What Killed and Ate a Very Large Shark? An Even Larger Shark, Scientists Say

When marine biologist James A. Sulikowski began observing the migration and mating habits of porbeagle sharks, he didn't expect to end up investigating a mysterious murder.

But when Penelope, a 2.4-metre-long porbeagle shark, disappeared from his research team's sight, he and his colleagues eventually concluded that she had suffered the grisly fate of becoming someone's lunch.

Their prime suspect is another, even larger shark.

“Penelope was a big shark, and the idea that she was eaten by something big is a little scary,” said Sulikowski, director of Oregon State University's Large Fish Lab. How does it happen? Hosted by Neil Koksal.

“The fact that this happened… just stuns us.”

Sulikovsky and his colleagues published a study in the journal Frontiers in Marine Science Their detailed account is the first evidence that porbeagles have natural predators, and a rare example of one large adult shark preying on another.

Scientists not involved in the study say the findings are plausible, but more research is needed to say for sure that Penelope was the victim of a shark bite.

“Something really weird happened.”

According to Sulikowski, porbeagles are fast sharks that can regulate their own body temperature, allowing them to survive in both cold and temperate waters. They can grow up to 3.5 meters in length and until now were thought to have no natural predators.

However, they are hunted by humans for their meat, and are sometimes caught as bycatch in tuna and swordfish longline fisheries. listed as vulnerable worldwide by the International Union for Conservation of Nature.

That's why Sulikowski and his colleagues capture, tag and track porbeagles, especially pregnant females, to better understand their mating and migration patterns in the name of conservation planning.

They tagged and released Penelope off Cape Cod in 2020. The satellite tags, which measure depth and temperature, are designed to last a year, Sulikowski said. But Penelope's tags fell off after just five months.

“They don't usually throw them out without warning,” he said. “When we started looking at the data, it was like, 'Wow, something really weird happened.'

The data showed Penelope was floating 600 metres below the surface when the temperature suddenly jumped from 15 to 27 degrees Celsius. She stayed that way for several days, “cycling up and down in the water,” he said.

That means the device was likely “in the belly of a predator,” Sulikowski said.

This predator, he said, must be larger than Penelope. Based on the depth and location of the tag, there are only two reasonable suspects: a great white or a shortfin mako shark.

“Another Murder”

Shark biologist Alison Towner, who was not involved in the study, told CBC the findings were “not just intriguing, but also entirely plausible.”

“White sharks have a varied diet, and while satellite data often shows patterns such as oscillatory diving in open ocean conditions, it is not always clear what they are hunting,” Towner, a research fellow at the International Marine Institute of South Africa, wrote in an email.

“In this case, the observed temperature increase is well consistent with what we would expect from a great white shark attack.”

Not everyone is convinced. Megan Winton of the Atlantic White Shark Conservancy in Chatham, Massachusetts, told Science News It's possible that the tag was eaten by another shark, but not Penelope.

Sulikowski says that's unlikely.

“When a shark goes to eat, it doesn't take a bite. It takes a big bite,” he said. “The tag it was carrying was in the middle of its body. So this event probably either tore it in half or tore a chunk out of its body.”

Moreover, he said that “another murder” took place in the same place, a year after Penelope's murder.

In this case, he said, another porbeagle they were tracking suddenly dove to the ocean floor, indicating it had died, and what was left of its body sank – along with the tag.

“If you put those two facts together, you'd probably come to the conclusion that something ate both sharks,” he said.

Sharks eat sharks, not people.

If the authors are right, this is not the first example of sharks attacking each other, said Toby S. Daley-Engel, director of the Florida Shark Conservation Lab.

Large sharks have been known to prey on smaller shark species or even smaller members of their own species, she said.

Still, Daley-Engel, who was not involved in the study, said: “It's a cool study.”

“The most unusual thing about this is that the shark that was eaten was an adult, so it must have been eaten by something big,” she told CBC.

Asked whether the study's horrific findings paint a negative picture of the very sharks he's trying to protect, Sulikowski says it's actually good news for those who fear being swallowed at sea.

“Sharks are shark food, right? We humans are not on the sharks' menu. If we were, there would be a lot more people disappearing into the ocean, that's for sure,” he said.

“We're much slower than the herring sharks and we like to splash around… so we'd be easy prey.”