It's one of the most basic biology facts taught in school: Birds and mammals are warm-blooded, while reptiles, amphibians and fish are cold-blooded. But new research is turning this well-known knowledge on its head with the discovery of the world's first warm-blooded fish - the opah.
In a paper published yesterday in Science, researchers from the National Oceanic and Atmospheric Administration (NOAA) describe the unique mechanism that enables the opah, a deepwater predatory fish, to keep its body warm.
The secret lies in a specially designed set of blood vessels in the fish's gills, which allows it to circulate warm blood throughout its entire body.
Scientists already suspected the opah was special, says Heidi Dewar, a researcher at NOAA's Southwest Fisheries Science Centre and one of the paper's authors. Most fish who live where the opah does - that is, hundreds of metres deep, in some of the ocean's darkest and coldest places - are sluggish, thanks to the low temperatures. At these depths, even predatory fish tend to be slow-moving, waiting patiently for prey to come by rather than actively chasing it down. But the opah, which spends all its time in these deep places, has many features usually associated with a quick-moving, active predator, such as a large heart, lots of muscle and big eyes. These characteristics made the opah "a curiosity", Dewar says.
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The opah's secret first started to come out when NOAA researcher and lead author Nicholas Wegner looked at a gill sample and noticed something intriguing.
All fish have two kinds of blood vessels in their gills: vessels carrying blood in from the body to pick up oxygen, and other vessels carrying oxygenated blood back out again. In the opah, the incoming blood is warm after circulating through the fish's body. This is because the opah swims by quickly flapping its pectoral fins, rather than undulating its body like many other fish do, to propel itself through the water - a process that generates high heat.
But outgoing blood, which has just been in contact with water in the gills, is cold.
Wegner noticed that in the opah's gills, the two sets of vessels are tightly bundled against each other, so that the incoming blood vessels can warm up the outgoing blood before it goes anywhere else. This set-up, known as "counter-current heat exchange", allows warm blood to be delivered throughout the body.
For now, though, the opah enjoys the spotlight as the world's first - and, so far, only - warm-blooded fish.
"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," Dewar says. "We've been very excited about it."
- Washington Post