Longest Lived Vertebrate? More Data Required

A new study published this week purports to have discovered the longest lived of all the vertebrates. The Greenland shark, Somniosus microcephalus, is a very large, very slow growing creature that lives in the far reaches of the north Atlantic. Full grown adults can reach in excess of five metres in length. The new research, published in Science, has been reported widely in the science news with the claim that they found an individual that was about 400 years old; if we look at the methodology of the ageing technique, however, I think there is reason to be a little sceptical of the claim.

During the 1950s the superpowers of the world were busy testing their new atomic weapons, it was an age of experimentation and displays of power. The fallout from those test detonations spread to every corner of the globe in the form of radioactive carbon atoms that worked their way into every organism and food chain. This is relevant because studying how these atoms decay over time can give clues as to the age of the organism harbouring them.

In bony fish there are certain bones that can be scrutinised to give a good idea of age but the greenland shark doesn’t have any calcified bones, just cartilage. The research team specifically analysed portions of the eye lens, then, in 28 shark specimens caught accidentally during surveys of other species. It’s worth mentioning that this is a new technique in fish, it’s normally used in mammals, and so baselines may not be well established to make an accurate estimate of age.

One problem of carbon dating using radioactive carbon from the nuclear bomb pulse is that it doesn’t allow us to go further back in time than when those atoms appeared in the environment which, for the north Atlantic, was the 1960s. How, then, can these researchers be claiming they found an individual that’s 400 years old? The answer is an amount of extrapolation that the data may not warrant.

One of the sharks they measured seemed to be born just at the time when radioactive carbon came to the environment. To calibrate a baseline they calculated how fast this particular greenland shark grew in the intervening fifty or so years. With a figure of about 0.5-1 cm of growth/year as their yard stick they estimated the age of a shark that measured in excess of 5 metres as 392 years. The error bars on this estimate are pretty huge, which shouldn’t come as a surprise when your whole premise is based on the measurement of a single individual. The range is +/- 120 years so it could be as young as 272 or as old as 512. Even the lowest figure would make that particular shark the oldest ever vertebrate recorded on earth.

No one is doubting that greenland sharks live a very long time indeed, and further data may show that they really are the longest lived of all the vertebrates. To crown them as such now, however, feels rather hasty with such a small dataset. More data from different teams using different techniques are going to be required before a full determination can be made.

NOAA Ocean Explorer: NOAA Ship Okeanos Explorer: Northeast U.S.
Greenland Shark, image courtesy of the US National Oceanic and Atmospheric Administration

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