Its squat body and flimsy-looking pectoral fins may not scream speed-demon.

But the opah, or moonfish, is actually quite fast, and can run with the big boys like tuna and swordfish. That's just one of many surprising revelations coming to light as more of these mysterious fish appear unexpectedly in scientific surveys along the southern California coast. (See pictures of other deep-sea animals.)

This unexplained surge is enabling researchers to study and photograph the camera-shy creatures.

While documenting a fishing survey, photographer Ralph Pace caught the roughly 130-pound (59-kilogram) fish on camera off the southern California coast in November 2014.

Researchers had accidentally caught the odd animal, and after it was released, Pace dove into the water to take some snapshots before it swam away.

He had only a couple of minutes with the opah, but they were memorable. It was big, Pace recalls—"probably bigger than a manhole cover." And he was lucky.

"Photos of them in the water free-swimming are pretty rare," says John Hyde, a fish-genetics researcher with NOAA Southwest Fisheries in La Jolla, California. "And Ralph Pace's images are better than the rest I've seen."

A Pricey Catch

Opah can be valuable commodities on the seafood market, says Owyn Snodgrass, a fisheries biologist with NOAA Southwest Fisheries, but there is no targeted fishery for them.

That's partly because the deep-sea dwellers don't congregate in large groups like other commercially valuable fish such as tuna. So focusing solely on opah won't make fishermen much money, Snodgrass explains. (Read about the plight of the Atlantic bluefin tuna in National Geographic magazine.)

Instead, moonfish are caught as bycatch in commercial tuna and swordfish fisheries. And despite the relatively small size of the catch, they bring in a good chunk of change. The 2012 Hawaiian opah market was valued at around $3 million. "They're very tasty fish," Snodgrass says.

Opah are unusual in that different parts of their body look and taste different, the biologist explains. The upper part of the fish looks like tuna and tastes like a cross between tuna and salmon, he says. But their pectoral muscles—the ones that power the fins on the side of the body—look and taste a bit like beef.

"[Opah] can be eaten raw, but they're also great on the barbecue or smoked," says Snodgrass.

Casson Trenor, who co-owns four San Francisco sushi restaurants, actually prefers a little sear on his opah. He doesn't offer the fish in his restaurants, though, since little is known about the sustainability of the fishery.

The pectoral muscles aren't just good eating—they also give researchers clues about the animal's speed and way of life.

Those muscles are about 17 percent of an opah's body weight, Snodgrass says, which is a relatively large percentage. "[So] despite what they look like, they can swim really fast when they want to, and they can swim long distances."

An Elusive Subject

Unpublished data Snodgrass obtained from a satellite-tagged moonfish showed that the animal swam from central California to Hawaii within an eight-month period. Snodgrass and colleagues are still analyzing their data—obtained from ten opah tagged between 2011 and 2013—so it's unclear just how fast the opah traveled.

It's also unclear if this kind of movement is normal for the fish, or if this particular opah was unusual. Other opah tagged off of central and southern California as part of the same study made their way down to Baja, California (map), Snodgrass says.

That's not the only thing researchers are in the dark about regarding moonfish. Population sizes, longevity, or even how many species there are remain mysteries. This is because of the fish's solitary ways and deep-sea habitat.

The surge in opah—an increase accidentally detected in the course of a long-term mako and blue shark survey off of southern California—is changing things, though. Researchers caught one opah in the first 15 years of this survey, says Snodgrass. "In the past five years, we've caught upwards of 60."

The increased number has allowed biologists to tag the fish and take small samples of flesh for genetics analysis. This has led to evidence suggesting that Lampris guttatus, the species commonly found around Hawaii, isn't a single species, Snodgrass says.

These studies aren't easy, though. The fish are strong and don't take kindly to being reeled in so people can attach tags to them. "They tend to thrash around and splash, and by the time you're done, everyone's soaked," Snodgrass says.

And those weak-looking pectoral fins? "If you get in the way of their fins, they'll smack you," says Snodgrass, who has firsthand experience. "They're pretty feisty."