Opah, the only known existing hot blooded fish, sustains its body heat using counter-current heat receptors found within its gills

Most fish, or all fishes for that matter, were thought to be cold blooded. It’s not surprising since they live in water. Well, for every rule there’s an exception, and there’s one fish species that doesn’t fit the niche. It’s called Opah. It’s a deep water sea creature and a cold blooded predator looking for prey in the frosty ocean depths. The Opah doesn’t need to resurface to heat some parts of its body, unlike tuna and certain types of sharks that do this to preserve their hearts and other vital organs from the negative effects of cold water.

The Opah can generate its own heat by continually flapping its oversize pectoral fins.  The Opah, also known as moonfish, is able to produce an average muscle temperature of 7 degrees to 9 degrees Fahrenheit over the surrounding water temperature at a time.

Opahs have a unique body structure that keeps heat within their body, preventing it from dissipating into the environment. They have a counter-current heat receptors within its gills to do it.

On the other hand, mammals and birds are known to be warm blooded animals. They are called endotherms due to their ability to produce their own body heat without relying on the environment. Cold blooded animals, also known as ectotherms, include amphibians, reptiles, and, of course, most of fish.

“With a more whole-body form of endothermy, Opah don’t need to return to surface waters to warm and can thus stay deep near their food source continually,” said fisheries biologist Nicholas Wegner of the U.S. National Oceanic and Atmospheric Administration’s National Marine Fisheries Service.

The Opah has a rusty reddish appearance with white spots dotting its bright red fins and can weigh up to 200 pounds or 90 kg, growing up to the size of a car tire. They are found in the oceans worldwide with their favorite habitat being 165 to 1,300 feet below sea level, foraging for squids.

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