Sleeper Sharks Slurp Snoozing Seals

Greenland sharks eat seals. On the face of it, that doesn’t sound surprising. These huge Arctic sharks swim the same waters as the blubbery mammals, after all. But sleeper sharks aren’t quite like the graceful and quick archetypal sharks that fascinate and terrify us. The Greenland shark (Somniosus microcephalus) is squat, cigar-shaped fish that grows up to 21 feet long and looks like a pumped-up dogfish. When it moves, the great fish looks like it’s swimming in slow-motion. Yet, somehow, this painfully sluggish carnivore manages to catch much faster seals. Scavenging doesn’t explain the condition of the fresh seal remains found in dissected shark stomachs, nor shark-bitten seals that have survived attack. Exactly how the shark manages to catch seals has been a long-running mystery.

Marine biologist Yuuki Watanabe and colleagues offer a possible solution to the puzzle in a pre-print Journal of Experimental Marine Biology and Ecology paper. The point of the study was to see if Greenland sharks have some special adaptation that allows them to pick up the pace when pursuing seals. Like many other fish, Greenland sharks are ectotherms. This means their body temperature is regulated by the temperature of their environment, and their body temperature affects how quickly the fish can move. In icy seas, the low temperatures makes for sluggish fish, and mammals – which remain highly active thanks to their physiological capacity to run hot all the time – take advantage of these conditions to hunt small fish. Since the big sleeper sharks are apparently able to catch the faster mammals, Watanabe and collaborators collected data on speed and tail-beat frequency in free-swimming Greenland sharks to see whether the predators are always so painfully slow.

The sleeper sharks didn’t hold any speedy secrets. They are the slowest fish of their size. Greenland sharks cruise at about a mile an hour, and top out at about 1.6 miles per hour. They move so slowly that it can take about seven seconds for one full beat of their stubby tails. In a flat-out race, the faster and more agile seals are going to win every time.

But speed isn’t everything. Stealth is another shark stock-in-trade, and, as Watanabe  and colleagues suggest, the sleeper sharks probably sneak up on snoozing seals. For the sharks, the availability of such easy prey is a fortuitous circumstance that they owe to another predator.

Greenland sharks consume ringed seals, harbor seals, hooded seals, and bearded seals, all of which are also prey for polar bears. To avoid being caught by the bears, the seals sleep in the sea, and this leaves them vulnerable to shark attack. Indeed, unlike whales and sea lions, the seals are not capable of resting one half of their brain while the other half remains alert. When a harbor seal sleeps, it’s out and oblivious to the presence of Greenland sharks. And a sleeping seal makes an easy, energy-rich meal for the horrifying sandman selachian. JAWS, it is not, but the thought of waking up in a Greenland shark’s maw is just as frightening as the scenarios Spielberg envisioned so many summers ago.

Reference:

Watanabea, Y., Lydersen, C., Fisk, A., Kovacs, K. (2012). The slowest fish: Swim speed and tail-beat frequency of Greenland sharks Journal of Experimental Marine Biology and Ecology, 426-427 DOI: 10.1016/j.jembe.2012.04.021