As we were taught in Biology, mammals and birds are warm-blooded; reptiles and fish are cold-blooded. But now scientists have discovered that the opah, a deep-water fish, is warm-blooded!
The large and colourful opah has become the first known “warm-blooded” fish, as scientists discovered it can regulate the temperature of its whole body.
The opah traps warmth from its flapping fins, which are well insulated by fat.
It uses that heat to keep its heart, brain and other organs warm while it swims to depths of hundreds of metres.
Other fish like tuna can warm specific body parts, boosting performance at key times, but whole-body “endothermy” has not been observed in a fish before.
The research is published in the journal Science.
Mammals and birds are traditionally thought to be the planet’s only warm-blooded animals, keeping their body temperature consistently warmer than the outside environment.Fish and reptiles are almost entirely cold-blooded, or “ectothermic”; they are at the mercy of the environment for their warmth and largely get by on slower metabolic rates.
Some fish are known to use so-called “regional endothermy”, including tuna, which can warm their swimming muscles for a burst of speed when pursuing prey.
But deep-water fish such as the opah are usually relatively slow and sluggish, tending to ambush their prey instead of making chase.
This makes the new findings quite a surprise, according to the study’s first author Nicholas Wegner.
“Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments,” said Dr Wegner, from the Southwest Fisheries Science Center run by the National Oceanic and Atmospheric Administration (NOAA) in the US.
“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.”