The mysterious Greenland shark is the world’s longest living vertebrate species capable of surviving in the deep icy waters of the Arctic for many centuries. But their long lives may be a curse rather than a blessing.
Greenland sharks are mysterious and fascinating creatures capable of living extraordinary long lives. Not only are they older than any other vertebrate alive on earth but some have been swimming around the Arctic since before the United States of America became a country. Their longevity is an extraordinary example of evolution, highlighted by the fact they’ve been alive since long before Darwin was even born. How they manage to achieve such a long lifespan is still relatively unknown but the fact that they do could put them in a precarious situation heading into the future.
A unique shark
The Greenland shark, Somniosus microcephalus, is not your ordinary shark. They are similar in size to great whites at around 6m long, but that is where any likeness comes to an end. Known as the ‘sleeper shark’ they move at an average speed of 0.7mph and are believed to grow at less than 1cm a year. They do not become sexually active until around 150 years old and very little is known about their reproduction. They have also never been observed feeding in the wild but due to their speed it is likely they are scavengers rather than predators. They reside in the deep waters of the North Atlantic Ocean in the Arctic Circle and prefer temperatures between -1oC and -5oC. Quite frankly they are bizarre compared to most other sharks and have even been shown to have copepods, small crustacean like creatures, attached to their eyeballs.
The study that determined the length of their lifespans was carried out in 2016 by a team led by Julius Nielsen from the University of Copenhagen. It was not an easy thing to do because of how different they are from any other shark, which meant having to think outside the box to work it out. Most sharks can be aged by counting rings of growth in vertebrate and otoliths (ear bones), similar to aging a tree. However the Greenland is what is known as a soft shark, its bones are too weak to have growth rings. But they do produce layers of protein growth in their eye lenses. I know what you’re thinking but don’t worry, the scientists didn’t go around removing the eyes of living sharks. Instead they used the eyes of 28 dead Greenland sharks, a majority of which had died from accidental bycatch on fishing vessels.
Unfortunately there was no way of knowing how often the protein layers were added to the lenses. So the researchers had to find a way to figure it out. Their solution came from an unusual source, cold war nuclear bomb tests. This may seem unlikely but increased tests of nuclear weapons in the oceans during the cold war, particularly 1955-63, significantly increased the concentration of carbon-14 isotopes in the oceans. This altered form of carbon was then used by the sharks to produce slightly different proteins during that time. So the scientists used these unique layers in the lenses to date the process and in turn use it to determine total lifespans. From there they figured out that most were at least 200 years old, but the approximate age of some of the individuals around 400 years old. This meant their date of birth was somewhere in the early 17th century.
What’s their secret?
Even though researchers have now determined how long these animals live, they are still quite unsure of how they are able to do it. One potential explanation is a high level of a chemical called trimethylamine N-oxide (TMAO). This is toxic to humans but in sharks it counteracts the high levels of urea that destabilise their proteins, especially at high pressure in deep waters. It could be that their high levels of TMAO are also somehow stopping damage to cells caused by the aging process. However the most likely reason is that they are just really cold. The extremely cold waters they live in and slow movement means the shark’s metabolism is very low. Shark biologist Chris Lowe from California State University joked that Greenland sharks have a metabolism “just above a rock”. You can think of it as a bit like how you can preserve life with cryogenics in humans by freezing the body. But even that is a best guess and without more research we are unlikely to ever know for sure.
Currently the Greenland sharks are healthy in numbers and in no danger of disappearing anytime soon. However they faced some serious problems in the early 20th century and continue to be effected by some minor threats today. Before WW2 hunters from Greenland, Norway and Iceland were catching 50,000 Greenland sharks a year for their valuable oils used in cosmetics. This put a strain on their population but that has since stopped since synthetic replacements were discovered. Traditionally they have also been caught to make a traditional Icelandic dish called Hákarl, where their meat is fermented and cured for 5 months to get rid of the toxic TMAOs. But as of 2018 the Greenland shark is now a protected species by the Northwest Atlantic Fisheries Organization (NAFO) meaning it is a no take species. Nowadays the main threat to the species is bycatch from fisheries.
A blessing or a curse?
You might think living for such a long period of time is a desirable trait, but it can also be very damaging to a species chances of survival. The longevity of Greenland sharks means it takes a long time to recover after a decline in the population. For example some juvenile sharks from the pre WW2 overfishing era may still not be sexually active and able to reproduce today. It also means that they will be slow to adapt to major ecological changes such as global warming. A Greenland shark born today will not be able to reproduce till the late 22nd century by which time there may no longer be any waters cold enough for them to live in. Although their 400 yearlong lives are an extreme example of shark longevity most species are capable of living for between 50 and 100 years and face the same sort of problems.
It is easy for us today to look around us and assume that the world we see is the way it has always been. But human lives are fleeting and have stopped us from seeing the bigger picture. In reality in the last 400 years the world has changed more than it had in the previous 4000. They may not be endangered now but will the Greenland sharks really be able to survive the next 400 years if we continue at our current pace. They have witnessed the impacts of the modern world better than any other creature on the planet. I wonder what would they have to say to us if they could talk?