Tue

Sep

2018:45

The Arctic Ocean’s top predator is a geriatric shark

 
Greenland sharks can live at least 272 years, according to a groundbreaking new study from the University of Copenhagen, which has shed light on the ancient apex predators that lurk beneath the Arctic ice

They have been hunted for hundreds of years for their meat and oil, but to this day, precious little is known about the Greenland shark. A member of the family of ‘sleeper sharks’, it grows up to seven metres long and moves at a crawl through the frigid Arctic Ocean, where it is most prevalent.

The shark’s slow pace doesn’t keep it warm. Instead, its meat and skin contain a chemical anti-freeze, TMAO. In cultures that eat the Greenland shark, such as Iceland, the meat is prepared over many months to remove this toxic chemical. Even then, though it is considered a delicacy, the meat is still widely considered to taste disgusting.

Little light reaches the sea floor, where it normally hunts for carrion, so it doesn’t mind that its eyes are often infected with a parasite that renders it blind. Instead, it uses its other senses to find dead animals that have sunk to the sea floor, or to ambush sleeping seals.

Greenland shark fishing from Sanna close to Upernavik 2014. Photo: Julius Nielsen

Greenland shark fishing from Sanna close to Upernavik 2014. Photo: Julius Nielsen

Their age has been shrouded in mystery. Fifty years ago, Danish fishery biologist Paul Marinus Hansen found that they grow no more than one centimetre per year, and put their maximum age at around 200 years.

But they can get older – much older – than that. In fact, according to a new study from the University of Copenhagen, we now know for certain that Greenland sharks are the longest-lived vertebrates currently alive.

We can thank the shark’s useless eyes for helping researchers from the University of Copenhagen’s Department of Biology to put a more accurate figure on the shark’s age. According to the Royal Society of Chemistry, scientists normally determine the age of animals by counting growth rings in calcified tissues that form on various anatomical structures, including scales and bones. This is not possible in Greenland sharks, however, as they have no calcified tissue.

PhD student Julius Nielsen, however, found that the sharks’ eyes contain tissue that does not change from the moment the shark is born. That means that the eye tissue can be measured for its levels of a heavy form of carbon known as carbon-14. The less it has, the older the shark is.

Greenland shark in the MR-scanner at Department of Forensic Medicine, Rigshospitalet, Denmark. Photo: Julius Nielsen

Greenland shark in the MR-scanner at Department of Forensic Medicine, Rigshospitalet, Denmark. Photo: Julius Nielsen

According to his results, which were published in the journal Science, Nielsen found that after testing 28 different Greenland sharks, their maximum life span was at least 272 years. But since the measurement has a margin of error of about 120 years, the sharks might live to be over 400 years of age.

This means that there are sharks alive today that roamed the oceans when Danish-Norwegian explorer Otto Fabricius first set out to make detailed observations of Arctic animal life in Greenland in the mid-18th century.

“Greenland sharks are among the largest carnivorous sharks on the planet, and their role as an apex predator in the Arctic ecosystem is totally overlooked,” Nielsen said.

“Thousands of them accidentally end up as by-catch across the North Atlantic, and I hope that our studies help to bring a greater focus on the Greenland shark in the future.”

Two Greenland shark entangled in the longline. Amasalik 2012. Photo: Henrik Schurmann.

Two Greenland shark entangled in the longline. Amasalik 2012. Photo: Henrik Schurmann.

The study also found that the sharks first reach sexual maturity at around age 150, meaning that they are vulnerable to overfishing. While project leader Professor John Fleng Steffensen wouldn’t speculate on the precise size of the current population, he speculates that it must be healthy.

“The Royal Greenland trade in shark oil gives an indication of the amount of sharks that used to be caught and sold. There used to be a significant trade. Logs show that the number of barrels of shark-oil traded annually from 1890 until World War II is equivalent to about 30,000 sharks per year”.

In 1963, the trade in shark oil ceased in Copenhagen, and there is currently almost no market for it.

“That’s a piece of good news for the Greenland shark,” Steffensen said. M

Tech

By Stubbe Wissing

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