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Opah, The First Documented Warm-Blooded Fish

Hear out, world. The first warm-blooded fish to ever have been discovered has been documented. The opah, also known as the moonfish, has been endowed with capabilities never seen in a fish before: it can generate its own body heat, and is thus faster, with higher metabolism.


Nick Wegner, a fisheries biologist, shows off an opah caught in the seas off California. Photo credits: NOAA fisheries/southwest fisheries science center.

The first known warm-blooded fish, the opah, also known as moonfish, has been documented in the journal Science.

The opah is a top predator with enhanced vision and the ability to swim and react really fast. It lives around 300 meters under the surface of the ocean. At this depth, the water is very cold, and dimly lit. As a matter of fact, being warm-blooded helps it achieve what other fish cannot.

“Before this discovery I was under the impression this was a slow moving fish, like most other fish in cold environments,” lead author Nicholas Wegner, a fisheries biologist at NOAA Fisheries’ Southwest Fisheries Science Center in La Jolla, said in a press release.

“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.”

The discovery of the amazing talents of the warm-blooded fish was triggered when a sample of its gill tissue was collected. The latter revealed the presence of blood vessels carrying warm blood to the gills. It was observed that the vessels coiled around those through which cold blood travels back to the body core after taking in oxygen from the water. Warm blood that leaves the body core heats up the cold blood coming from the gills where absorbs oxygen.

The researchers learnt that the fish flaps its fins to generate body heat. This motion also speeds up its metabolism, movement, and reaction times. As a consequence, its body is 5° Celcius higher than the water in its environment.

“There has never been anything like this seen in a fish’s gills before,” Wegner said. “This is a cool innovation by these animals that gives them a competitive edge. The concept of counter-current heat exchange was invented in fish long before we thought of it.”

“Nature has a way of surprising us with clever strategies where you least expect them,” Wegner said. “It’s hard to stay warm when you’re surrounded by cold water but the opah has figured it out.”

Also, the Opah is growing into a popular food source. It is caught in the seas near Hawaii. Now that its uniqueness has been proven by researchers, it might be easier to control its population and prevent overfishing.


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