A new study published in the journal Science has identified the world's first fully warm-blooded fish: the car tire-sized opah, or moonfish. The opah is a large, deep-sea fish, commonly found off the coast of Hawaii and west Africa. It’s also the first and only fish known to be warm-blooded, or endothermic. The opah averages 100 pounds, has a diameter of 3 feet and can grow to up to 6 feet long.
Usually, a fish's overall temperature matches the water it's in. Researchers said in the journal ‘Science’ on Thursday that this deepwater denizen is the first fish known to be fully warm-blooded, circulating heated blood throughout its body, enabling it to be a vigorous predator in frigid ocean depths. The fish has blood vessels in its gills that carry warm blood from the body's core. These vessels wrap around other vessels near the gills, where the fish breathes, bringing in oxygenated, cold blood.
Here’s more on the opah from ‘National Geographic’: “Most fish have body temperatures that match the surrounding water. A small number of them can warm specific parts of their bodies… But none of these animals can heat their entire bodies. Their hearts and other vital organs stay at ambient temperature, so while they can hunt in deep, cold waters, they must regularly return to the surface to warm their innards. The opah has no such problem. It can consistently keep its entire body around 5 degrees Celsius warmer than its environment. It doesn’t burn as hot as a bird or mammal, but it certainly outperforms its other relatives… The opah’s brain is warm. Its muscles are warm. And perhaps most importantly, its heart is warm—a first for a fish. Not even a great white shark has a warm heart.”
..... Devashree Goenka