Being sluggish usually isn’t the best hunting strategy, and the Greenland shark (Somniosus microcephalus) with an average cruising speed of 1.2 kilometers per hour (.76 miles per hour) seems an unlikely threat to agile Arctic seals.
Yet Greenland sharks, also called sleeper sharks, are known to consume ringed seals (Pusa hispida), harbor seals (Phoca vitulina), hooded seals (Cystophora cristata), and bearded seals (Erignathus barbatus). Marine biologists originally thought the sharks fed on the carcasses of dead seals, but fresh seal chunks found in sharks’ digestive system as well as bite wounds on living seals showed that the sharks were eating live seals.
“Arctic seals sleep in water to avoid predation by polar bears (Ursus maritimus), which may leave them vulnerable to this cryptic slow-swimming predator,” wrote the authors of a recent study on Greenland sharks’ speed, published in the Journal of Experimental Marine Biology and Ecology.
The marine biologists tagged six Greenland sharks with instruments to measure their speed and the environment around them. The sharks could hit a maximum burst of only 2.7 kilometers per hour (1.7 mph). That makes them the slowest shark relative to its body size.
The cold-blooded sleeper sharks may be super slow because the water around them averages only about 2 degrees Celsius (36 degrees F).
A 30-foot sleeper shark submerges a one-meter bait cage into the ocean floor mud. (Ralph White, Corbis)
Marine biologists tag a Greenland shark. (Courtesy Yuuki Watanabe)