In the frigid waters of the sub-Arctic ocean lurks a mysterious and slow-moving beast known as the Greenland shark. It's a massive animal that can grow up to 20 feet in length. Now, new research suggests it may have a massive lifespan as well.
According to a paper published Thursday in Science, the Greenland shark could live for well over 250 years, making it the longest-living known vertebrate on Earth.
"I am 95% certain that the oldest of these sharks is between 272 and 512 years old," said lead author Julius Nielsen, a marine biologist at the University of Copenhagen. "That's a big range, but even the age estimate of at least 272 years makes it the oldest vertebrate animal in the world."
The oldest animal record holder is a clam called Ming that was dredged up from the ocean floor off the coast of Iceland. It was said to be 507 years old when it died in 2006, but there are other animals that have been known to live for more than a century.
Shortraker rockfish from off the Alaskan coast and orange roughy off Namibia are both estimated to live up to 200 years or longer. Harriet, a Galapagos tortoise from the Australia Zoo lived to be about 170 years old.
Still, if Nielsen's estimations are correct, the Greenland shark would be a record breaker.
Greenland sharks are among the largest sharks on the planet. They are dark brown or purple with small, beady eyes. They inhabit the Arctic and sub-Arctic waters, as well as cold, deep water in other oceans throughout the world.
Because they spend most of their time in the darkness their eyesight is thought to be very poor, but a vast network of neurons in their snouts suggest they hunt and scavenge using their powerful sense of smell.
"They are basically a giant swimming nose," said Aaron Fisk, a professor at the University of Windsor who has studied the Greenland shark for two decades.
Scientists have long suspected these lethargic giants of having extremely long lifespans in part because previous research shows that they grow very slowly — possibly as little as a centimeter per year.
"In colder temperatures, growth slows and fish tend to get older," said Fisk, who was not involved in the study. "It's not hard to imagine that they could be 200 or 400 years old."
But determining the exact age of the Greenland shark is a tricky business. When scientists determine the age of fish like cod, rock fish and salmon, they usually look at the otolith — a bony structure that grows in the ear of a fish. Otoliths have seasonal growth rings, kind of like the rings in tree trunks. If researchers can figure out how long it took the animal to lay down one ring, they can easily determine the age of the fish.
Sharks and rays don't have otoliths, so scientists have found other ways to determine their ages. For some species of sharks, it's possible to tell how old they are by looking at growth layers deposited in calcified parts of their vertebra or fin spines. But the Greenland shark doesn't have fin spines, and its cartilage skeleton is extremely soft with almost no calcified material, so there are no layers to count.
To overcome this hurdle, Nielsen and his collaborators turned to a more complicated technique called eye lens radiocarbon dating, which has been used to determine the age of other animals.
The eye lenses of all vertebrates continue to grow with the animal through its life, adding layers like an onion. However, the core of the eye lens is formed before the animal is born and remains metabolically stable throughout its life, Nielsen explained. That means that embedded in this small piece of tissue in the center of a shark's eye is a chemical signature from the environment just before it was born.
In the late 1950s, atmospheric tests of thermonuclear weapons caused a big and easily detectable spike in the amount of radiocarbon that eventually made its way into the sea. Scientists call this bump "the bomb pulse," and it has become a handy way to verify the age of marine organisms.
If the amount of radiocarbon in a shark's lens represents post bomb-pulse levels, that's a pretty clear indicator that the animal was born after 1960. (It took a few years for the radiocarbon to filter down into the deep water).
For this study, Nielsen examined the eye lenses of 28 female specimens that were caught off the coast of Greenland between 2010 and 2013.
The radiocarbon levels in the lenses of the two smallest sharks had a clear post-bomb pulse signature, suggesting that these animals were 50 or younger. The radiocarbon levels of the third-smallest shark put it right on the onset of the bomb pulse. The researchers say this means the third shark was likely born in the early 1960s.
However, the center of the eye lenses of the 25 larger sharks all had pre-bomb-pulse radiocarbon levels, leading the authors to conclude that they were more than 60 years old.
The group's next step was to determine how long before 1960 the other 25 sharks were born and here's where they had to get creative. They measured the radiocarbon levels in each of the remaining eye lens samples and then compared them to a published reference of how radiocarbon levels in the ocean have changed over time. This chronology of much more subtle radiocarbon fluctuations goes back 50,000 years and is usually used to date corals and other organisms that are thousands of years old. When it is used to date more recent organisms, it shows a wide range of error.
To further constrain their results, the authors made the assumption that the longer a shark is, the older it is. When they added the lengths of the specimens to their model, they found that the biggest shark in the data set — a 16-footer — would have been 392 years old, give or take 120 years.
The authors concede that the margin of error is still very large, but they say their findings demonstrate that the Greenland shark is extremely long-lived and that its population would take a long time to bounce back to normal if the animals were exploited by humans.
Aaron MacNeil, a research scientist at the Australian Institute of Marine Science who was not involved in the work, said the study represents an interesting approach to a difficult biological problem, but added that the findings are not necessarily conclusive.
"I don't think this is the final word on Greenland shark ages," he said.
Other researchers agreed.
Simon Thorrold, a biologist who studies sharks at Woods Hole Oceanographic Institution in Massachusetts took issue with the authors' assumption that the length of a shark can be used to determine its age.
"We know that in sharks and bony fish size is a very poor predictor of age," he said. "That's why we spend millions of dollars every year to age fish — because we simply can't use size."
Allen Andrews, a biologist at the National Oceanic and Atmospheric Administration's Pacific Island Fisheries Center who specializes in age validation of marine animals, noted that previous research in bony fish shows that the largest fish are not necessarily the oldest fish.
"With orange roughy we've seen that a maximum-size fish could be 20 years old or over 100," he said.
Andrews didn't doubt that the Greenland sharks might live as long as 300 years, but he said further work was needed to definitively nail down their ages.
"The data and analyses are not iron-clad and certainly deserve scrutiny," he said.
Nielsen agreed that it is possible for Greenland sharks of the same size to be different ages. However, he still thinks the calibration model used in the study is sound. He notes that only the smallest sharks were affected by the bomb pulse, making them the only ones younger than 50 years. This helps validate the decision to equate size with age, he said.
"At the end of the day, we completely agree it is possible that Greenland sharks of the same size can be different ages," he said. "We still believe the calibration is sound and allows for an estimation (not determination) of Greenland shark life spans."
Fisk, the Greenland shark expert from Windsor University said that although the study did not necessarily validate the age of these sharks, it still provided good evidence that they are indeed very long-lived animals.
"I wouldn't want to put a date on it, but I think 400 or 500 years is in the realm of possibility," he said. "And that's pretty extraordinary."
MORE IN SCIENCE