When an ocean sunfish washed ashore in Australia a few days ago, the people who found it couldn’t help but gawk. The ocean sunfish is fundamentally bizarre, and the weirdest thing about it isn’t the teeth in its throat. It isn’t the way it plays dead on the ocean surface. It isn’t even the way its googly eyes make it perpetually look as if it’s just realized it left the stove on at home.
The strangest thing about an ocean sunfish, at least to Natasha Phillips, is its shape. Phillips, who studies the creatures at Queen’s University Belfast, called them a “giant pancake of a fish,” because they can grow to more than 5,000 pounds. Essentially, ocean sunfish look as if somebody began building a fish, added enormous vertical fins just behind its head, and then started laughing too hard to continue. And for scientists, these fish, with their unusual shape and inscrutable life, remain a “big bag of mysteries,” Phillips says.
Ocean sunfish, also called mola, are a genus of bony fish. They have a pair of normal, side-to-side fins that they can use to hover when they’re just chilling, but the true stars are their dorsal and anal fins, which protrude vertically from the fish’s body. Those majestic, ridiculous flappers can stretch up to 14 feet from tip to tip.
Despite their ungainly appearance, those fins are the mola’s primary way of getting around the ocean and can actually be a rather speedy mode of transportation. Ocean sunfish have been recorded swimming as fast as 21.6 feet (6.6 meters) per second over short periods. That’s fast enough to rival the cruising speed of more conventionally shaped (read: unoriginal) swimmers like yellowfin tuna.
What’s unclear is exactly how the fish manage this dashing feat. It’s been suggested that those big fins work like wings, Phillips says, but that’s not yet accepted as scientific fact. She also says the fish’s clavus—that still giant but oddly truncated butt where a tail should be—seems to function like a rudder, but researchers are trying to figure out its exact function.
Just last year, Phillips and other scientists discovered that the muscles that drive the fish’s fins are also connected to a layer of gelatinous mystery meat that lies just under its skin. That layer is called the capsule, and Phillips says it functions as “a wetsuit and a life jacket and a skeleton all in one.” She and her team found that the exoskeleton-like nature of the capsule helps the ocean sunfish move, but precisely how is anyone’s guess. It could make the fish’s strokes more powerful, Phillips said, but it could also just be an “evolutionary quirk.” (“Evolutionary quirk,” coincidentally, is a description that could apply to the entire Mola genus, not just its squishy super-suit.)
Part of the reason scientists still know so little about how the fish moves is that, despite its coquettish looks, the animal is notoriously shy. Marine biologists, including Philipps, have managed to tag and track a few over the years, but tagging can only reveal so much.
Researchers haven’t yet figured out how the ocean sunfish’s dorsal fins counterbalance when they’re moving at different times. They’re not sure how many different species there are, or even how many individual fish. Scientists don’t know how much the fish eat, how long they live, or how to distinguish juveniles from adults. It’s a riddle how and why the ocean sunfish’s pufferfish-like ancestors moved into the open ocean. And it remains to be seen where the fish go to breed, and where their normal ranges are. “They’re being found in places we didn’t expect,” Phillips says. That includes the Australian specimen, a member of a species that rarely shows up in that part of the world.
It seems odd that the fish continue to grab the world’s attention, despite the fact that different ocean-sunfish species are found pretty much everywhere except the poles and the equator, and humans have known about them for thousands of years. Phillips blamed the proliferation of smartphones: Fisherman’s tales are now accompanied more often with proof. But I suspect that these things are so purely weird that, perhaps, we’ll never get tired of being excited by them.
Of course, it’s not the ocean sunfish’s fault it’s so weird. Like most of us, it seems to have its family to blame. “When you start with the normal body plan of a pufferfish” and plunk it in the open ocean, Phillips says, “this is kind of as good as you’re going to get.”