A student job with the Greenland Institute of Natural Resources inspired Julius Nielsen, lead author of the new study, to research the Greenland shark, which roams the chill waters of the North Atlantic. "I encountered the sharks here for the first time, and I was fascinated that so little were known about such large sharks," Nielsen said.
They are the largest fish native to Arctic seas, with adults typically measuring between 13 and 16 feet and females consistently outgrowing the males.
Still, their biology was mostly a mystery, explained Nielsen. "For example, their age was unknown but expected to be great," he said, based on studies from more than 50 years ago. The extremely slow growth rates of these sharks -- less than a centimeter per year -- suggested they must live for many years."
So how do you discover the age of a mute primordial shark? Nielsen and his colleagues turned to radiocarbon dating.
In the mid-1950s, the testing of thermonuclear bombs left a huge amount of radiocarbon in the atmosphere. It became incorporated in both terrestrial and marine food webs throughout the globe and so naturally entered the diets of animals and fish. In fact, radioactive isotopes in animal tissue reflect this "bomb pulse," which has become a well-established time stamp for age validation of marine animals.
To determine the age of Greenland sharks, Nielsen and his colleagues examined the eye lens nuclei of 28 females, caught during scientific surveys. "We were inspired to use the eye lens from other age determination studies conducted on whales," he said.
He and his colleagues estimated that the two largest sharks in the study, measuring a little more than 16 feet, 2 inches, and 16 feet, 5 inches, were 335 and 392 years old, respectively. Further analysis suggested the lifespan of Greenland sharks was at least 400 years, with sexual maturity reached around the age of 150.
"The biology and life history of the deepwater shark species is mostly unknown," said Cindy A. Tribuzio, a research biologist at the National Marine Fisheries Service. She explained that these "sleeper sharks" are notoriously hard to study and said attempts to determine their age have shown little promise. "This study on Greenland sharks appears to open a new avenue of study into aging these deepwater shark species."
Though the age estimates cannot be proved -- only the sharks know for sure -- Nielsen said other evidence, including high levels of accumulated contaminants, supports the results.
"Our results demonstrate that the Greenland shark is among the longest-lived vertebrate species," concluded Nielsen.