When the stomach contents of Greenland sharks have been analyzed in the past, scientists have found a lot of fish, but also fresh seals that weren't just scavenged.
"How Greenland sharks hunt seals remains a mystery," Watanabe said. "We hypothesize that sharks hunt seals sleeping in the water because of two reasons. First, seals in captivity can sleep in the water -- at the surface or on the bottom."
"Second," he added, "although how seals sleep in the wild is not known, we in the field observed a seal floating at the surface. At first we thought it was a dead seal, and approached it by boat and actually touched it. Then the seal suddenly started moving and dove down."
There's a good chance, then, that the slow-moving Greenland sharks are simply able to approach a snoozing seal and enjoy a relatively easy dinner.
As for the Greenland shark's own predators, they don't have many because of their very large body size. Some individuals can grow to become 21 feet long, weighing 2,200 pounds. Killer whales might be able to eat them, Watanabe suspects, but Greenland sharks usually stay within deep water that killer whales do not frequent.