Tuesday 02 September, 2014

Slowest Greenland sharks `hunt sleeping seals`

Published On: Sat, Jun 23rd, 2012 | Marine Biology | By BioNews

Researchers have measured the speed of the ocean’s slowest shark – the Greenland sharks, which “cruise” at 0.34m per second that is less than 1mph and revealed that they hunt sleeping seals.

The study, with the help of data-logging tags, showed that even when the ocean’s slowest 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 believe that it probably “sneaks up on them” as they sleep under the water.

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,” the BBC quoted him as saying.

“Because [it would seem] impossible for them to catch fast-swimming seals,” he said.

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 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 initially thought that Greenland sharks simply fed on the carcasses of dead seals on the seafloor, but the team recently discovered evidence that they were consuming 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,” Dr Watanabe said.

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.

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,” the researchers wrote.

Vincent Gallucci, a shark expert from the University of Washington, US, explained that Greenland sharks may not need “to get 100 percent 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 said.

“This does make it a bit easier for a lie in wait ambush predator to consume prey that pass near its mouth,” he added.

The study was published in the Journal of Experimental Marine Biology and Ecology.

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