In an evolutionary sense, sharks are among Earth’s oldest survivors; they’ve been roaming the oceans for more than 400 million years. But some individual sharks boast lifespans that are equally jaw-dropping. Incredibly, deepwater sharks off the coast of Greenland appear to have been alive and swimming back in Shakespeare's day 400-plus years ago—making them the longest-lived of all known vertebrates.
Bristlecone pines can live to be 5,000 years old. Sea sponges can live for thousands of years. One quahog, a hard-shelled ocean clam, died in 2006 at the age of 507. But among vertebrates, the long-lived skew much younger. Bowhead whales and rougheye rockfish can live for up to 200 years, and a few giant tortoises may also approach the two century mark. Now it seems that Greenland sharks more than double even these remarkable lifespans, scientists report today in Science.
The reason for the sharks’ unfathomably long lives has to do with their lifestyles. Cold-blooded animals that live in cold environments often have slow metabolic rates, which are correlated with longevity. “The general rule is that deep and cold equals old, so I think a lot of people expected species like Greenland sharks to be long-lived,” says Chris Lowe, a shark biologist at the California State University at Long Beach. “But holy cow, this takes it to an entirely different level.”
Lowe, who wasn’t involved in the research, adds that Greenland sharks must have a metabolic rate “just above a rock.”
Greenland sharks spend their time in the remote, freezing depths of the Arctic and North Atlantic oceans, making it difficult for researchers to parse the details of their lifestyle and reproduction. Determining their birthdates is even harder. Until now, scientists have been thwarted in their efforts to date this elasmobranch species—a group which include sharks, skates, and rays—by the fact that the animals lack calcium-rich bones, which can be radiocarbon dated.
Faced with a dearth of calcium-rich material to date, the authors of the new study employed a creative solution: They searched the sharks’ eyes. The nucleus of the shark’s eye lens, it turns out, is made up of inert crystalline proteins that are formed when the shark is an embryo and contain some of the same isotopes used to date bones and teeth. Measuring the relative ratios of these isotopes enabled scientists to determine the year when each shark was aged zero.
Scientists examined 28 female sharks—all acquired as bycatch from commercial fisheries—to find that many seemed to have lived longer than two centuries. (Scientists discarded the youngest animals, because they showed signs of radiocarbon released by Cold War-era nuclear bomb testing.) The biggest shark of this group, which measured about 16.5 feet, was believed to be 392 years old—placing her in the era of astronomer Galileo Galilei. Yet Greenland sharks are known to grow well over 20 feet, meaning many are likely even older.
Given that the study produced such striking conclusions and relied on unorthodox methods, scientists will likely question its findings. But Lowe said the idea to use radiocarbon in the eye lens is “creative and bold, but I think a safe approach to take,” adding that the results are “mind-boggling.” “If this dating is accurate there are Greenland sharks swimming around now that were swimming around long, long before the U.S. was even founded,” he says. “I have a hard time getting my head around that.”
The key to sticking around longer may have to do with growing slowly. Thanks to several tagging studies dating as far back as the 1950s, we knew that Greenland sharks grow at a snail’s pace, expanding by 1 centimeter a year. Yet they live so long that they still reach typical lengths of 400 to 500 centimeters, or 13 to 16 feet, by the time they attain full size. By contrast, great white sharks—a reasonable comparison in terms of size, says Lowe—can grow a foot a year during the first few years of their lives.
Matching the sharks' ages to their sizes produced another insight. Because previous studies have revealed that females become sexually mature only when they exceed lengths of 400 centimeters, it now appears the sharks don’t reach reproductive maturity until they are 156 years old. From a conservation standpoint, that’s concerning: Such a slow rate of reproduction means that each individual shark may be far more important to the species as a whole than scientists previously realized.
Fishermen once hunted Greenland sharks for their valuable liver oil, which could be used in lamps. A century ago, Greenland alone landed 32,000 sharks a year according to studies compiled at the time. Iceland and Norway also fished the sharks for their oil, which was also used in industrial lubricant and cosmetics. Although the oil lamp industry—and thus most of the Greenland shark trade—is now a relic, that violent history could still have ramifications today.
“One of the possible reasons for large Greenland sharks being rare could be because of [that] targeted fishery for them,” says Richard Brill, a fishery biologist at the Virginia Institute of Marine Science and a co-author on the study. “It's possible that the original age structure of the population has not had time to recover in the intervening years ,as the sharks are so slow growing.”
Lamp oil isn’t the only use humans have found for this marine methuselah. While its flesh is toxic, laced with an unpalatable natural antifreeze of urea and trimethylamine oxide, that hasn’t stopped us from eating it. In Iceland, shark meat is drained of fluids, dried outside for months, and served in small pieces as a traditional and notoriously pungent hors d'oeuvre called hákarl or, by some, “rotting shark.” Fortunately, this delicacy creates only a small demand for shark meat according to the BBC, but again, every shark counts.
In fact, the biggest human threat to sharks is unintentional. Many Greenland sharks, including the ones dated in the study, meet their deaths on boat decks when they are picked up as bycatch by coldwater fisheries that catch creatures like shrimp, halibut and other fish with trawling nets and longlines. Preventing that bycatch will have a major bearing on the future outlook of the Greenland shark.
That these fish have survived under pressure for so many years is a testament to their resilience—but not something to be taken for granted. Lowe raises an interesting possibility for how these sharks have managed to survive despite centuries of fishing: “They may have natural refuges where people haven't been able to access them historically,” he says. But as Arctic ice recedes and the seas and fisheries at the top of the world shift, many areas where these ancient animals might have once been safe could open up to new fishing pressures.
Researchers are now planning a shark-catching expedition for next spring, says Brill, “with the hope of getting some eye lens samples from some exceptionally large animals so we can confirm their ages.” But as those exceptionally large sharks aren't often captured, the expedition may rely on something that's even harder to pin down than an exact age: good fortune. “This will take some considerable luck,” Brill says.