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I was wondering, as I began working on this story, whether to eat my subject as part of the research. I imagined a bold opening: “This is the longest-lived animal in the world – and it tastes great.”

Since the animal in question is a species of clam, I visualised a spaghetti alle vongole with plenty of garlic. But setting aside the ethical considerations of killing and eating a fellow animal, and the ecological damage we are doing in over-exploiting the ocean, I realised there is another consideration. This special animal – the ocean quahog – can live for at least 500 years. Killing it just seems wrong. So, no, I won’t eat this mollusc. As such, let me amend my introduction: this is the longest-lived animal in the world – and my mission is to discover its secret.

You can be forgiven if you haven’t heard of the ocean quahog, also known as the Icelandic cyprine: it isn’t the sort of animal that gets much TV time. It is a large bivalve mollusc that lives buried in the sand on both coasts of the Atlantic, from the southern warmth of Florida and Cadiz in Spain, to the colder waters of Quebec in Canada and Norway. If you have had clam chowder in the US, you will have almost certainly eaten it. Its shell is fine-lined like the rings of a tree trunk, and like tree rings, you can count these lines to tell its age.

The oldest known specimen was called Hafrún by researchers, an Icelandic name that means “mystery of the ocean”. Hafrún was born in 1499 and lived as its ancestors had done for generations, quietly on a modest diet gathered off the coast of Iceland. In that sense, its life was unremarkable, but for the fact that it went on and on – and on. It ended, in fact, only in 2006, when it was dredged from the sea by a team from the University of Exeter, UK. Sclerochronologist Paul Butler was the researcher tasked with ageing it. Sclerochronology involves analysing bivalve shells to construct timelines for their surrounding environments.

“Its age was initially published as just over 400 years, but closer reading of the growth lines and comparison with other shells showed it was in fact 507 years old,” he tells me. It is likely there are even older ones still out there, especially in the cold waters around Iceland, where they seem to grow more slowly and live even longer. Is there an upper limit to their age? “It’s hard to believe they live a lot longer,” says Butler, “although we did once get the ages of a few individuals analysed by a mathematician who said in principle they could live forever.” Well, that’s mathematicians for you.

The key to the longevity of the quahog appears to be in its mitochondria – the structures in our cells that use food to provide us with energy. By “us”, I mean us eukaryotes – all complex organisms, from yew trees and mealworms to jellyfish and rabbits.