The deep ocean is the last frontier on the planet, filled with strange creatures that have evolved in spectacular fashion to accommodate life in one of the most unique environments on Earth. This includes the opah, a deep sea predator that has been recently discovered to be the first fully warm-blooded fish.
Fish are cold blooded, which means that their body temperature matches that of their environment. In the deep sea where little or no sunlight reaches, the depths become quite cold and, as a result, the fish that live at there tend to be sluggish. Even the predators prefer to let prey come to them rather than actively chasing food (think the bioluminescent lure used by the anglerfish). Some fish, such as sharks, can use a quick burst of warmth with their swimming muscles during periods of high activity, but eventually have to return closer to the surface to warm back up.
The tire-sized opah, or moonfish, doesn’t look like a particularly adept predator, but its unusual body is what gives it its edge.
The opah has evolved in such a way that it can not only live and hunt exclusively in these deep waters, but is able to chase down prey at speed. Its wing-like fins generate heat, and an adaptation in its gills allows it to recycle that heat. The blood vessels that carry warm blood from the body are actually wrapped around other vessels carrying cold blood from the gills. This is an adaption other animals have used, such as the feet of penguins or arctic foxes, but has never been seen in a traditionally cold-blooded animal.
This means the moonfish is completely warm blooded and, as such, can move and react quicker than the other denizens of the deep it preys on.
“Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments,” said Nicholas Wegner, a biologist with the National Oceanic and Atmospheric Administration and the author of the opah study 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.”
Wenger added, “Nature has a way of surprising us with clever strategies where you least expect them.”
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