Forget Everything You Knew: Some Fish Are Warm-Blooded

The opah, which is about the size of a large car tire, can keep its whole body warm. NOAA FISHERIES/SOUTHWEST FISHERIES SCIENCE CENTER

Another day, another way nature forces us to change something fundamental about how we see the world.

Scientists have long assumed that all fish are basically cold-blooded. Sure, there are some limited exceptions: tuna, great white sharks and a few others can warm up parts of their body a bit when they vigorously swim. Generally, though, fish that live in the deep cold water of the sea were thought to be exclusively sluggish and slow-moving.

Naturally, then, biologist Nick Wegner of the National Oceanic and Atmospheric Administration's Southwest Fisheries Science Center in La Jolla, Calif., assumed that a large silvery fish called opah, which hangs out in the depths, was cold and lethargic.

This turned out to be wrong, and researchers have discovered that the opah can actually continually keep its whole body, including its heart and brain, warm. This ability, known as endothermy, was thought to be unique to mammals and birds. A warm body temperature allows these animals to have a higher metabolism, and a better-performing brain and more active muscles, according to the study.

"Because the opah 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," Wegner said in a statement.

In a study published today in the journal Science, Wegner and colleagues found that opahs produce heat by continually flapping their wing-like pectoral fins. This movement generates warmth in the fish's enormous pectoral muscles, which take up nearly one sixth of its body mass.

Opahs also have an ingenious way of conserving and circulating this heat, Wegner says. The veins leaving the muscles with warm blood are wrapped around those leaving the gills, carrying cold blood. This heats up the colder blood, and limits loss of heat. This actually looks a lot like a design humans use to warm or cool things, called a "counter-current heat exchange," found for example in car radiators. It's amazing that this setup was "invented in fish long before we thought of it," Wegner says.

"Nature has a way of surprising us with clever strategies where you least expect them," he adds.