The warm-blooded opah fish

One of the first things children learn in biology lessons is that mammals and birds are warm-blooded while reptiles, amphibians and fish are cold-blooded. Now that rule has been broken: US researchers have declared the opah to be the world’s first fully warm-blooded fish.

The opah, also known as the moonfish because of its silvery round body, heats up its blood by constantly flapping its red fins. An extraordinary heat exchange system within the gills then ensures that this warmth is distributed through the opah’s body. Scientists at the Southwest Fisheries Science Centre in California, part of the National Oceanic and Atmospheric Administration, reported the discovery in the journal Science.

The tyre-sized opah lives hundreds of feet beneath the surface of oceans around the world. Most fish swim slowly in cold, deep water, conserving energy by ambushing prey instead of chasing it. The opah, in contrast, is a high-performance predator that moves and reacts quickly, thanks to its warm blood.

“Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments,” says Nicholas Wegner, lead author. “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.”

Nick Wegner holding an opah

The “counter-current heat exchanger” in the opah’s gills has a dense network of blood vessels. The ones carrying blood from the body, which has been heated by rapid fin flapping, wrap around the vessels carrying cold blood from the gills and warm it up. The principle is similar to a car radiator. In addition, insulating fat helps the opah retain heat.

Monitors attached to live opah swimming at depths between 50m and 300m off the US west coast show that they consistently keep their muscles 5C warmer than the surrounding water (which is 8C to 11C). While this is not impressive by the standard of marine mammals such as dolphins, which maintain their temperature between 36C and 37C, the researchers say the opah is the first fish known to keep its whole circulation warmer than the environment. A few other species, such as some sharks and tuna, heat up swimming muscles to boost performance but their internal organs including heart are not warmed significantly.

NOAA research suggests that opah are becoming more common off the Californian coast, though no one knows why. They are turning up more frequently in seafood markets where buyers like their rich, fatty flesh.

Photograph: NOAA Fisheries / Southwest Fisheries Science Center; Dorling Kindersley

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