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."