In May of 2015, the first warm blooded fish was discovered. Lampris guttatus (otherwise known as Opah or Moonfish) is a deep-sea dwelling endotherm,  meaning it internally regulates its body temperature. This is very unique because all other fish are ectotherms and rely on the environment as their heat source. The Opah’s endothermic nature means that it can swim faster, see better, and respond to its environment quicker.

So this isn’t 100% correct and it’s a case of the media misinterpreting scientific research in order to make it more accessible and easier to digest for the public (which I am all for) unfortunately that means they often completely twist the research. 

Not all fish are exothermic and nor is the Opah the first endothermic fish discovered.  Sharks from the Lamnidae family, some species of tuna and billfishes are all endothermic. But none of these species are homeothermic like mammals or birds. 

The reason the Opah however, is different from other endothermic fish is that it has a warm heart that allows it to warm it’s heart and brain, and keep most of their body constantly above ambient temperature. Which other endothermic fish haven’t been observed to be able to do.

Other endothermic fish have such as tuna, temporarily heat up their swim muscles, the Opah however is able to generate enough heat with it’s swim muscles that it can then heat it’s heart and brain. 

Here’s the full paper of it for those that are interested.