In 1994 on Flagler Beach, a picturesque stretch of sand on Florida’s Palm Coast, a shark attacked a 21-year-old surfer named Jeff Weakley. He survived with lacerations on his foot and a mystery that would not be solved for another 24 years. Last fall, when Weakley chanced upon an article about scientists using DNA to identify sharks that attacked humans, the case finally began to close.

“That was about the same time I pulled the tooth out of my foot,” Weakley tells Inverse.

He’d read about a team led by Gavin Naylor, Ph.D., director of the Florida Program for Shark Research (FPSR) at the Florida Museum of Natural History, that had identified the species of shark that had bitten a child off the coast of Fire Island, New York, in 2018. Weakley, now an editor at Florida Sportsman, thought Naylor’s team would be able to identify his attacker using the tooth fragment he’d tweezed out of the small bulge growing on his foot.

He tracked Naylor’s team down, “curious to see if they could use the same DNA process on my sample.”

He was in luck. As Naylor and other shark experts report in a Wilderness and Environmental Medicine study published July 2, the tooth proved to be a reliable source of shark DNA, allowing the team to reveal the identity of the attacker.

“We all thought that because the tooth had been in Jeff’s body for so long that there was almost no chance of retrieving any of the shark’s DNA,” Naylor tells Inverse. “However, we all also thought that it would be fun to try!”

Lei Yang, Ph.D., lab manager at the FPSR and first author of the new study, worked with Naylor to clean the tooth fragment and scrape out its pulp, which somehow had not been degraded by Weakley’s immune system during its 24-year sojourn in his foot. Yang, who thought this case would be “a long shot,” tells Inverse that the success of their DNA technique on the sample was unexpected.

“As museum researchers, we specialize on working on difficult samples,” Yang says. “We were very pleasantly surprised to find intact DNA remained that could be suitable for making an identification.” After extracting and purifying DNA from the tooth, the team sequenced it, comparing the sequences against a database of genetic information on sharks and rays.

Usually, says Naylor, the team uses this technique to estimate evolutionary relationships among different species of sharks, understand their population structure, and reconstruct their historical demography — “in much the same way as is done by companies like 23 & Me for humans.” In a 2015 PNAS study, they applied it to archival river shark specimens over a century old. Unlike the tooth, those samples had been carefully preserved by a museum, not inside a surfer’s foot.

The team’s success, says Lei, “made us realize that the techniques we are using may be more powerful and generally applicable than we originally thought.”

In the database, the tooth DNA had a clear match: Carcharhinus limbatus, better known as the blacktip shark. At first, the team was skeptical of the result, fearing they had accidentally contaminated the sample. But the DNA they sequenced was consistent with DNA from other degraded tissues and didn’t carry a signature that suggested contamination. “Also,” Naylor points out, “the sequence perfectly matched that of the most abundant species off Flagler county where Jeff had been bitten 24 years earlier.”

After 24 years, Weakley’s suspicion was confirmed. “I long suspected it had been a blacktip shark bite, and it was satisfying to learn the truth,” he says.

DNA is becoming an increasingly useful way to identify the sharks behind attacks on humans, pointing to specific populations that must be monitored. As for shark attacks themselves, they appear to be on the rise — but not because sharks seem to be any testier than usual.

A PLOS One study published in February showed, using the grisly data from the Florida Museum of Natural History’s International Shark Attack File, that shark attack rates have doubled over the past 20 years in highly populated areas. The authors concluded that the sharks were not becoming more hostile but that more visitors were flocking to beaches.

Still, beach goers should heed the preventative measures recommended by experts like Naylor. “Be sensible,” he urges. That means follow beach rules, don’t swim at dawn or dusk, and avoid swimming alone.

Another useful tip involves staying alert for weird activity among other sea animals: “Get out of the water if you see large schools of baitfish jumping in shallow water,” he says. “It is highly likely they are being chased by a predator like a jack or a shark and you wouldn’t want to get in the way of that interaction.”

All these years later, Weakley harbors no hard feelings toward the shark and continues to surf. The bulge in his foot is gone, its contents now a key addition to the annals of shark science.

“How do I feel? I’m continuing to marvel at the achievements of modern science,” says Weakley. “I feel happy to think perhaps I’ve made some contribution.”

Abstract:

Shark attacks are rare, but when they occur they generate substantial media, public, and other interest. The aftermath of an attack has the potential to influence public attitudes toward sharks and policy decisions surrounding shark attack mitigation and conservation. After an attack, the media often generate interest in determining the species of shark involved. This information can fulfill curiosity, complete medical and case records, and inform shark attack mitigation strategies. However, determining the identity of the species responsible is not an easy task.