Scientists Discover The World's First Warm-Blooded Fish

The large, round fish—about the size of a manhole cover—uses its warm blood as an advantage in the ocean's freezing depths

Southwest Fisheries Science Center Biologist Nick Wegner holds captured opah.

Deep-water fish called opah appear to be the first fully warm-blooded fish species ever discovered, according to a new paper published in Science. Researchers say the unique biology behind opah, also known as moonfish, allow the species to operate at peak performance even within frigid ocean depths.

Being warm-blooded has its perks. Birds and mammals (or endotherms) conserve their internal heat to maintain high body temperatures, which helps them flee predators, chase prey and thrive in sub-zero climates.

But fish—and other cold-blooded animals, like reptiles and amphibians—aren’t so lucky. Most deep-sea fish move slowly, preferring to ambush prey rather than give chase, as their low body temperatures (and reaction speeds) mirror the cool ocean water. But now, scientists say they have discovered one exception to this rule: opah.

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“It’s a real advantage if you’re in this deep, cold habitat and you’re swimming around with a warm body,” says Nick Wegner, an NOAA fisheries biologist and lead author on the paper. “It increases the rates of all the reactions that occur within the body—you can swim faster, see better, react faster and capture cold-bodied prey that are not able to respond nearly as quickly.”

Opah, which resemble large, colorful tires, thrive in the deep sea and are an increasingly popular seafood.

Although Wegner had been studying opah for years, he recently noticed that warm blood vessels leaving the fish’s heart wrap around cooler blood vessels returning from its gills.

Later, he determined that opah generate heat by flapping their pectoral fins, and retain that heat through this dense layer of blood vessels.

Opah’s internal heating system is capable of keeping the fish’s heart and brain at peak performance, even at depths of up to 1,300 feet.

Certain tuna and shark species also retain body heat, and warm select muscles for high-performance hunting

but these species are far from warm-blooded: Most of a shark or tuna’s body (including its heart and other vital organs) remains cold.

“Opah is the first fish that can circulate warm blood throughout the entire body, and that gives it some advantages over tuna and shark species,” Wegner says. “Since they can keep their entire bodies warm, they can stay down deep, continuously close to their forage base.”