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Today on Animal Fact Files we’re discussing opahs. The opah is a round shaped fish also known as the moonfish, which might get them confused with the ocean sunfish, but these animals aren’t closely related. There are a handful of species in the opah genus. The one most commonly known as the opah is a cosmopolitan species, meaning it’s found worldwide, some of the other species may, however, have more restricted ranges. These are oceanic pelagic fish, meaning they spend most of their time in open ocean waters and tend to swim anywhere from the ocean’s surface to about 1,640 feet (500m) below the waves. They may dive deep for food like other fish and crustaceans, however, their main diet consists of squids.

Opahs look like they would be lumbering giants. In size they average three feet (91cm) in length and 100 pounds (45kg) in weight, though they can nearly double that length and have been reported more at more than triple that weight. So that’s a pretty big fish to move around. And they come in a shape that doesn’t seem as streamlined as something like a billfish, but opahs can move quickly. Their primary prey source, squids, are fast moving creatures, so opahs have to be able to keep up in order to catch them. And while the ocean gets quite cold the deeper they dive, opahs don’t have to concern themselves with close to freezing temperatures.

Opahs are considered whole body “warm-blooded” fish. Now, we’ve talked about tunas and how they can produce enough heat to cook themselves from the inside, however, tunas don’t disperse this heat around their bodies. Opahs, on the other hand, do. Some scientists argue that the opah’s extremities, like the tailfin, remain cold, so it’s a bit of a stretch to call them whole-body warm blooded, but, at the end of the day, opahs can warm themselves many degrees above their surrounding environment. While this trait of being “warm-blooded” is mostly seen in birds and mammals, it also appears in these oceanic fish! They get warm by rapidly moving their pectoral fins as they swim. Also of note is that opahs maintain neutral buoyancy in the water via a swim bladder and porous, oil-filled bones!

These fish have a disc shape to their bodies and in life appear silvery blue with bright red fins and lips. They may also have white spots and a red belly not unlike the red-bellied piranha we’ve talked about in a previous episode. In death, opahs appear even more red throughout their bodies. Predators to opahs include other fish like sharks, however, they’re also eaten by people! While generally a rare catch, opahs may be snagged in fishing operations after other types of deeper dwelling fish.

There isn’t much known about opah reproduction, though it’s believed they spawn year round in warmer tropical waters and seasonally, likely in spring, in cooler temperate waters. They may spawn multiple times throughout a season and are believed to grow rapidly. Most caught in longline fisheries are estimated to average about five years in age.

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