In 1620, the Mayflower set off from Plymouth, carrying hopeful pilgrims to the New World. As it sailed over the Atlantic, it passed over deep, cold waters, where baby Greenland sharks were starting out their lives. Those youngsters slowly grew into giants. And if a new study is right, some of them are still alive today.
The Greenland shark is similar in size to a great white but the points on its body are rounded, giving it a much less fearsome countenance. It’s sluggish too, cruising at a typical speed of 0.7 miles per hour—a pace that has earned it the nickname of “sleeper shark”. Its skin looks like a charcoal etching, and its eyes usually have parasitic crustaceans hanging from them. Its stomach can contain the scavenged remains of everything from fish to moose to polar bears, but no one has ever seen one hunt. Indeed, it’s an enigmatic and rarely seen animal, which prefers to stick to the almost sub-zero waters of the deep North Atlantic.
In 1936, a Danish researcher managed to measure and tag one of these reclusive beasts. When he recaptured it in 1952, it had grown by just 8 centimeters—a rate of half a centimeter a year. If these sharks grow steadily, it would take centuries for one to reach its maximum 7-meter length. That estimate, however, has been hard to verify. You can often work out how old a shark is by counting the growth rings in its vertebrae, as you would the rings in a tree trunk. But the Greenland shark “is a soft shark,” says Julius Nielsen from the University of Copenhagen. Its vertebrae are like candles—too soft to hold growth rings.
Instead, Nielsen and his colleagues used a creative method, based on the fallout from the nuclear bomb tests of the 1960s, to carbon-date the eyes of recently caught Greenland sharks. And they estimated that these creatures have a maximum lifespan of anywhere from 272 to 512 years, with a best guess of 392.
Even the lowest of those figures would make the Greenland shark the world’s longest-lived vertebrate, beating the previous champion—the bowhead whale, with an estimated lifespan of 211 years—by a significant margin. A 272-year-old shark would have been almost 170 when the Titanic hit the iceberg, and almost 90 when Charles Darwin set sail in the Beagle. It would have swum through centuries, cruising through icy waters while empires rose and fell around it.
Sadly, these ancient giants are sometimes snagged by fishing boats and research vessels. That’s how Nielsen first saw one. He was a graduate student on a research boat that accidentally hauled in a Greenland shark, while measuring cod stocks. “Everyone was up on the deck pushing it back into the ocean,” Nielsen recalls. “It was an amazing experience. When I got back, I thought: What was that animal? And hardly anything about its biology was known.”
A few months later, he was listening to a lecture on Greenland sharks. The speaker, John Steffensen, suggested that it might be possible to work out how old these animals get by studying the lenses of their eyes. These structures are made of proteins that are added in layers throughout the shark’s life. Peel away the layers and you can eventually find molecules that were laid down at the animal’s birth. The only problem with this idea was that Steffenson didn’t have enough lenses. “I raised my hand and said that I was on a ship that had caught and released a Greenland shark,” says Nielsen. “He said we should talk.”
Hunters in Greenland and Iceland used to catch some 50,000 Greenland sharks a year, and although fishing is uncommon, many are still caught as by-catch. Most are released without harm, but some suffer fatal injuries. And although the deaths of such old animals are tragic, Nielsen was determined that they shouldn’t be wasteful. So he and his team collected lenses from 28 of them.
For each lens, they measured the amounts of carbon-14—a mildly radioactive form of carbon. They then compared these measurements against a graph showing how carbon-14 levels have varied in the oceans over the past millennia. Such graphs are commonly used to carbon-date specimens on land, but the oceans complicate matters; there, carbon-14 levels haven’t changed much in the past centuries, and can vary from place to place depending on currents. To deal these uncertainties, Nielsen’s team had to learn to love the bomb.
Between 1955 and 1963, the world’s superpowers started testing their nuclear arsenal. The exploding bombs created huge amounts of carbon-14, doubling the usual levels in the atmosphere. As the carbon-14 spread around the world, it also worked its way into the food web. Animals, including marine ones, incorporated the stuff into whatever tissues or body parts they built at the time, effectively date-stamping themselves. Thanks to the brief era of nuclear testing, scientists have been able to accurately carbon-date everything from trees to elephant ivory to human brain cells. And now: Greenland shark eye lenses.
One of the lenses was pivotal. The third smallest of the 28 animals, it belonged to a shark that must have been born just around 50 years ago, just at the end of the bomb-testing era. That precise estimate allowed the team to ground-truth the others. Christopher Ramsey from the University of Oxford, an expert at carbon-dating, did this by creating a mathematical model that accounted for not just the 50-year-old’s age, but also how fast Greenland sharks grow, their size at birth, where they live, and other known factors.
The results? “We can say with 95 percent certainty that they’re between 272 and 512 years old,” says Nielsen. “But it’s most likely that it’s approximately 390.” With ages that big, there will always be uncertainty. But whichever of those numbers you pick, the Greenland shark is astonishingly long-lived. Among animals, it’s second only to the ocean quahog—an edible clam known to live to 507 years.
Chris Harvey-Clark from the University of British Columbia, who has probably clocked more time swimming with Greenland sharks than anyone else, says that the results are convincing, but will undoubtedly stir controversy. “Not a lot of people have published not a lot on radiocarbon dating for fishes,” he says. “The same thing happened with the bowhead whale: [the age estimates] were greeted with everything from hoots of delight to derision, but they stood the test of time.”
How do the sharks get so old? No one knows. “Part of the explanation, of course, is that it has a very big body—and a cold one,” says Nielsen. “They’re the same temperature as their surroundings and their preferred swimming temperature is -1 to 5 degrees Celsius. They have a slow metabolism. Maybe they have anti-ageing mechanisms, too, but I don’t know about that. I’m just a Greenland shark nerd.”
You can see why. There’s so much about them that we don’t know. “No one has ever seen a Greenland shark capture live prey,” says Nielsen. “And yet sometimes I’ve found entire seals inside their stomachs, and fast-swimming fish like Atlantic cod and halibut.” He thinks these remains were unlikely to have been scavenged, and yet it’s hard to imagine how an animal that’s supposedly so lethargic could taken down a speedy, agile seal. “I think they must be ambush predators that can sneak up on their prey. But no one knows.”
Meanwhile, a lot of “facts” about Greenland sharks are more like myths. The vast majority of them, including the one in the image above, have parasitic copepods (a type of crustacean) hanging off their eyes. That much is true, but according to Nielsen, the idea that the sharks are therefore blind is not. “When you look at hundreds of Greenland sharks, as I have done, you see that most of them have the parasite but they don’t have damage.”
The animals are not endangered. But Nielsen hopes that the results from his study will motivate people to find ways of reducing the numbers that are accidentally caught, and inspire scientists to learn more about the species. “We don’t know where they mate, where they give birth, how many there are,” he says. “When we go out, every time we go out, we learn something new.”
And in the meantime, if any studio would like to greenlight a centuries-spanning comedy drama about an aging Greenland shark that gets increasingly curmudgeonly and exasperated with each new human invention—the sequence with the first submarine alone would be golden—call me. Pixar?
We want to hear what you think about this article. Submit a letter to the editor or write to firstname.lastname@example.org.