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The 150 Year Old Virgin

In the past, they were a rather obscure species - not as cute or charismatic, nor as feared or famous as some of their cousins - but Greenland sharks (Somniosus microcephalus) have hit the news in a big way in recent years... With the revelation that they are the longest-living vertebrate on our planet, scientists and the public alike have become increasingly curious about these elusive and strangely beautiful sharks. So do Greenland sharks really live so many hundreds of years? How is it possible that they can live so long? And what does this mean for their conservation?


Scientists suspect Greenland sharks can live to 600 years of age (Image Credit: Dotted Yeti / Shutterstock)

The Green Room

Ranging throughout the bitingly old waters of northern Canada, Greenland, Iceland, to Scandinavia and Russia, Greenland sharks are the Arctic's apex pradator. Yet, unlike some of their equatorial relatives, Greenland sharks do not put on high-octane hunting displays. Mostly snacking on squid, fish and other, smaller species of sharks, these top predators are slow swimmers, only reaching maximum speeds of 2.6 km/h. They often rely on scavenging carrion and they have even been known to make hunting that little bit easier by sneaking up on their prey whilst it is asleep! Doing anything more energetic would be far too much of a strain in such cold conditions (Compagno, 1984, Watanabe et al, 2012, Neilsen, 2017).


Perfectly adapted to life in cold water, Greenland sharks are very slow-growing and have a very low metabolism. Scientists think they grow by as little as 0.5 – 1 cm each year and they only need to consume on average 193 g of prey each day to sustain themselves. In fact, they can go many weeks without eating anything at all (Augustine et al, 2017, Ste-Marie et al, 2022).


This lazy lifestyle and their remote and inhospitable habitats, mean Greenland sharks have featured in far fewer scientific studies compared to some of their cousins and there is still a lot we don't know about them. You would think they would be the star of the show considering they are actually one of the largest sharks on our planet! They can easily reach 5 metres in total length (TL) - the same size as a great white (Carcharodon carcharias). You needn't fear though - Greenland sharks' have never been recorded assaulting a human being (Neilsen et al, 2016, Stork, 2019).



Green with Envy

Greenland sharks made major headlines in recent years when scientists discovered that they are the longest-living "vertebrate" (an animal with a spinal column) on our planet. Quite a claim to fame! But how is it possible to know how old a shark is or figure out how long they can live? The answers lie in "vertebral band analysis" and "radiocarbon dating"...


Greenland sharks are very elusive, so we have very few images of them in the wild (Image Credit: Dotted Yeti / Shutterstock)

Sharks never stop growing throughout their lives. Each year their cartilagenous skeleton adds a new layer of growth over the old, so that the shark can expand. With many species of sharks this means that scientists can look at the large bones in the spine (the "vertebrae") under a microscope and count the bands of growth to determine how old the animal was when it died. Much like counting the rings on a tree (to learn more, check out What's My Age Again?). This is known as vertebral band analysis. However, this method was inaccurate for Greenland sharks (Neilsen et al, 2016, Neilsen, 2017).


So instead, researchers turned to "radiocarbon dating". Carbon isotopes (Carbon-14 for example) are incorporated into living tissues as an organism grows and then degrade at a predictable rate over time. This means we can measure the ratios of the different isotopes to determine the age of organic matter. As if this wasn't cool enough, scientists can use big events as isotopic time markers - in this case the scientists knew that a lot of Carbon-14 was produced in the 1950s and 1960s during nuclear testing programs. This meant they could use this distinctive Carbon-14 pulse as a place-marker in time to more accurately assess the sharks' ages (Neilsen et al, 2016, Neilsen, 2017).


Inspired by medical techniques developed for use on human beings, scientists chose to analyse tissues from Greenland sharks' the eyes. Because these organs develop when a shark is an embryo and then do not change throughout the their lives, radiocarbon dating their eyes meant the researchers were able to actually estimate when a particular shark was born! (Neilsen et al, 2016, Neilsen, 2017).


3D rendering of the Greenland shark (Image Credit: Dotted Yeti / Shutterstock)

The scientists discovered that the oldest Greenland sharks they assessed had lived at least 400 years! In fact, the largest animal they sampled - a 5 metre TL female - could have been more than 510 years old... meaning she would have been born before Shakespeare (Neilsen et al, 2016, Neilsen, 2017, Augustine et al, 2017).


"The Greenland shark is the longest living vertebrate animal in the world... I estimate the oldest shark to be between 272 years and 512 years."

- Neilsen, 2017


Scientists think that their adaptions to life in a cold environment, including their slow metabolism, may be the Greenland shark's secret to such a long life. Scientists are now also studying their brains and hearts to investigate why these organs don't degrade with age, as they do for so many animals (including us)... but we don't actually know why Greenland sharks are able to stay so vital for so long... we may never completely understand how it is possible for them to live for hundreds of years (Augustine et al, 2017).



The Green Light

With a lifetime spanning many centuries, Greenland sharks are incredibly slow-growing and this has implications for their reproduction. Greenland sharks are "ovoviviparous", meaning that after fertilisation their eggs hatch inside their bodies, to then be nurtured by the mother until their birth. Litters are thought to range between 200 – 324 pups per pregnancy (depending on the size of the mother), meaning that a female Greenland shark may be able to have as many as 200 to 700 pups over the course of her lifetime! (Neilsen et al, 2016).


3D rendering of the Greenland shark (Image credit: Dotted Yeti / Shutterstock)

So it sounds like they are super fertile, right!?... Maybe not!


Remember how slow-growing these sharks are? Scientists think that Greenland sharks actually do not reach sexual maturity until they are at least 150 years old! Maybe as high as almost 180! Males must reach about 2.8 metres TL and females around 4 metres TL before they are fertile. This means that these sharks have to survive for a couple hundred years before they are even capable of producing offspring (Neilsen et al, 2016).


This could have serious implications for the conservation of these fabulous animals. Greenland sharks are flagged on the the Red List of Threatened Species as vulnerable and their populations are noticably declining in the wild. This is because they are often the victims of "bycatch" in fisheries targetting other commercially valuable species. If Greenland sharks continue to die in fisheries this way, it is very unlikely that they will be able to breed quickly enough to recover their dwindling numbers. Armed with the new discoveries about their longevity, scientists and conservationists are now pushing for stricter measures to protect Greenland sharks, to ensure that they are given the chance to survive for hundreds more years to come (Kulka et al, 2020, IUCN, 2022, NAFO, 2022).



References

Augustine S, Lika K, & Kooijman SA. (2017). Comment on the ecophysiology of the Greenland shark, Somniosus microcephalus. Polar Biology, 40:12, 2429-2433. Access online.


Compagno LJV (1984). Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date. FAO Species Catalogue. Access online.


IUCN (2020). The International Union for the Conservation of Nature red list of Threatened Species. Access online.


Kulka DW, Cotton CF, Anderson B, Derrick D, Herman K & Dulvy NK (2020). Somniosus microcephalus. The IUCN Red List of Threatened Species 2020: e.T60213A124452872.


NAFO (2022). Northwest Atlantic Fisheries Organization. Report of the NAFO Commission Ad hoc Working Group to Reflect on the Rules Governing Bycatches, Discards and Selectivity (WG-BDS) in the NAFO Regulatory Area Meeting. Access online.


Neilsen J (2017). The Greenland shark (Somniosus microcephalus): Diet, tracking and radiocarbon age estimates reveal the world’s oldest vertebrate. Doctoral dissertation, University of Copenhagen. Access online.


Nielsen J, Hedeholm R, Heinemeier J, Bushnell P, Christiansen J, Olsen J, Ramsey C, Brill R, Simon M, Steffensen K & Steffensen J. (2016). Eye lens radiocarbon reveals centuries of longevity in the Greenland shark (Somniosus microcephalus). Science, 353:6300, 702-704. Access online.


Nielsen J, Hedeholm R, Lynghammar A, McClusky L, Berland B, Steffensen J & Christiansen J. (2020). Assessing the reproductive biology of the Greenland shark (Somniosus microcephalus). PLOS ONE, 15:10, e0238986. Access online.


Ste-Marie E, Watanabe YY, Semmens JM, Marcoux M & Hussey NE (2022). Life in the slow lane: field metabolic rate and prey consumption rate of the Greenland shark (Somniosus microcephalus) modelled using archival biologgers. Journal of Experimental Biology, 225:7, jeb242994. Access online.


Stork K (2019). Reproduction, distribution, and feeding ecology of the Greenland shark (Sominosus microcephalus) in relation to climate change and human activities in the Arctic. Access online.


Watanabe YY, Lydersen C, Fisk AT & Kovacs KM (2012). The slowest fish: Swim speed and tail-beat frequency of Greenland sharks. Journal of Experimental Marine Biology and Ecology. 426–427, 5–11. Access online.


By Sophie A Maycock for SharkSpeak.

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