Researchers shocked after 8-foot shark is eaten by a predator. But who's the culprit?

For the first time ever, researchers have found evidence of a top-of-the-food chain shark being killed by a predator, according to a study published Tuesday in the peer-reviewed journal Frontiers in Marine Science.

The team of scientists who wrote the study had set out to research pregnant porbeagle sharks and track their movements because they're endangered in many parts of the world. The scientists come from Oregon State University, Arizona State University and the Rhode Island-based Atlantic Shark Institute.

One of the sharks studied measured 8 feet and had been tracked for five months, said James Sulikowski, director of the Coastal Oregon Marine Experiment Station at Oregon State University.

What the researchers found is that the large, warm-blooded shark had fallen prey to another warm-blooded predator − likely another shark. 

Porbeagle shark was eaten by a warm-blooded predator, team says

Sulikowski said the team put two different tags on their subject sharks.

The first tag, called a finmount tag, is located on the shark’s fin and gives scientists “a very accurate geolocation when the fin comes out of the water,” he told USA TODAY on Tuesday.

The second tag, called a pop-off satellite archival tag, notes the temperature, as well as the shark’s depth in the ocean.

“That's how we knew the shark was actually eaten or attacked,” Sulikowski said, referring to the archival tag.

A second shark, also a porbeagle, was killed nearby a year after the first shark and sank to the bottom of the ocean before its tag surfaced, the scientists said.

Shark had been tracked hundreds of miles

The shark that was killed had been tracked for hundreds of miles as it made its way from New England to Bermuda, scientists say.

The shark had been spending time at depths between 1,640 feet and 3,280 feet. Because the shark had been swimming so far away from the sun, its temperature readings were much cooler. Suddenly, while still deep in the ocean, one of the shark’s tag readings went from 15 degrees Celsius to 25 degrees Celsius.

“We knew that something happened,” Sulikowski said. “We knew that the tag was inside of a warm-blooded creature … And we knew that it wasn't a whale or mammal, because mammals are much warmer than that.”

The predator that ate the porbeagle shark was most likely another lamnid shark, Sulikowski said, adding that the “iconic three” lamnid sharks are porbeagles themselves, white sharks and mako sharks. 

Their body temperatures are typically between 25 and 27 degrees, he said.

“My guess is probably a mako or a white shark because they do get larger than a porbeagle,” he said. 

Second shark sank to the bottom of the ocean

There was also another shark the researchers collected data from that they found interesting. A porbeagle shark was swimming around a depth of about 1,968 feet when it suddenly sank closer to the bottom of the ocean, Sulikowski said.

The team thinks something killed the shark without eating it or its tags. After the shark sank, its tag − set to pop off when the sharks are stagnant for an extended period of time − resurfaced after about three days.

“Both sharks were attacked at about the same depth, at about the same location, a year apart from one another,”  Sulikowski told USA TODAY.

What does this mean for porbeagle sharks and science?

Sulikowski said that with sharks as big and as fast as porbeagles, the only animals that prey on them are other sharks that are bigger than them. 

Researchers expect a smaller shark to fall prey to bigger sharks, but one measuring 8 feet was pretty unexpected, he said. 

The fact that this happened is a testament to how little is known about the ocean, he said.

“It makes us want to study more and learn more about how susceptible other large sharks are to be eaten and who is the top dog out there."

Saleen Martin is a reporter on USA TODAY's NOW team. She is from Norfolk, Virginia – the 757. Follow her on Twitter at@SaleenMartin or email her atsdmartin@usatoday.com.