Remarkably, there exists an animal species that has persisted since the dawn of the 1600s, a group of dark, slow-moving giants that live deep in the world’s coldest oceans and still grasp to life today.
The Greenland shark, also called Somniosus microcephalus, is now thought to be the longest living vertebrate on Earth. An international team of scientists, including ones from Scandinavia and Greenland, recently used radiocarbon dating of metabolically inactive eye lens tissue to show that the mysterious and poorly understood Greenland shark can live for more than four centuries. This is around twice the previous record of longevity in a vertebrate, which belonged to the Bowhead whale. What’s more, it is estimated that it takes 15 decades for the Greenland shark to reach sexual maturity, about seven times the age when humans reach sexual maturity (20 years).
Little is known about these sharp-toothed elders of the ocean, but the new findings have catapulted Greenland sharks into the mainstream and made them stars of the research world. It all started in 2010 when Professor John Steffensen, a marine biologist at the University of Copenhagen, and his graduate student, Julius Neilsen, spent three years sampling the sharks close to Greenland, trying to discern their age. Normally, the age of a fish can be determined by counting the layers of calcium carbonate in their ears, similar to dendrology (counting tree-rings). But Greenland sharks do not possess calcium in their ears, so the researchers turned to the sharks’ gray-black eyes—specifically the carbon dating of the proteins in the lenses of their eyes—to gauge the age of the great Somniosus microcephalus.
Carbon dating has long been used to estimate lifespan. The technique is based on the fact that carbon-14, which is found in all living things, degrades over time at a known rate, meaning that the relative amount of C-14 within a specimen is reflected in its age. Of course, there is a degree of uncertainty associated with carbon dating, but it continually proves to be the most reliable method of dating currently available for specimens in this age-range.
Carbon slowly accumulates in the eye lenses of a shark. This process begins when the shark is in its mother’s womb, with the carbon deposits first appearing in the middle of the lens. As the shark gets older, the carbon deposits slowly spread to the outer edges of the eye. In the 1950s, atomic bomb tests doubled the amount of carbon-14 in the atmosphere, creating a useful “C-14 time stamp,” which scientists could detect in the eye proteins of Greenland sharks. The smaller sharks had the highest levels of C-14 deposits, indicating they were born post 1950. As well, the size of the sharks increased with age and sex, with females being notably longer than males.
And long they were. One female was a terrifying five metres (as long as a big great white shark) and estimated to between 390 and 512 years old. Moreover, females can only produce young after they reach at least four metres in length, which takes a whopping 150 years.
That an animal can survive for half a millennium is miraculous. But how? Greenland sharks are very slow moving. If you watch a YouTube video of one in action, you’ll see very little action —a sluggish, sleepy giant peacefully gliding through the deep. This slow-motion existence is thought to be an adaptation to the extreme cold of the Arctic Ocean and North Atlantic, resulting in a low metabolism, a slow growth rate, and an extended lifespan. Scientists are also exploring the DNA sequences of Greenland sharks to see if they harbour any anti-aging genes.
Apart from having the right biological make-up, achieving centuries of longevity requires a lot of good luck. Like anything living, Greenland sharks need to eat, avoid injury and disease, and survive the various dangers and challenges of their environment. The ocean is full of predators, but big sharks are at the top of the food chain. Nevertheless, food scarcity and disease are blind to breed and size and are a major reason why sharks die every year, but Greenland sharks appear to be particularly hardy and healthy.
Whatever the reasons for their longevity, it is remarkable to think that there exists an animal alive today that has persisted throughout all the major wars, discoveries, and technological advancements over the last four centuries—an animal that might have witnessed some of the earliest vessels arrive in North America as well as today’s modern tankers and cruise ships. Although the sharks are shy and reclusive, there is a chance that early Inuit might have spotted a Greenland shark while fishing hundreds of years ago and that a modern tourist has photographed that same shark with an iPhone. If the Greenland shark can teach us anything, maybe it’s that it is best to move slowly and peacefully through the cold currents of life.
Neetin Prabhu is an undergraduate science student at the University of Western Ontario. David Smith is an assistant professor in the biology department at Western; he can be found online at www.arrogantgenome.com.