Researchers from the National Oceanic and Atmospheric
Administration (NOAA) determined that unlike other fish, opah generate heat as
they swim and distribute the warmth throughout their entire disc-shaped bodies
by special blood vessels. Special "counter-current heat exchangers"
in their gills minimize heat loss, allowing the deepwater predators to keep
their bodies several degrees above the water temperature 250 feet down.
"There has never been anything like this seen in a
fish's gills before," said biologist Nick Wegner, the lead author.
Though some species of fish can temporarily warm their swim
muscles, including tuna and some sharks, "whole-body endothermy" has
distinguished mammals and birds from fish and reptiles, which draw heat from
their environments.
"The opah appears to produce the majority of its heat
by constantly flapping its pectoral fins which are used in continuous
swimming," Wegner told Live Science.
His colleague Heidi Dewar told The Washington Post "I
think that it's really exciting that we spend so much time studying especially
these larger fish to find something that's completely unique and has never been
seen before in any fish."
Their team's findings are published in the May 15 issue of
Science.
Also known as the moonfish, the opah averages 100 pounds,
has a diameter of 3 feet and can grow to up to 6 feet long. While deepwater
fish are slow moving because of the cold, the opah's warm-blooded uniqueness
results in faster swimming, better vision and quicker responses, giving it an
edge in the survival sweepstakes.
"Before this discovery I was under the impression this
was a slow-moving fish, like most other fish in cold environments," said
Wegner, of the Southwest Fisheries Science Center, in La Jolla, Calif.
"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
Opahs, which don't swim in schools, are regularly caught
either by longline fishermen from California to Hawaii to New Zealand seeking
tuna or unintentionally in commercial driftnets.
An old fish tale held that opah brought good luck, NOAA
says, so fishermen would give the colorful catch away rather than sell it. But
times and marketing have changed, and food fetishists are increasing demand for
its "rich, tasty meat."
Two years ago, recreational anglers in Southern California
caught a 125-pound opah during a rockfish outing. The "mystery fish"
put up a 45-minute fight, leading the captain to think it was a shark, GrindTV
reported.
