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The oldest denizen of the sea

The oldest denizen of the sea
No  well bred scientist can ask a lady shark how old she is, says S.Ananthanarayanan.

The problem however, goes beyond good manners, for it is challenging to tell the age of some living things that last more than a human lifetime. The legendary Redwood trees are hundreds to thousands of years old. We can work out their age , however, from the ‘growth rings’ once the tree is felled, or even from records of people, long dead, who were familiar with the tree.

But how do you tell the age of an animal, which shows no annual growth markings and does not stay in one place, like a Redwood tree? Julius Nielsen, Rasmus B. Hedeholm, Jan Heinemeier, Peter G. Bushnell, Jørgen S. Christiansen, Jesper Olsen, Christopher Bronk Ramsey, Richard W. Brill, Malene Simon, Kirstine F. Steffensen and John F. Steffensen from institutes and universities in Denmark, Greenland, Norway, USA and UK report in the journal, Science, that they have used a combination of radioactive dating and the growth rate of shark tissue, to find that there are female sharks in the sea which are more than two hundred and seventy years old!

That radioactive dating could be used to tell the age of a contemporary living thing may come as a surprise. This established technique of finding the age of long dead fossils, archaeological specimens, and so on, is based on the decay of the nuclei of radioactive carbon. The normal carbon nucleus, which is stable, consists of six protons and six neutrons, or twelve particles in all. But there is also a tiny percentage of atoms whose nuclei have eight neutrons instead of six. The atom, however, is still carbon, as chemical properties depend on the number of protons, which is the same. But this form of carbon, known as C14, is unstable and one of the neutrons, sooner or later, radioactively changes to a proton and with seven protons, the atom becomes one of nitrogen.

We can imagine that any living thing, which contains carbon, would keep showing radioactivity whose rate would gradually slow down, as the number of atoms of C14 gets depleted. The fact, however is that in the atmosphere and the environment, there is also generation of C14 atoms, by the action of cosmic rays, and there is a balance. And as living things keep exchanging carbon atoms with the environment through intake of food and exhalation of breath, the percentage of C14 stays constant in living things too. But once the organism dies, the exchange stops, and as the C14 content depletes, the level of radioactivity starts to fall.

Age of a living thing
This is the way we can tell how long ago it was that a once living thing died, the skeleton of dinosaur, or the wood used to build a temple, for instance. But how do you use this to tell how long an animal has been living before it died? This is also possible, thanks to a part of the body that forms just before the animal was born, and crystallises, or stops participating in metabolism, so that the C14 content in that part started depleting and keeping count of passing time, almost from the moment of birth. Julius Nielsen and others, the authors of the paper in Science, used this method to estimate the age of the Greenland shark, which inhabits the North Atlantic and grows to be over 5 metres long. As the shark shows a very slow rate of growth, less than a centimeter every year, it clearly grows to this length over a very long lifetime! “The biology of the Greenland shark, however, is poorly understood, and longevity and age at first reproduction are completely unknown,” the authors say.

The part of the body that acts as the timekeeper in the Greenland shark is a bit of crystallised protein within the eye lens. “In vertebrates, the eye lens nucleus is composed of metabolically inert crystalline proteins, which in the center (i.e., the embryonic nucleus) is formed during prenatal development,” the authors say in the paper. This tissue thus has C14 content that was the same as its surrounding at the time of the shark’s birth, but has been depleting by radioactivity ever since. Measuring the level of radioactivity at the time of the shark’s death then provides information of how long ago it was that the shark was born.

The team was able to inspect the eye lens centre of twenty eight female Greenland sharks, measuring from 81 cm to 502 cm (the females being larger than male Greenland sharks), which were accidentally caught in the course of fishing during the years 2010 to 2013. Analyses of the eye lens material showed that the source of the carbon was the diet of the mother and the date of the crystallized centre was nearly the same as the birth of the young shark. The ages estimated by the level of radioactivity were seen to agree with the idea that smaller sharks were likely to be younger sharks. This was also in agreement with expected high levels radioactivity in carbon ingested during the period of thermonuclear tests, in the late 1950s - early 1960s – the three smallest sharks were found to have radioactivity levels that corresponded to post-nuclear tests environmental levels, in two, and a spike, corresponding to the test period, in one case, which places the age of that shark as approximately fifty years in 2012. The remaining twenty five sharks showed lower levels, indicating greater age, and corresponding to larger sizes.

Another marker to help corroborate the findings was the Suess effect, or the change in the atmospheric C14/C13 levels as a result of large scale burning of fossil fuels during the last century, which is imprinted in the marine food web. Accordingly, sharks which were less than three metres long and estimated to be less than a hundred years old, showed lower levels of C13, in keeping with the atmospheric trend.

The estimation of age was adjusted for known variation in the environment carbon levels in the last four hundred years as well as the effect of mixing of waters from different parts of the ocean. The statistical method used for this calibration uses a presumed age distribution, which was derived from a formula, known as the Von Bertalanffy equation, to relate fish size, whose increase gradually slows down, with age.

Oldest member
The findings hence place the longest Greenland sharks as at 392 years, with an error margin of 120 years, or at least 272 years old. This is more than the lifetime of the Bowhead whale, which is estimated to live for 211 years. While the only longer lived animal is the ocean quahog, a species of clam that lives for 507 years, the Greenland shark is the longest lived vertebrate. Being 272 years, which is the lower bound of the estimated age, places the birth of the largest Greenland sharks at around 1740, the time of the declining Mughal empire, Nadir Shah’s sacking of Delhi and the beginning of the steam engine and the industrial revolution!

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