Scientists Solve a ‘Murder Mystery’ After a Pregnant, Tagged Shark Got Eaten

The food web is one of the first concepts students learn in ecology. Animals higher up on the web prey on the ones below. At the very top are the apex predators—nothing is supposed to eat them. The porbeagle shark is one of those top hunters, but a new study published Monday in the journal Frontiers in Marine Science finds that one such shark might have unexpectedly become another creature’s meal.

“It really was like a shark murder mystery,” Brooke Anderson, a marine biologist for the state of North Carolina, tells Science News’ Jason Bittel.

The porbeagle had been tagged by researchers in October 2020 in the northwestern Atlantic Ocean as part of an effort to track shark migration. Since the animal was a pregnant female, scientists hoped she would reveal important habitats for porbeagle mothers and their offspring, which could inform conservation efforts for the endangered species. But 158 days after tagging and releasing the shark, the scientists got some surprise data.

Researchers had affixed two transmitters to the porbeagle—one that would send a signal anytime the shark surfaced and another that recorded data continuously until it was set to pop off, float to the surface and send its data to a waiting satellite. Unexpectedly, the pop-off tracker came online in late March 2021—far earlier than the scientists had planned.

Large sharks may be hunting each other - and scientists know because of a swallowed tracking tag

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The data showed that the shark had spent about five months living a fairly consistent routine. During the day, she cruised around 2,000 to 2,600 feet below the surface. At night, she swam up to water depths of 330 to 650 feet. During that time, the water temperatures she swam in ranged from 43.5 to 74.3 degrees Fahrenheit.

Then, four days before the tag surfaced and sent in data, that pattern was disrupted. The water depth ranged over those days from about 500 to 2,000 feet, but the temperature recorded stayed constant at 71.6 degrees. That consistency didn’t track with what researchers knew about the water column—that it gets colder as you go deeper down.

“As soon as I got the data from that tag, I immediately knew something weird had happened,” Anderson tells CBS News’ Mary Cunningham. “The only explanation for that data is that this tab is now in the stomach of a predator.”

But, the researchers wondered, is it really possible that an apex predator could get eaten? Porbeagles would make unwieldy prey, since they grow large and can weigh up to 500 pounds. Scientists had never previously recorded a porbeagle shark getting eaten by another animal.

“I honestly kept trying to come up with alternative explanations,” Anderson says to Science’s Phie Jacobs. “I couldn’t believe that an 8-foot-long porbeagle shark—a pregnant porbeagle shark, nonetheless—would have gotten eaten by something.”

To find the culprit, the team first ruled out that the shark had been eaten by an orca, which are known to sometimes hunt other predators. For one thing, orcas are warm-blooded marine mammals, so the temperature the tag recorded is too cold for being inside a “killer whale.”

The most likely candidate is a larger shark, probably a great white shark or shortfin mako. A mature female white shark is a promising candidate, per the paper, since the water depths recorded match those previously documented for the species.

“In the open ocean, size matters,” Jon Dodd, a marine biologist at the Atlantic Shark Institute and co-author of the study, tells CBS News. “But there is always something bigger.”

Some other scientists remain skeptical that the data shows clear evidence of shark-on-shark predation. “There’s no question that something ate the tag,” Megan Winton, a fisheries scientist at the Atlantic White Shark Conservancy who was not involved in the research, says to Science News. “But it’s hard to tie that necessarily to the mortality of the porbeagle.”

Winton adds that it’s possible a predator somehow ate the tag, but not the porbeagle itself—or that another porbeagle ate its unfortunate relative.

If something really did successfully hunt the porbeagle, as Anderson tells New Scientist’s Madeleine Cuff, that predator probably had a really good day. “In this scenario, if you can pull it off, a large pregnant porbeagle shark would be a lot of bang for your buck in terms of a meal.”

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Rudy Molinek is Smithsonian magazine's 2024 AAAS Mass Media Fellow.