Scientists Discover First Warm-Blooded Fish in the World: The Opah

Scientists have discovered the first warm-blooded fish in the world: the opah, or moonfish. They've found that this fish circulates heated blood throughout its body much like mammals and birds, giving it a competitive advantage in the icy depths of the ocean.

The silvery fish is roughly the size of a large automobile tire; it's known to exist in oceans around the world at depths that reach hundreds of feet beneath the surface. It swims by rapidly flapping its large, red pectoral fins like wings through the water.

While fish that typically inhabit cold depths are slow and sluggish, though, the opah's constant flapping makes it unusual. In fact, the researchers have found that the flapping fins heat its body, speed its metabolism movement and reaction times.

Because the fish is warmer, it can also react more quickly; this, in turn, turns it into a high performance predator at depth.

"Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments," said Nicholas Wegner, one of the researchers, in a news release. "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."

The researchers made their discovery after sampling a bit of gill tissue. This gill tissue revealed blood vessels that carry warm blood into the fish's gills wind around those carrying cold blood back to the body core after absorbing oxygen from the water. Known as counter-current heat exchange, this system means that warm blood leaving the body core helps heat up cold blood returning from the respiratory surface of the gills.

"There has never been anything like this seen in a fish's gills before," said Wegner. "This is a cool innovation by these animals that gives them a competitive edge. The concept of counter-current heat exchange was invented in fish long before we thought of it."

While a few other fish like tuna and some sharks warm certain parts of their bodies, such as muscles, internal organs, including their hearts, cool off quickly and begin to slow down when they dive to cold depths. This shows how the opah, in particular, is so unusual.

The findings are published in the journal Science.

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