Greenland sharks live in by far the coldest waters of any shark species. That's bad news for a cold-blooded species, and the best way to conserve their energy is to move as little and as slowly as possible.
That's why it's so strange that scientists have found seal remains inside the stomachs of these Greenland sharks. These sharks have an average speed of about a mile per hour, and if they are really gunning it, they can hit a top speed of about 1.6 miles per hour. Everything about these sharks is in slow-motion - it even takes seven whole seconds for these creatures to complete a single tail sweep that propels it forward. By comparison, the world's fastest known shark, the shortfin mako, can cut through the water at speeds over twenty miles per hour.
The neighboring seals, on the other hand, average about 2 miles per hour - still not particularly fast, but easily enough to outpace even the fastest Greenland shark alive. And yet the sharks do still dine on these seals, and analysis by the Norwegian Polar Institute indicates the seals were eaten alive, so this isn't a case of sharks turning to scavenging. No, what we're looking at here is something pretty much right in the middle between hunting and scavenging.
The sharks simply wait until the seals fall asleep, and then they go to work as slowly as they like. Even then, as University of Washington shark expert Vincent Galluci explained to BBC News, the sharks behave in a manner that's...well, I don't exactly want to say lazy, but:
"[It doesn't need] to get 100% of its mouth onto its prey. It can get an assist from a sucking action as part of its feeding process. This does make it a bit easier for a lie in wait ambush predator to consume prey that pass near its mouth."
Ironically, these seals take the slightly unusual step of sleeping in the water precisely because they are trying to avoid being eaten by a predator. The Greenland shark and these seals live in the waters right off of polar bear country, and so the seals avoid sleeping on land so that they don't have to deal with the ursine threat. While a good idea in theory, it leaves them vulnerable to being eaten quite possibly the least intimidating predators on the planet.
For more, check out BBC News. Original paper at the Journal of Experimental Marine Biology and Ecology. Image via Wikimedia.