Warm Blooded Fish (?)

My drawing of the opah

I love how what we accept as scientific "fact" is constantly changing. It just goes to show how amazing the universe is and how little we really know about it.

Recently scientists Nicholas C. Wegner, Owyn E. Snodgrass, Heidi Dewar, and John R. Hyde, discovered  the first known warm-blooded fish. When I first heard the news I was shocked. "But, fish are cold-blooded! They're not mammals!" Then, I read an article on the subject and I went from shocked to intrigued.

The fish, known as the opah (Lampris guttatus) lives in the very depths of the ocean where it is extremely cold. Most fish living at this depth are slow moving, but the opah is a fast moving predator with characteristics similar to predators that live nearer the surface. It's sluggish prey barely stands a chance.

The opah generates its heat by beating its pectoral fins constantly and quickly. This warms up the blood which warms the body circulates. A mechanism in the opah's gills allows the warm oxygen-poor blood, to transfer its warmth to the cold oxygen-rich blood. This process, known as "counter-current heat exchange," keeps the warmth concentrated in the opah's core. 

The opah is not as warm as mammals, but it does maintain a body temperature five degrees Celsius above the temperature of the surrounding water. Being warm has many evolutionary advantages, especially for a predator, as it allows the animal to move faster and have bigger eyes.

No fish yet discovered are as endothermic as the opah, but some fish like tuna have regional endothermy in their eyes, liver, and swimming muscles. Some people like Professor Diego Bernal, think that the opah is just another ectotherm with regional endothermy. Since the opah is warmest at its core, but gets colder near the outer edges, this is a possibility, but no matter what it's a pretty unique fish.