It has long been suspected that whale sharks, the biggest fish in the sea, are long-lived, and now this has been confirmed using a carbon dating technique. It turns out these animals live for at least 50 years, and probably far longer.
Knowing how long animals live is important for conservation, says Steven Campana of the University of Iceland in Reykjavík. “It makes a big difference whether they are fast-growing and short-lived, or slow-growing and long-lived.”
But working out how long sharks and rays live is difficult. They do not have the bony structures called otoliths in their ears that are used to work out the age of most fish. Instead, sharks are aged based on growth rings in their cartilaginous vertebrae – but these growth rings form at different rates in different species, and may stop forming after sexual maturity.
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Two decades ago, Campana’s team showed that growth-ring-based age estimates for some long-lived animals could be checked by looking at levels of the carbon-14 isotope in the rings. The nuclear bomb tests carried out from the 1950s onwards created distinctive peaks in carbon-14.
This technique has shown that the age estimates for many sharks were wildly off. For instance, it was thought great white sharks lived only 12 to 15 years but recent studies have revealed individuals as old as 73.
Now the carbon-14 technique has been applied to the preserved remains of two adult whale sharks, one washed up dead in Pakistan and the other caught in Taiwan before a ban was introduced in 2007. It shows that one of these whales was at least 50 years old. Other individuals may live even longer.
In 2016, a study using the same method reported that Greenland sharks are the longest living vertebrates, possibly living as long as 500 years. Campana is sceptical about such extreme lifespans, but says it is clear they can live more than a century.
Journal reference: Frontiers in Marine Science, DOI: 10.3389/fmars.2020.00188
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