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Greenland sharks can live for hundreds of years, drifting through some of the coldest and darkest waters on Earth. Once dismissed as slow, clumsy and nearly blind, these deep-sea giants are now giving up new secrets – at a time when climate change and commercial fishing are encroaching further on their world.
Some of the Greenland sharks swimming today were alive during the French Revolution, and a few may even date back to the time of Shakespeare. Yet despite their age, scientists still know remarkably little about them. Where they reproduce, for example, remains a mystery. So does their number, and how their populations are changing.
For decades, these sharks were believed to be almost blind. Their eyes, often cloudy and covered in parasites, helped reinforce that view. Along with their slow movements and life far below the surface, this fed the idea of a sluggish scavenger drifting through the darkness.
That assumption has now been turned on its head. In research published in January, scientists examining Greenland sharks estimated to be 100 to 134 years old discovered that their eyes showed no signs of the damage normally associated with ageing.
“Usually tissue just kind of degrades over time. But we found evidence that there is a functional visual system in the Greenland shark, and it seems to be really well adapted for life in dim light,” Lily Fogg, a marine biologist at the University of Basel, who led the research, told RFI.
“With ageing, the DNA in the cell usually starts to break. So we did a test and we couldn’t find any evidence for it. This suggests there’s no ongoing cell death in the eye, which is quite incredible for an animal that’s over a century old.”
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Hidden lives
Despite their name, Greenland sharks are not geographically confined to Greenland. They roam both the Arctic and Atlantic oceans in waters that remain near freezing. Their presence has been observed from the surface down to depths of more than 2 kilometres.
Fully grown, the sharks can reach up to 7 metres in length and weigh more than a tonne. Much of what scientists know comes from animals caught accidentally in fishing gear.
Studying them under controlled scientific conditions, however, is challenging. Their habitat is remote, research expeditions are expensive and handling animals of such size is a daunting task in itself.
However, the research on their eyes offers a clue as to why these animals remain so mysterious: a visual system that works for more than a century doesn’t evolve quickly, and neither does anything else about them. Greenland sharks are fine-tuned for stability.
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The cost of longevity
Greenland sharks grow extremely slowly, and scientists believe their gestation period may last between eight and 18 years, although firm data is lacking. The last pregnant Greenland shark documented was caught back in 1950, and more than 70 years later scientists have yet to discover where they breed.
They do know, however, that they produce very few offspring.
“They live a very, very long life. But this life is also linked to a very, very late sexual maturity – about the age of 150 years,” explained Alessandro Cellerino, an evolutionary biologist at the Scuola Normale Superiore in Italy.
He’s part of the team that sequenced the Greenland shark’s genome in an effort to better understand how an animal can live for centuries.
While living for so long may sound like an advantage, it also comes with risks.
Cellerino says the sharks likely inhabit the entire bottom of the ocean, an icy abyss where temperatures remain more or less constant – meaning they could simply retreat to deeper waters as surface conditions change.
“It is very difficult for us to foresee what the effect of climate change could be on this specific species,” he told RFI. “Unless their reproduction grounds are in regions that are getting warmer, which we don’t know.”
Their slow biology also means the species may take generations to recover from population losses. A shark caught before it reaches maturity never reproduces. Even small losses can echo for generations.
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A fast-changing world
When an animal’s environment shifts in the space of decades, that kind of biology can become a weakness – and the pressure on Greenland sharks is growing.
As Arctic sea ice contracts, previously inaccessible waters are opening to commercial fishing, exposing the sharks to greater bycatch risk. Around 3,500 are caught each year in nets set to catch cod and halibut.
Scientists do not know how many survive after being released.
“For any species, the rapid human-caused changes to the planet are going to present nearly unprecedented challenges,” Catherine Macdonald, a marine ecologist at the University of Miami, told RFI.
“But for Greenland sharks that have such long generation times, those challenges are going to be even greater because the timescales on which evolutionary processes can act are so much longer.”
Even low levels of mortality can have serious effects on a population that replaces itself so slowly. “It takes so long for adults to mature that the loss of reproductive adults is going to be really harmful,” Macdonald said. In 2020, the Greenland shark was listed as Vulnerable on the IUCN Red List.
Scientists encounter Greenland sharks in Arctic waters partly because shrinking sea ice changes where both animals and researchers can operate.
With researchers studying the places that are easiest to access rather than those where animals may actually spend most of their lives, Macdonald compares deep-sea research to “looking for lost keys under a street lamp, because it’s light there”.
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