Albeit slow and sluggish, the Greenland shark (Somniosus microcephalus) is currently the world's longest-living vertebrate, a new study suggests.

In fact, these very characteristics contribute to the marine animal's longevity, which can stretch to at least four centuries, researchers say.

The new report, which is published in the journal Science, details how scientists calculated and measured the Greenland shark's age for the first time.

Determining The Age Of Sharks

Greenland sharks move through the frigid waters of the North Atlantic and the Arctic at such a sluggish pace that they've earned the moniker "sleeper sharks."

But aside from their underwater languor, these sharks also grow at a slow rate — at about 1 centimeter or four-tenths of an inch every year.

This has led some scientists to believe that the animal can live for a long time. However, they've had no idea how long that might be, until now.

Julius Nielsen, a Danish marine biologist from the University of Copenhagen and a co-author of the study, says determining the age of any shark is not easy.

What's more, doing so on the Greenland shark is particularly challenging, he says.

In order to track the age of sharks, most scientists study the animal's bony structures, but there aren't that many.

Some shark species have calcified fin spines or vertebrae that contain stripes used to assess the age of a shark — similar to how scientists count tree rings to determine growth.

Unfortunately, Nielsen says Greenland sharks are "soft sharks" that do not possess those many bone structures. He says the team had to take something new into consideration to solve the mystery.

The solution lies upon a technique that has been used to find the age of certain whale species: by peering into the lenses of the eyes.

Nielsen says the transparent tissue in the shark's eye lens is not active in terms of metabolism, with new layers being added throughout the lifetime of the animal.

Reaching Maturity

Through radiocarbon dating, scientists measured the carbon isotope absorbed by the tissue. They were able to estimate the age of several sharks that were captured in the North Atlantic as bycatch.

Two of the largest Greenland sharks they caught were at least as big as 4.9 meters (16.2 feet) and 5 meters (16.5 feet), and were about 335 and 392 years old, researchers said.

Female sharks are known to typically reach sexual maturity when they measured approximately 4 meters (13.1 feet) in length, but based on the research, scientists calculated that Greenland sharks become sexually mature at 156 years old.

When researchers analyzed the lens tissue from all the sharks, they found that the Greenland shark's lifespan ranges from 272 to 512 years, with an average longevity of 390 years.

Nielsen says there is still uncertainty behind this estimate. However, it's still an incredible feat for an animal because the lowest age range still makes Greenland sharks quite old.

The Greenland sharks have now replaced the bowhead whale, which had a lifespan of about 200 years, as the longest-living vertebrates.

Nielsen says the study should bring the shark "much-deserved respect."

"This species is completely overlooked," says Nielsen. "Only a few scientists in the world are working with this species."