This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
In a jaw-dropping revelation, scientists announced today in the journal Science that radiocarbon dating of structures in the eye lenses of 28 female Greenland sharks reveal that they do not reach sexual maturity until around 156 years of age, and live to be at least 272 -- and possibly as much as 512. Although most of the sharks were less than 200 years old, two were far older and the oldest female in the study was judged to be around 392 (+/- 120) years old, a number that, if accurate, beggars belief and crowns the Greenland shark the unquestioned vertebrate longevity champ-een.
The tradeoff for these eye-poppingly venerable ages is a life confined to the black, icy waters of the coasts of northern Europe, Greenland, and northeastern North America. There, Greenland sharks cruise at depths up to 6000 feet searching for any prey or carcasses that happen into their path. They eat mostly fish, but pieces of seals, horses, moose, polar bears(!), and reindeer (even whole carcasses), have all been found inside them. But they have rarely, if ever, consumed a human, possibly because most people do not fancy pleasure-swims on the Greenlandic Riviera -- or perhaps the sharks rarely visit the surface.
As the centuries roll by, the sharks typically reach 16.5 feet long and weigh several thousand pounds. There is no question they are the largest fish that haunts Arctic waters, and they are among the largest sharks in the world. In a Science podcast interview(shark segment starts at about 15:00), Nielsen said he feels it is the combination of near-freezing body temperatures and hence a low metabolic rate with the tremendous size these sharks attain that leads to their supernumerary lifespan.
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A special structure in the lens of the vertebrate eye called the nucleus was used to date the sharks in this study. Traditionally, mineralized bones reveal the age of vertebrates, but sharks famously lack these. Their skeleton is made of softer cartilage that cannot be dated as bone can.
To get around this, scientists turned the optical lens nucleus, which is made of crystalline proteins that are not metabolized during the life of the organism. Its center is formed during fetal development. The shark can thus be dated by measuring how many carbon-14 isotopes remain in the center of the lens.
The carbon-14 concentration of seawater is known and, like all radioactive isotopes, carbon-14 has a stable and known decay rate into nitrogen-14. When the shark is born, a fixed amount of carbon-14 is stashed there. Over the centuries, it decays and is not replaced. By measuring how much carbon-14 is left in the lens nucleus of a Greenland shark’s eye, we can get a rough idea of how old it is.
Intriguingly, the team was able to detect the effects of hydrogen bomb tests of the 50’s and 60s, which began to show up in marine life in the early 1960s. Exploding hydrogen bombs ejected enormous quantities of carbon-14 at that time. This surge appeared as a “radiocarbon bomb pulse” that formed a convenient, if unfortunate, calibration point for the study.
The first shark to show signs of the pulse must be about fifty years old. It turned out to be around two meters long -- and there were only three sharks this size or younger out of the entire 28 sharks. That shocked the scientists because they knew that females don't reach sexual maturity until they attain four meters. 50-old sharks were still effectively children.
To date sharks older than the radiocarbon pulse, the scientists created a shark age model based on a pre-existing radiocarbon calibration curve called Marine13. Although any model is only as good as the data that goes into it and its designers’ skill -- and the designers had to contend with many challenges like potential regional carbon isotope variations -- the ages obtained are not outside the realm of plausibility. Scientists had already guessed the sharks were very old because growth rates have been calculated to be less than a centimeter a year, and the largest Greenland sharks measure north of 5 meters (500 centimeters).
Although bowhead whales may live in excess of 200 years and are mammals to boot, it seems clear now that alone among vertebrates, Greenland sharks are “Eldest”. What would it be like to live for 400 years? I think I can safely say that if it entails swimming around alone in freezing, murky water subsisting on a diet of carrion and raw fish, I am content with my lot.
