Shark biologists stumble into a deep-sea murder mystery

When biologists in Massachusetts placed tracking tags on porbeagle sharks, they hoped to learn more about the massive hunters’ habitats.

What they got was a murder mystery. The victim was a pregnant shark nicknamed Penelope.

And the murderer, to their surprise, might have been another type of shark – even though porbeagles don’t have any known predators.

After catching Penelope and nearly a dozen other porbeagles near Cape Cod, Massachusetts, in October 2020, marine biologists recorded their length and used an ultrasound to check if the females were pregnant. They then attached a tracker to each animal to follow its location, and a tag to collect information on the depth and temperature of the water it swam through.

Battery-powered straps on the depth and temperature tags were programmed to automatically disconnect after one year, allowing the sensors to float to the surface and transmit their data to waiting satellites. But the tags were also set to automatically disconnect if they recorded the same water pressure for three days, which can be a sign that a shark has died and sunk to the ocean floor.

If a shark were to die in this case, the researchers figured it wouldn’t be by cannibalism. Porbeagles, which can grow roughly 12 feet long and weigh more than 500 pounds, were believed to be apex predators who could live more than 40 years. The International Union for Conservation of Nature lists fishing and harvesting as porbeagles’ biggest threats.

After measuring Penelope to be more than 7 feet long and attaching her tags, the researchers released her into the Atlantic Ocean. They hoped Penelope’s tag data would shed light on where porbeagle mothers – who carry four babies at a time, on average – settled down with their newborns.

The researchers followed the sharks’ journeys using the location trackers, which transmit data when the sharks rise above the sea surface. Penelope was spotted at Georges Bank, an elevated area of seafloor between Cape Cod and Nova Scotia, in November 2020.

Then in April 2021, Penelope’s depth and temperature tag surfaced southwest of Bermuda. Brooke Anderson, the lead researcher, said she thought Penelope’s tag had fallen off, but her hypothesis changed when she examined the data it had collected.

“I immediately knew something weird had happened that we had never seen before,” Anderson told the Washington Post.

Penelope had been swimming for about five months at depths between 1,900 and 2,700 feet during the day, Anderson said, and depths between 300 and 700 feet at night. She swam in waters between 43 and 75 degrees.

But the tag data showed that in late March 2021, the temperatures suddenly stopped changing. For about four days, the water surrounding Penelope remained around 72 degrees – even when the tag appeared to move deeper in the ocean toward cold water.

All the data pointed to Penelope being eaten and her tag later being excreted, Anderson said. The temperature recorded those final four days was similar to the gut temperatures of other large fish, Anderson said. She studied large marine mammals that preside near Bermuda, she said, and put on her “murder mystery” hat.“There were only a handful of suspects,” Anderson said.

Some suspects, including toothed whales, were ruled out because their internal temperatures are too high – closer to 100 degrees, Anderson said. Researchers narrowed the suspects to a shortfin mako shark – a species that typically eats small sharks, swordfish, tuna and sea turtles - and a white shark, a predator that often feasts on small sharks, whales, rays, seals and sea lions.

While large sharks often eat small sharks, Anderson said, big sharks rarely prey on other big sharks.

“I couldn’t believe it,” said Anderson, who works for the North Carolina Division of Marine Fisheries. “I was trying to think of alternative explanations, because in my head, there was no way that this 8-foot hefty mama had been eaten.”

Sora Kim, a life and environmental sciences associate professor at the University of California at Merced who was not involved with the study, said discovering proof of sharks’ predation habits is difficult since they live deep underwater.

“I’m sure that it happens in nature all the time,” Kim said of large sharks preying on porbeagles. “But that we as humans actually have evidence of it and that it’s documented, I think, is what’s so cool.”