Opah, also known as moonfish, are comically rotund, but not to those who live in fear of them. Opah are deadly deep predators: plying cold, dark waters as far down as 300 meters. Most predatory fish are slowed by the cold at the depths opah hunt, waiting for meals to come to them. By contrast, moonfish are nimble pursuers.
The adaptation that makes the moonfish lifestyle possible is endothermy. Opah are the only fish known to maintain their body temperature in the chilly waters where they live. Other top sea predators, such as tuna and sharks, must reheat their bodies in warmer waters near the ocean surface after diving deep for food.