You’d be forgiven for mistaking it for a mouldy sausage that got lost under the sofa for several years, but there’s more to the Greenland shark than meets the eye.
Cruising through the cold, dark waters of the Arctic deep sea, this blotchy grey shark is the longest-lived vertebrate on the planet. Scientists have long believed that these ancient creatures – which can live for around 400 years – are almost blind, but a new study challenges that idea.
The idea that Greenland sharks have poor vision “is because of their extremely long lifespan (some of the sharks swimming around today are older than Isaac Newton’s laws of motion!) and little tassel-like parasites that hook onto the front of their eyes,” says Lily Fogg, a researcher at the University of Basel. “What we found is, basically, the opposite.”
Working with Walter Salzburger at the University of Basel and Dorota Skowronska-Krawczyk at the University of California Irvine, among other colleagues, they examined the sharks’ eyes and were stunned by what they saw.
“Their eyes and retinas are actually intact and highly specialised for life in very dim light,” says Fogg. “Even in individuals over a century old, the cells and molecular machinery needed for vision are still there and still look healthy.” The findings are published in Nature Communications.
Previous research has suggested that Greenland sharks had impaired vision, so Fogg wasn’t expecting to discover anything remarkable. “Then we looked at the eyes and realised they were still beautifully intact, even after more than a hundred years,” she says. “That was the moment we realised the Greenland shark had been seriously underestimated.”
Unlike humans, who have rods (for seeing in dim lighting) and cones (for bright light) in their eyes, these sharks only have rods.
This suggests that they likely don’t see much detail or colour, explains Fogg. “A good comparison is how humans see at night when we get up without turning on the light. You can’t see colours or fine detail, but you can still make out shapes and navigate your surroundings,” she says.
“My guess is that’s probably similar to what the Greenland shark experiences. It might not sound like much, but if you live most of your life in near darkness, it’s exactly what you need.”
How these ancient sharks retain their vision over their long lifespan isn’t yet fully understood, but the researchers have a theory that they want to dive into further.
"One of the big problems with ageing tissues is that DNA gets damaged over time, which eventually leads to cell death,” says Fogg. “Our suspicion is that [Greenland sharks] may have evolved a very effective system for repairing DNA, almost like a 'super DNA repair' kit.”
As well as being fascinating, this surprising discovery could benefit human healthcare. “Just think about how many people experience deteriorating eyesight every year,” says Fredrik Myhre, a marine biologist at WWF-Norway who wasn’t involved in the study. “Nature can help us with solutions to human-related health challenges.”
But protecting mysterious species such as Greenland sharks is not just important because humans might benefit. Learning more about this enigmatic fish “would enable us to protect the species more effectively against overfishing,” says Myhre.
Even though most of us will never see a Greenland shark in our lifetime, we don’t want them to disappear from our waters. “We need to take better care of the ocean’s sharks if we are to have healthy and well-functioning seas in the future,” he adds. “Humans are completely dependent on the ocean, and therefore also dependent on sustainable shark populations.”
Top image: Greenland shark in Arctic waters off the coast of Greenland. Credit: Ghislain Bardout from the Under the Pole expedition.
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