The opah, also known as the moonfish, is able to stay 7 degrees to 9 degrees Fahrenheit warmer than the water that surrounds it thanks to the constant flapping of its powerful pectoral fins. That may not sound like an impressive feat to you, but that's because you're a mammal and staying warm is all part of a day's work. For a fish, this is a big deal. There are only a couple other fish that can raise their bodies' temperatures, and they're predators like tuna and sharks. But those fish are only able to temporarily become warmer during hunting before they have to recede to warmer waters and bring their bodies back to normal.