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It's one of the most basic biology facts we're taught in school growing up: 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 Thursday in Science, researchers from the National Oceanic and Atmospheric Administration (NOAA) describe the unique mechanism that enables the opah, an unusual-looking 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 the fish to circulate warm blood throughout its entire body.
Scientists already suspected the round-bodied 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 - which is a tasty if uncommon table fish - "a curiosity", Dewar says.
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 strange.
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.
WATCH: The opah has a 'competitive edge' over other fish in its environment

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.
Some other types of fish, such as tuna, have similarly designed blood vessels in certain parts of their bodies, allowing for "regional endothermy" - warm-bloodedness that's limited to certain organs or muscles, such as the eyes, liver or swimming muscles.
But the opah is the only fish scientists know of that has this design in its gills, where most fish lose the majority of their body heat to the surrounding cold water.
By warming up the blood in the gills before it goes anywhere else, the opah achieves not just regional endothermy, but whole-body endothermy.
Testing showed that the opah is able to maintain a body temperature about 5 degrees Celsius warmer than that of the surrounding water.


Big catch: world's first known warm-blooded fish unveiled by scientists

Marine biologist Nick Wegner displays an opah. Photo: Reuters

Marine biologist Nick Wegner displays an opah.Photo: Reuters

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Marine researchers locate an exception to the conventional wisdom in the opah, a deep sea predator with blood that's warmer than water


It's one of the most basic biology facts we're taught in school growing up: Birds and mammals are warm-blooded, while reptiles, amphibians and fish are cold-blooded.


Big catch: world's first known warm-blooded fish unveiled by scientists

.css-1c6uqr6{color:inherit;font-weight:inherit;font-size:inherit;font-family:inherit;line-height:inherit;overflow-wrap:break-word;}Marine researchers locate an exception to the conventional wisdom in the opah, a deep sea predator with blood that's warmer than water

Marine researchers locate an exception to the conventional wisdom in the opah, a deep sea predator with blood that's warmer than water