Scientists just named the longest-living species of vertebrate animal. It's a creature that can live up to 400 years old, almost twice the lifespan of the oldest giant tortoises or bowhead whales, nature's next runners-up. These lords of longevity are the Greenland sharks, Somniosus microcephalus.

"Every time you think science has figured something out, you find something like this."

Although the Greenland sharks are not a newfound species, their absurdly long lifespan has only now been discovered. Julius Nielsen, a marine biologist at the University of Copenhagen in Denmark, and a team of fellow researchers have found that the mysterious sharks don't even reach maturity until 150 years. The slow-swimming sharks are endemic to the cold polar waters of the north Atlantic and Arctic oceans and can grow up to 17 feet in length. The scientists describe how they discovered the Greenland sharks' lifespan today in the journal Science.

"We were blown away by the lifespan of these sharks," says Peter Bushnell, a biologist with the team at Indiana University, South Bend. "Frankly, we sort of looked at this at first and asked ourselves, OK, so where did we go wrong?"

A Greenland shark near the surface after its release from research vessel Sanna in northern Greenland. Julius Nielsen

In Their Eyes

While relatively little is known about the elusive Greenland shark, Nielsen demystified their long lifespans by performing a fascinating experiment with 28 dead sharks—each accidentally netted by Scandinavian fishermen as bycatch. Using radiocarbon dating, Nielsen's team peered into sharks' eyes to find their age.

Radiocarbon dating is a clever way of finding the age of something by measuring the unique levels of elemental carbon in a material. You're probably heard about it in reference to dating old artifacts or bones. Here's how the process works: Certain isotopes of carbon over time will naturally and spontaneously decay. Because scientists know how frequently this decay happens, they can tell how old something is based on its ratio of carbon atoms. The older it is, the more heavy isotopes have transformed away.

So why the lenses of the eyes? Unlike like cartilage, teeth, scales, or various other tissues, the center of the sharks' eye lens doesn't keep growing as the creature ages. That would give the scientists a false reading. For example, you may be in your 20s, 40s or 60s, but your fingernails and skin are mere weeks old, which wouldn't tell future scientists much about your age. These shark eye lenses are formed of an inert crystal-like protein, which is first formed while the shark is still in its egg.

"One traditional way to age fish is to look at the cartilage and bone, which is grown in a regular pattern, and basically read the tissue like you're counting the rings on a tree trunk," Bushnell says. But that method just wouldn't work for these slow-growing, long-living fish because of peculiarities in how the sharks grow cartilage. But the eyes have it.

"It's actually pathetic how little we know about these creatures."

Of the 28 fish the team dated, the oldest is estimated to be around 400 years old—possibly even up to 500 years old. (The radiocarbon dating process isn't exact, but gives a fuzzy age range.) With the exception of the smallest two, all the sharks were older than 60. There's another, strange reason scientists can be so sure of this: Cold War-era nuclear testing radically increased the number of heavy carbon isotopes the world over thanks to radioactive fallout spreading throughout the atmosphere during that time, creating another scientific way to confirm dates.

These shark's ultra-long lifespan makes sense, says Bushnell, when you consider how slowly the creatures grow and how big they can end up. "We understand that on average they grow about a half an inch each year, and we've found individuals that are over 17 feet," he says.

A Greenland shark slowly swimming away from the zodiac, returning to the deep and cold waters of the Uummannaq Fjord in northwestern Greenland. The sharks were part of a tag-and-release program in Norway and Greenland. Julius Nielsen

Grow Cold, Grow Slow

So how the heck are these fish living such biblical lifespans?

"That's one question we really can't say we know the answer to," Bushnell says. Because the fish live their lives in the polar waters, never (or rarely) surfacing, little is known about them. "We don't know how big the babies are, where they're born, we've never seen pregnant females, so there's a lot of questions we have about that... I mean, it's actually pathetic how little we know about these creatures."

But Bushnell and his colleagues have inklings as to what's driving the Greenland shark's astonishing longevity, and they all go back to the creature's chilly home and creeping metabolism. "There are two things I think are sort of working in the animal's favor. First they're living in water 37 to 41 degrees Fahrenheit," he says. "That temperature has a profound sluggish effect on the chemical reactions and metabolism of the sharks. And secondly, there's also a sort of scaling effect, where the larger an animal is, the lower the metabolic rate progresses. These sharks are getting up to 17 feet."

"This type of discovery, it really makes you think," Bushnell says, "every time you think science has figured something out, you find something like this, and learn you've figured nothing out."

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