“The opah… 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 secret is in the blood vessels of its gills.
“Wegner’s team confirmed this by catching opah, implanting them with small thermometers, and then releasing them. The instruments inside the fish recorded consistently higher temperatures than those dropped into the surrounding water. 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 [which warms some of its muscles] has a warm heart.”
Our human attempts to shove every creature into a neat little category are once again defeated by Nature. The world is so much bigger than we realize. That makes me happy.