The Greenland shark’s scientific name is Somniosus microcephaly, which means ‘sleepy small brain’. They live in the cold waters of the North Atlantic and Arctic Oceans, and are members of the family Somniosidae – the Sleeper Sharks. This name implies their slow growth and low levels of activity. In some ways they live up to their name, but in many other ways they are anything but sleepy, small-brained creatures!
How old is old?
These sharks were known by Greenlanders to grow slowly and the fact that they can reach lengths of 5.5 metres implied they may also be very old. It wasn’t until a few years ago that a Danish team led by Prof John Steffensen was able to confirm their extreme longevity, and publish the findings last year in the journal Science.
Calculating the age of a cartilaginous shark is more complicated than it is for a bony fish. This is because bony fish have otoliths, bones of the inner ear which grow in rings, much like that of a tree. By counting the rings of the otolith, you can determine the fish’s age. Cartilaginous fish like sharks (and rays) do not have otoliths, nor do they have any other true bones.
To overcome this challenge, John and his team used carbon dating techniques to determine the age of 28 female sharks (81 to 502 cm total length) collected during expeditions to Greenland between 2010 and 2013. Carbon dating is famously used to determine the age of fossils, but they used this same technique on the carbon in lenses in the sharks’ eyes. This carbon comes from the “bomb-pulse” that entered the ecosystem following the nuclear tests of the 1950s. The carbon in body parts formed during that time is in a different form.
From these sharks, growth curves were established linking a shark’s age to its size. Using these, we can now calculate the age of live sharks from their measurements, which we took on the expedition. Current estimates suggest that the average Greenland shark grows less than one cm per year. That makes animals longer than 5 metres between 275 and 510 years old!
How does living so long affect the shark’s bodies?
Aging is decay, at least in humans. Indeed, the biggest risk factor for a large number of diseases like heart disease and cancer is advancing age. So how is it that the Greenland shark can live for so long?
At the moment, we know very little. Very early evidence from previous work suggests these sharks do not have any special strategies for surviving damage from free radicals. Another health concern specific to top oceanic predators is accumulation of toxic substances in their bodies. However, recent work on accumulation of organic pollutants does not suggest that they accumulate with age in the Greenland Shark either. We hope to learn much more about how these sharks age as we process the samples from our expedition. For instance, Takuji Noda, Bob Shadwick and Diego Bernal are studying movement and skeletal muscle function in sharks of different ages. This could tell us whether Greenland sharks deal with frailty, another plague of human aging.
Some aspects of a shark’s long life bear no comparison to humans. Although dimming eyesight might feel like an inevitable part of aging to us, Greenland sharks contract a parasite on their cornea, so they may need more than a pair of reading glasses as they age! A 4-6 cm long crustacean dangling from the centre of a shark’s eye is off-putting, but also intriguing. It raises obvious questions about how the parasite affects vision and how the fish survives with potential vision-impairment. General consensus is that the parasite severely impairs the ability of the eye to form images, but how important is vision in animals living 1 kilometer below the surface? Perhaps they rely more on their other senses, like smell. In any case, gut contents show these sharks eat everything from small fish to whole seals, suggesting they forage successfully even with a large crustacean dangling from their eye!
Conserving this old shark
The Greenland shark is slow-growing and thus also slow to reach reproductive age. A member of our team on the expedition, PhD student Julius Nielsen, from the University of Copenhagen has been trying to understand how body length relates to reproductive maturity in both males and female sharks. Their current estimate is that females reach sexual maturity at lengths of over 400 cm, which makes them at least 150 years old. Males, who are smaller than the females, may reach reproductive maturity at slightly shorter lengths.
This slow maturation means it takes at least 150 years for shark pups to start reproducing! This has big implications for the population, and for conservation. Indeed, the Greenland shark population may still be recovering from being over-fished before World War II, when their livers were used for machine oil. Calculating back from the amount of machine oil produced suggests that between 50,000 and 150,000 animals were caught per year between 1900 and 1938. Prices of shark-liver oil fell in 1949 as other options became viable, and the fishery then collapsed. Hatchlings born to parents who were caught for their livers are now teenagers and still not quite at reproductive age. That may be the reason why there are very few reports of juvenile Greenland sharks.
Slow growth leading to late reproductive maturity is a big factor for conservation strategies in this animal. At present, the Greenland shark is a common by-catch, meaning it’s unintentionally caught along with other fish, in the North Atlantic and Arctic Oceans. This suggests that its population is still quite strong. But as it takes so long to increase the population of adult animals, they are vulnerable to increased fishing pressure in a warming Arctic environment. Thus they are listed as near-threatened on the ICUN list and there is a growing need to understand their physiology, life history and their role as top predator in the Arctic marine ecosystem.
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This expedition was made possible by funding from the Danish Centre for Marine Research, the Greenland Institute of Natural Resources, The Danish Natural Science Research Council and the Carlsberg Foundation.