread through ice and snow and ducks can swim in a frigid pond without lowering core temperatures.

But the opah’s heat exchanger is in its gills. Normally, a fish’s gills would work just like a car’s radiator, with blood returning for oxygenation exchanging heat with the cold of the sea. That gets rid of internally-generated heat real quick. However, with the opah, the rete mirabile in its gills dump heat into the ocean-temperature blood coming in from oxygenation, rather than the environment. As a result, the opah can maintain a higher overall body temperature, and therefore a lifestyle — the first warm-blooded lifestyle found in a fish — that gives it an edge over other slower, fully cold-blooded predators of the deep.

“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.”

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STUDY: Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus

IMAGES: Lee Fish USA; N.O.A.A. Fisheries/Southwest Fisheries Science Center