Greenland sharks: old, cold and fascinating
SHOW NOTES
Greenland sharks are the world’s longest-living vertebrate, with a longevity that can be measured in centuries. But what do they do during that long life? In this episode, we speak with scientist and “Greenland Shark Detective” Dr Julius Nielsen, who has studied these mysterious creatures for many years. Julius was part of the first team to estimate their extraordinary lifespan, and explains exactly how they did it. He also walks us through some other fascinating aspects of their biology and ecology, including what they eat, how deep they can go, their reproductive habits, and some very unusual parasites…
We start as always with Julius’ most memorable ocean experience [6.20]. In the spirit of this episode, Julius remembers a very special encounter with a Greenland shark in the field. Having just completed and defended his PhD thesis (which aimed to investigate the longevity of the Greenland shark), he still had some tags left and had headed back into the field in the hope of finding some more sharks. Julius worked mainly with trawlers and longliners, researching the sharks they caught accidentally as by-catch. On this particular day, they caught a relatively small Greenland shark – too small to tag, but the ideal size for Julius to release it by hand. Usually, Greenland sharks are so big – reaching lengths of 4-5m – that it is not possible to handle them. Being able to hold a Greenland shark, and then release it into the wild, was a very poetic way to close that chapter in Julius’ life, saying goodbye not only to that particular shark but the huge body of work dedicated to understanding the species as a whole.
Julius first came across Greenland sharks as a biology student at the University of Copenhagen, Denmark, while working a summer job on the research vessels of the Greenland Institute of Natural Resources, monitoring commercial fish and shrimp stocks [10.32]. It was during this time that Julius learnt of another vessel from the institute, which was studying Greenland sharks. He knew nothing of the species, but had heard that they were enormous and was intrigued. The following year, he decided to join that trip to see Greenland sharks for himself. They were lucky – seeing a number of individuals, most over 3m and the largest weighing up to 1,000kg! He recalls being amazed by them, but not driven to work with them at that moment. Rather, they were part of an all-round incredible experience exploring arctic waters, and seeing many amazing things for the first time. It wasn’t until years later, during a lecture on Greenland sharks by Professor John Steffensen, that the stars began to align. Steffensen was working, among many things, on ageing the Greenland shark for the very first time. However, he had only been coming across small to medium-sized sharks (less than 2.5m) and was seriously needing larger (and presumably older) individuals. Of course, there was a student in the class who had seen 3-4m sharks in Greenland: Julius! He raised his hand, volunteered what he knew, and this sparked the conversation that eventually led to Julius’ master’s thesis, and then his PhD, researching the same question.
Before John and Julius came up with an age estimate, no one really knew anything about Greenland sharks [14.10]. There was a study from 50 years ago that suggested the sharks were growing very slowly, using catch-and-release methods to measure the same individuals over time. However, the scientist who originally tagged the sharks never came across them again. Rather, most of the data came from fishing vessels who did not know how to measure a shark with scientific accuracy. Consequently, the results varied widely – one shark was even said to have shrunk by a metre! The only reliable piece of data came from another scientist, who had been able to bring a shark ashore and measure it accurately. And their findings were astounding – the shark had only grown 8cm over 16 years! The study concluded that, while data was limited, it did seem that Greenland sharks grew extremely slowly, which would also suggest that they lived a very long time.
Living life in the slow lane is the Greenland shark’s speciality [18.35]. They are part of the sleeper shark family, the Somniosidae, a group of sharks known for their inactive, sluggish lifestyles and slow swimming speeds. Julius describes the Greenland shark as “slow and docile”, with very small eyes and soft bodies. As a deep-water species, they are rarely seen at the surface, except to feed on the floating carcasses of whales or seals. Another characteristic is having parasites attached to their eyes [21.34]. It is common for Greenland sharks to be found with a parasitic copepod (which kind of looks like a long, white worm) attached to the cornea of their eye. The parasite hangs on until it is ready to breed, and falls off after producing eggs before being replaced by another. You can see the evidence of these tiny hitch-hikers by looking at all the scar tissue on the shark’s eye, which is left behind after each parasite. This has led to the popular myth that Greenland sharks are blind. Julius doesn’t buy in to that theory, however – his observations seem to suggest that the scar tissue gets gradually more transparent over time. So, while the sharks may not have perfect 20/20 vision, they are still likely to have some ability to see. But as a deep-water shark who can hang out at depths of at least 2.9km [25.18], who needs perfect sight?
Of course, one of the biggest headlines about the Greenland shark – which comes from Julius’ PhD and later publication in the journal Science – is that they can live for hundreds of years [26.00]. Julius and his team used radiocarbon dating to estimate the age of 28 individual Greenland sharks, suggesting the oldest to be between 272 – 512 years old! Julius is quick to stress that this is an estimate, based on the best available methods at this time. However, there is no doubt that Greenland sharks are capable of getting very old, and that their longevity can be measured in centuries.
So how did they get such an estimate [30.09]? As Julius explains, conventional methods of ageing fish did not apply to the Greenland shark. One method used for bony fishes is to count the growth rings of the otolith (ear stone), calcium carbonate structures found in the head of fishes like salmon and pike. The growth rings show just how many summers and winters they have been alive – much like the rings found inside a tree. However, sharks and rays do not have otoliths. Ageing sharks and rays is much more difficult, and the methods depend on the species. Some species have calcified cartilage skeletons, and you can count the growth rings present in the vertebrae. Others, like the spiny dogfish, have a spine that you can take a part of and analyse to see how many years they have been alive. Greenland sharks lack this spine, and have a much softer skeleton, so neither of these methods work.
Instead, Julius and his team had to use radiocarbon dating of the eye lens to get their estimate [32.34]. The eye lens of every vertebrate is made of a unique material, sometimes called “biological glass,” as it is made up of crystalline proteins. These proteins do not change over time, meaning that at the centre of the eye lens there is material that has been present since the shark was born. Julius analysed this part of the eye lens to assess the level of radioactive material present, in a technique known as “bomb pulse carbon dating”. In the late 1950s, thermonuclear testing released elevated levels of carbon14 into the atmosphere – and we can see evidence of this in the tissues of every living thing that was alive during that time. For example, imagine a tree that was planted in 1900, and went on to grow for 100 years. If you cut it open in the year 2000 and analysed it, what you would find is a spike in the levels of carbon14 at the 50-year mark, which would then steadily decrease until present day. This is known as the bomb pulse, and Julius found evidence of this in the Greenland sharks sampled for his study. In fact, they found evidence of it even in the smallest individuals – sharks of around 2.5m – which we know now to be just teenagers. This meant that the sharks that had not even reached sexual maturity and were still technically sub-adults, were at least 50 years old! This would mean that the 4 and 5 metre individuals were much, much older.
So what do Greenland sharks do with that long life? Well, Julius has also looked at aspects of their reproductive biology [30.09]. His research estimates that they only reach sexual maturity at an astounding 150 years, which is a red flag for conservation. But there may be a glimmer of hope. For a long time, it has been thought that Greenland sharks only give birth to around ten pups – but Julius’ work as “the Greenland shark detective” has suggested a different idea. The former belief comes from just one account of a female Greenland shark caught on a longline. Fishermen say they witnessed her give birth to ten pups, which they dutifully delivered to scientists. However, this was decades ago – and there is no information on how long the female had been left on the line for. Sharks are known to give birth under stress [53.20], something Julius has witnessed himself in black dogfish. If the foetuses are large enough and capable of swimming, then the female will give birth prematurely if she feels threatened, or that she is dying – presumably to give them a fighting chance. Julius believes the female Greenland shark did the same thing, and that the species may be more fecund than first thought.
There is evidence to support his theory, some of which comes from Julius’ own work and observations. He has observed female Greenland sharks with very large ovaries, containing 400-600 unfertilised eggs – all of the same size. This seems to support the theory that Greenland sharks are capable of giving birth to hundreds of pups at a time, corroborated by the reproductive traits of other species within the same family that are much better understood.
We also get into their feeding habits [54.48]. Greenland sharks are known to be scavengers, feeding on a huge variety of foods from whales, to fishes, to seals and even polar bears and moose when opportunity allows! Julius has seen evidence of this scavenging behaviour when analysing the stomach contents of Greenland sharks (a very interesting but smelly job!), finding other scavengers that would also be present on a carcass including brittle stars, snails and amphipods. However, he has also found freshly digested seal meat with no other scavenging fauna, suggesting that Greenland sharks are also capable hunters. They may be slow, but there are other ways to score a meal than chasing it down. One theory is that Greenland sharks sneak up on seals while they’re sleeping. Julius has also found sharks with bellies full of cod, after finding them in amongst aggregations of the fish. This suggests that Greenland sharks are capable of short bursts of intense activity, followed by long periods of rest.
So many mysteries still surround the Greenland shark, and solving them could help safeguard the species [1.04]. One of the main threats to Greenland sharks is bycatch, which could be a serious problem if it overlaps with areas where larger individuals are found. Larger sharks are at a later stage of maturity, which if they are caught could disrupt the reproductive cycle.
ABOUT OUR GUEST
DR JULIUS NIELSEN
Julius Nielsen is a marine biologist who has worked for the Greenland Institute of Natural Resources for many years, as a researcher specialising in the deepsea fishes of eastern Greenland. For the past 10 years he has researched multiple aspects of the Greenland shark (Somniosus microcephalus). This encompasses satellite tracking, age investigation, feeding ecology, reproductive biology and genetics which has been carried out in close collaboration between University of Copenhagen, Greenland Institute of Natural Resources and UiT the Arctic University of Norway.
For his PhD, Julius used radiocarbon dating of the eye lens to estimate the longevity of the Greenland shark. He has since published in the journals Science, Frontiers in Marine Science and Plos One, and his work has been featured by National Geographic, Discovery Channel, The New Yorker and The Guardian.
You can follow him on twitter and Instagram: @juniel85
