La Jolla scientists publish paper on rarely seen megamouth sharks following September sighting

A megamouth shark is pictured off the coast of San Diego.
(David Stabile)

Fishermen’s video of a pair of megamouths swimming together near the surface 24 miles off San Diego — ‘the only time we have seen two together’ — seemingly shows mating behavior, a marine biologist says.


Despite being one of the largest fish in the sea, sightings of the megamouth shark are few and far between. Really far between. Thus research of the species is also scarce.

But last Sept. 11, a pair of megamouths estimated at 12-15 feet long were seen swimming together near the surface about 24 miles off the coast of San Diego. Fishermen filmed the pair, providing first-of-its-kind documentation of the behaviors of the little-known shark.

The megamouth, once thought to be extinct, is found largely in the open ocean. The first known encounter was in 1976, when a shark got tangled in Navy gear. Subsequent sightings occurred in the 1990s and more in the 2010s.

There now have been a total of 273 confirmed megamouth sightings, most of them individual sharks caught up in commercial fishing nets.

Following the September sighting, researchers and collaborators at La Jolla’s Scripps Institution of Oceanography published a paper that scientists believe will advance understanding of the species.

“It’s important to know what they are like in the wild, but we rarely see them,” said Zachary Skelton, a marine biologist and contractor with NOAA Fisheries in La Jolla and a co-author of the paper. “Even scientists have never seen them before in the open ocean and we have had to rely on people that frequent the ocean. So this [video] is very exciting.”

In the early 1990s, Skelton said, an individual shark was caught off California and scientists tagged it to track its movements. The data they collected suggests megamouths stay in the deep sea during the day and go to the surface at night. Scientists believe that nocturnal activity has a lot to do with the rarity of sightings.

Perhaps even rarer than a daytime sighting, the September video shows the two sharks seemingly exhibiting mating behavior.

Two megamouth sharks are documented in September about 24 miles off the coast of San Diego.
(David Stabile)

“What makes the sighting so important is there were two … and they were found during the day, which is abnormal to what we know about megamouth biology,” Skelton said. “We think the male was trying to mate, but the encounter was only 10 minutes long and only four minutes of video. The smaller shark had claspers [male reproductive organs], which helped us determine it was a male, and there were tooth scrapes on the female ... because the males try to bite the female during mating. The scars looked fresh and the male was trailing under the female and close to her. The male had some abrasions near its tail near where the claspers are, which could have been from mating. It’s really exciting because it’s the only time we have seen two together.”

Skelton said he was “shocked” when he saw the video. “I feel like a lot of people don’t understand how rare it is,” he said. “I knew right away that this was a first occurrence.”

Many of the other sightings have been in fisheries or when the megamouths are brought up as bycatch. In 2010, fisheries started to report more of their findings. Thus, 80 percent of all reported megamouth shark sightings in fisheries and elsewhere have come since 2010.

“Even scientists have never seen them before in the open ocean and we have had to rely on people that frequent the ocean. So this [video] is very exciting.”

— Marine biologist Zachary Skelton

Though the megamouth’s biology is largely a mystery, Skelton said it is believed the shark can reach over 20 feet long; the largest one captured was 23 feet.

As their name would suggest, the sharks have large mouths but small teeth and fine gill rakers comparable to whale sharks. Megamouths have a broad, flat head, bluish-gray skin, floppy dorsal fins and signature white tips on their pectoral fins and are considered slow-moving.

They eat krill and other pelagic (open ocean) plankton.

The fishermen who spotted and recorded the sharks off San Diego were listed as co-authors on the local paper.

“This was a community discovery,” Skelton said. “Because we know so little about the [sharks’] movement, people need to know how important it is so [they] tell us about it when they see megamouths. If you are fishing off the coast, keep an eye out.”

The new study gives only “a brief glimpse” into the shark’s biology, Skelton said, but he hopes it will provide a “snowball effect” of public interest, funding and understanding.

“We hope this paper pushes that snowball down the hill and encourages people to learn more,” he said. “Without the public knowing what they are … we wouldn’t have known it was seen. These observations and public curiosity are really important.” ◆