In 2015 many sources, including USA TODAY, Discover Magazine, and National Geographic, ran stories about the Opah fish (Lampris guttatus) being warm blooded.
Headline from USA TODAY 5/14/2015: First warm-blooded fish identified
In a discovery that defies conventional biology, a big fish that lives deep in the Pacific Ocean has been found to be warm blooded, like humans, other mammals and birds.
Article from National Geographic 5/14/2015: Meet the Comical Opah, the Only Truly Warm-Blooded Fish
The opah’s wonderful nets are in its gills, and that makes all the difference. The blood vessels carrying warm blood from heart to gills flows next to those carrying cold blood from the gills to the rest of the body, warming them up. So, while a tuna or shark might isolate its warm muscles from the rest of its cold body, the opah flips this arrangement. It isolates the cold bits—the gills—from everything else.
This allows its huge pectoral muscles, which generate most of its heat, to continuously warm the rest of its body. It also keeps that heat with the help of thick layers of fat, which insulate the heart from the gills, and the pectoral muscles (which produce most of the animal’s heat) from the surrounding water.
According to the Merriam-Webster Dictionary warm blooded is defined as:
having warm blood; specifically : having a relatively high and constant internally regulated body temperature relatively independent of the surroundings
This definition seems to agree with the stories about the Opah fish being warm-blooded. However, it seems different from humans, mammals, and birds which generate and maintain body heat when muscles are at rest too. Are there different definitions of warm-bloodedness that would fit the Opah fish that is different from humans, mammals, and birds?