Slowest Greenland shark hunts sleeping prey
Researchers have measured the swimming speed of the ocean's slowest shark.
Data-logging tags revealed that Greenland sharks "cruise" at 0.34m per second - less than 1mph.
The study showed that, even when the languid fish embarks on a burst of speed in order to hunt, it is far too slow to catch a swimming seal.
Since the species is known to eat seals, the scientists think it probably "sneaks up on them" as they sleep under the water.
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The Greenland shark was already known to be the world's slowest swimming shark, but its sluggishness surprised the scientists.
Yuuki Watanabe from the National Institute of Polar Research in Tokyo, who took part in the study, said that, when you account for the size of its body, it is the slowest fish in the ocean.
Previous research had revealed seal remains in the stomachs of the sharks.
"It was hard to understand," he told BBC Nature, "because [it would seem] impossible for them to catch fast-swimming seals."
The researcher joined Dr Kit Kovacs and Dr Christian Lydersen from the Norwegian Polar Institute, to tag Greenland sharks in the waters off Svarlbard.
The study, published in the Journal of Experimental Marine Biology and Ecology, was the latest part of an ongoing mission by the Norwegian researchers to find out what has been killing the harbour seals off Svarlbard's coast.
It was thought that Greenland sharks simply fed on the carcasses of dead seals on the seafloor, but the team recently discovered evidence that they were taking live seals.
The tagging study found that, while seals swim at about 1m per second (2mph/3km/h), the sharks' maximum bursts of speed reached only 0.7m per second - far too slow to catch a swimming seal.
"The [tags also] told us how fast the shark moves its tail," said Dr Watanabe.
It takes seven seconds for a single full tail sweep that propels the shark forwards.
The scientists also recorded the temperature of the water that the fish were swimming in, which were an icy average of 2C (36F).
The energy cost of regulating their body temperature in the almost freezing depths could be the reason for the sharks' very limited speed.
Biggest, fastest, fiercest
- The fastest shark is thought to be the shortfin mako. The species has been recorded in excess of 20mph and can chase down some faster bony fish, such as tuna
- The world's largest shark is the whale shark (pictured), which can grow to over 13m long. This huge filter-feeder shark sifts zooplankton from the water
- The smallest shark is the dwarf lanternshark. This deepwater species is found in the Caribbean Sea and reaches a maximum length of just 21cm
- The shark with the largest teeth relative to its size is the largetooth cookiecutter. At only 42cm in length, this species has 17-19 rows of large lower teeth and feeds by cutting circular pieces of flesh from larger marine mammals, fish and other sharks. It has even caused problems for submarines
Source: Shark Trust
These Arctic fish live further north than any other shark species.
In this frozen habitat, the researchers explained in their paper: "Arctic seals sleep in water to avoid predation by polar bears.
"This may leave them vulnerable to the cryptic slow-swimming predators."
Vincent Gallucci, a shark expert from the University of Washington, US, explained that Greenland sharks may not need "to get 100% of its mouth onto its prey" in order to eat it.
"It can get an assist from a sucking action as part of its feeding process," he told BBC Nature.
"This does make it a bit easier for a lie in wait ambush predator to consume prey that pass near its mouth."
In the future, the scientists who embarked on this study hope to use underwater cameras, in order to record what could be the world's slowest chase scene.
Ali Hood from the Shark Trust pointed out that, historically, Greenland sharks were targeted for oil and meat.
"It's a long-lived species considered highly vulnerable to fishing pressure," she told BBC Nature. "[We] welcome further research to illuminate the behaviour of this elusive species."