Living 500 years or more, a species of shark has swum away with the medal for world's longest-lived vertebrate, scientists from Denmark have discovered.
Animals like fish can be difficult to age because, unlike other species which lay down the bony equivalent of tree-rings as they grow, sharks are built from cartilage, which just enlarges as they do.
To solve the problem, the University of Copenhagen's Julius Nielsen and his colleagues have resorted to radioactivity and carbon dated the animals' eyeballs, with surprising results.
Working with 28 Greenland shark specimens, some of them 5 metres long and all landed accidentally as by-catch, the researchers removed the lenses from the animals' eyes and employed carbon-dating to estimate their ages. Eyes are ideal for this purpose because the lens material is composed of metabolically inert and very long-lived proteins called crystallins. These are laid down during embryonic development and turnover only very slow during life. This means that the atoms they are made from were those around in the environment at the time when the animal was developing inside an egg.
The analysis on the shark specimens revealed an average age of 272 years, with one large female predicted to be 400, and possibly even as old as 520 years old.
Critically, only the animals under 220cm in length had "bomb" blips in their radioactive carbon fingerprints. These are increased levels of radioactivity caused by radioactive fallout from bomb tests in the 1950s. Based on the growth trajectory of the animals, this means that Greenland sharks reach sexual maturity aged over 150 years and appear to be the longest-lived vertebrates (animals with backbones) on the planet.
Elsewhere in the longevity league, bowhead whales, with a life expectancy of over 200 years, are snapping at the sharks' tails, although the reigning champion of the long-lived is actually a shellfish species called quahog (Arctica islandica), the oldest example of which was 507 years old.
According to the Danish team though, the discovery of the extreme life expectancy of the Greenland shark, and the late age at which the animals reach reproductive maturity, renders the animals very vulnerable, writing in Science, where the work is published, "our estimates strongly suggest a precautionary approach to the conservation of the Greenland shark, because they are common by-catch in arctic and subarctic ground-fish fisheries and have been subjected to several recent commercial exploitation initiatives."