Giant sunfish sets new record for world’s largest bony fish

As soon as he saw the hulking, bone-white carcass of an ocean sunfish sloshing in the waves offshore of the Azores archipelago, José Nuno Gomes-Pereira suspected the animal might be a record-breaker.

It was December 9, 2021, and Gomes-Pereira was responding to a call as part of his marine biology work with the Azores' marine strandings network and the Atlantic Naturalist Association, a nongovernmental organization focused on ocean monitoring.

Massive, rotting carcasses pose a threat to human health, says Gomes-Pereira. They are also a danger to passing ship traffic, which is why the network keeps close tabs on any large remains that drift near shore.

Likewise, human vessels frequently injure or kill big ocean creatures in surface collisions. In fact, it’s suspected the sunfish, also known as a mola, may have died from a ship strike, owing to a large contusion on its head. The wound contained fragments of red antifouling paint, which is commonly used on the underside of ships.

Without a marine veterinarian on hand, though, it was impossible to determine whether the strike happened before or after death, says Gomes-Pereira.

After towing the animal back to shore, a forklift hoisted it into position so a crane scale could measure the behemoth’s heft more accurately. After a few minutes of calibration, the scientists knew they had a whopper. (Read about one of the world's biggest great white sharks.)

At 6,049 pounds, the mola weighed more than the full starting lineup of an American football team. The specimen was also nearly a thousand pounds heavier than the previous Guinness World Record holder for the heaviest bony fish—another sunfish of the same species, Mola alexandrini, which was caught near Japan in 1996. (Fish are divided into two large classes, with sharks and rays classified as cartilaginous fish and mostly everything else as bony fish.)

According to Gomes-Pereira, who is lead author of a study publishing the findings in the Journal of Fish Biology, the discovery is both a cause for hope, as well as a warning.

“It means that the marine ecosystem is still healthy enough to sustain these large animals,” he says.

However, because the animal may have died due to humans, the mola should also serve as a wake-up call that we have more work to do when it comes to conservation, says Gomes-Pereira.

Mysterious mola

Molas are easy to spot at the water’s surface. The animals can be 10 feet long and tend to enjoy sunbathing. But scientists still have a lot of unanswered questions about these curious creatures.

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“We don't know how long they live or how fast they grow in the wild,” says Tierney Thys, a marine biologist with the California Academy of Sciences and a National Geographic Explorer. “We also don't have a good sense of how many individuals there are in the world or how big regional populations are.”

Thys has been chipping away at such questions since 2000, when she received her first grant from the National Geographic Society to study the giants.

As far as the record-breaking animal, it was likely female and 20 years old or more, says Thys, who is also the founder of the Adopt A Sunfish Program.

“Mola alexandrini is truly one of the ocean's great oddballs and while its external appearance may seem somewhat awkward and cumbersome, it is in fact a gentle giant and swims through the water with surprisingly graceful winglike strokes,” she says.

Another fascinating feature of these goliaths? They begin their lives as larvae that are less than one quarter of an inch in size. Where spawning takes place, or exactly how the pencil-eraser-size larvae survive long enough to turn into the biggest bony fish in the sea, scientists still don’t know. (Read about the recent discovery of the world's largest freshwater fish.)

“The only larvae specimen to be genetically verified as M. alexandrini was found off New South Wales, Australia, so that area may be one of the spawning grounds,” says Thys.

“A colossal reminder”

While most people are probably more familiar with M. alexandrini’s close cousin, the Mola mola, ocean sunfish are just one of those animals that inspire “primal curiosity”, says Thys.

“I so love going to the Monterey Bay Aquarium because sometimes we have the sunfish shown on display in the million-gallon tank alongside the hammerheads and tuna. And then when the sunfish appears, people are just like, ‘Oh. Wow! Why?!’” laughs Thys. “It’s an animal that just begs so many more questions.”

To start to try to answer some of them, Thys recently co-wrote the first ever academic book on the Molidae group, The Ocean Sunfishes: Evolution, Biology and Conservation.

The new world-record-holding mola will also help further understanding of the species. Not only did Gomes-Pereira and his colleagues collect scales, intestinal contents, and genetic samples from the animal, which will contribute to future research, but they also buried the carcass in the hopes that it can be exhumed one day and assembled as a museum specimen.

“It’s a colossal reminder that our ocean still holds so many mysterious surprises,” says Thys.