Opah- The World’s First Warm-Blooded Fish Species

The opah is the first fish species found to be fully warm-blooded, circulating heated blood throughout its body much like mammals and birds, research has revealed.

The fish, found in the waters off the US, Australia and several other countries, generates heat by constantly flapping its fins and has developed an internal “heat exchange” system within its gills to conserve the warmth.


This adaptation means warm blood that leaves the opah’s body core helps heat cold blood returning from the surface of the gills where it absorbs oxygen, maintaining an average body temperature of about 4C to 5C.

This system, likened by scientists to a car radiator, is similar to that used by mammals and birds, which are known as endotherms for their ability to maintain body temperature independent of the environment.

While tuna and some sharks can warm certain parts of their bodies, such as their brains and eyes, fish are generally classed among cold-blooded animals, known as ectotherms.

Opahs, which are an oval-shaped fish roughly the size of a car tyre, use this adaptation to become more effective predators. It swims faster, reacts more quickly and sees more sharply than the other marine life that, like the opah, dwell 50m to 400m underwater, NOAA said.

The species’ rich meat has become popular among seafood aficionados but NOAA said there is no evidence opahs are in decline and populations may even be growing.

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