History 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,

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, was

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.

"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."