Nobody knows if the Mayflower sailed by a baby Greenland shark when the Pilgrims made their crossing to the new world in 1620. But if the Mayflower did pass by a baby Greenland shark, and if sharks could talk, this one could still tell the tale. That's because, according to a new study in Science magazine, Greenland sharks have just copped the award for longest-lived vertebrate, with an estimated lifespan reaching 400 years.
Greenland sharks, as their name suggests, favor the waters of the cold North Atlantic. They are a widely distributed species and are among the bruisers of the shark world, measuring 13 to 16 feet at maturity (400 to 500 cm). They attain that great length only very slowly, however, growing at a rate of just .39 in. (1 cm) per year. Since they're only about 15 in. (39 cm) at birth, it stands to reason that the adults are pretty old.
Of course, "stands to reason" is not how science is done. In order to determine if the sharks are indeed as old as they seem, a team of researchers led by marine biologist Julius Nielsen of the University of Copenhagen tried a more exacting approach, working with the remains of 28 female Greenland sharks, measuring 31 in to 16.4 ft. (81 cm to 502 cm). The sharks had been killed as by-catch—or the innocent bystanders of fishing fleets hunting for other species.
A common way to determine age is to measure growth bands in calcified tissue like bones. That would be fine here too except sharks don't have calcified tissue. Instead, Nielsen and his colleagues turned to the lenses of the animals' eyes.
The shark's lens is formed in utero, meaning that the proteins that make it up originally came from what the mother shark ate. What the mother shark ate, in turn, was influenced by her environment, including the amount of radioactive carbon-14 that was extant at the time. That means that carbon-14 dating—or how much the isotope has decayed away since the lens of the eye was formed—can serve as an accurate market of the animal's age.
The first thing the researchers had to do was discard the three smallest sharks, but not because of their size alone. The carbon-14 levels in their lenses was not just high but unusually high, a marker of what's known as the "bomb pulse," or a period of elevated radioactive isotopes that corresponded with atmospheric testing of nuclear weapons in the 1950s and 1960s. Bomb pulse signatures are often used in dating marine animals that are about 60 years old or younger, and in this case the readings were indeed proof that the three shark's small size was an indicator of youth.
Of the other 25 sharks, size and age did move in lockstep. The largest two specimens were carbon-clocked at 335 and 392 years old. The average age of the entire sample group was 272 years. Since none of the sharks died natural deaths, it is reasonable to assume that the biggest and oldest one would have managed at least a few more years, making the four-century figure a pretty safe bet.
Of all of the advantages to living so long, there is one downside—at least for any of the females hankering to be a mother. Greenland sharks are known not to reach sexual maturity until they're about 13 ft. (400 cm) long. Like most animals, they do their fastest growing earlier in their lives, so they reach this threshold comparatively quickly. But in the case of a Greenland shark, "comparatively quickly" means when they're about 150 years old. So no rush at all to paint the shark nursery.