There are Greenland sharks alive today that had already been born as Britain rolled out the cotton mill, as America pushed for independence, and as Mozart played a piano for the first time. Through generations of human lifetimes they swam in the chilled depths of the Arctic Ocean, slowly coming of age, growing just a centimetre or two every year. By adulthood, Greenland sharks are up to 5 metres long, and one of the largest carnivores in the ocean - a dappled, grey-eyed predator.
‘What we can say for sure is that this is an animal whose longevity is measured in centuries, not decades’ says Julius Nielsen, Marine Biologist at the University of Copenhagen
This year, an international group of scientists, lead by marine biologists from the University of Copenhagen, have revealed that the Greenland shark has a life expectancy at least 272 years, superceding the bowhead whale as oldest vertebrate animal. Its true longevity, however, is still unknown.
‘What we have is a window of life expectancy of 272 to 512 years,’ says Nielsen, a self confessed ‘fish-nerd’ and lead author of the research, ‘which is incredible - it is the first time an animal's age has been estimated with 240 years of uncertainty, and still considered a success! The lower estimate already means that we are dealing with the oldest animal known to science.’
The uncertainty stems from the creature's biology. The Greenland shark does not accumulate any hard tissues, so its age has little or no reference points. Instead, the team had use carbon dating of their eye lens proteins. By analysing the innermost centre of the lenses – which remain the same since birth – they could search for a particular hallmark: the hydrogen bomb era. The vast releases of radiocarbon into the atmosphere by atom bomb testing in the 1960s created a consistent time stamp for scientists. Referred to as the ‘bomb pulse’, it can be used to determine whether an organism, such as the Greenland shark, was born before or after 1960. ‘We realised only the very smallest ones were affected’ says Nielsen. ‘It was the first moment we had evidence of that we were be dealing with a very old animal.’
The bomb pulse showed Nielsen's team that most sharks were over 50 years old, but not by how much. In order to get a more accurate estimate, they had to combine the bomb radio-carbondating with another archeological form of radiocarbon dating, which deals in terms of thousands of years. ‘Such a combination of methods have never been used on a living animal before’ says Nielsen.
‘Providing strong evidence that these sharks are exceptionally old was our main objective,’ he continues. ‘It is important now to simultaneously improve dating methods and raise their profile, so that they can be recognised and protected for the old, wild animals that they are.’