First warm-blooded fish identified

Michael Winter | USA TODAY

Show Caption Hide Caption Scientists discover the first fully warm-blooded fish Scientists say the deep-water predatory fish known as the opah is the first fully warm-blooded fish ever identified. Jen Markham (@jenmarkham) has the story.

Cold fins, warm heart? Strange but true, scientists say.

In a discovery that defies conventional biology, a big fish that lives deep in the Pacific Ocean has been found to be warm blooded, like humans, other mammals and birds.

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 of the report.

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 toldLive Science.

His colleague Heidi Dewar toldThe 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 demanding it 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.