Greenland sharks are the longest-living vertebrate known to science, topping out at more than 400 years old, and scientists have largely believed they were nearly blind. But new research suggests they actually can see, and, remarkably, maintain their vision for more than a century.
Greenland sharks (Somniosus microcephalus) mostly live in the cold waters of the Arctic and North Atlantic, in the ocean’s dimly lit twilight zone, at depths of 200-1,000 meters (660-3,300 feet). Their dark habitat led scientists to believe that the sharks could barely see. Many Greenland sharks have also been found with parasites in their eyes, raising the possibility they may even be blind.
Lily Fogg, who researches fish vision at the University of Basel in Switzerland, told Mongabay in a video call that shark biologist John Fleng Steffensen approached her to study the Greenland shark’s vision.
Fleng Steffensen originally discovered Greenland sharks’ incredible longevity, and had 10 shark specimens from an ongoing study. “He said, ‘I’ve got these eyes, would you like to do a study on them?’ And we said, ‘Why not? That’s a great opportunity.’ If they’re going in the bin, then that would just be a waste,” Fogg said.
So, Fogg and her team synthesized the shark’s genome and found that the genes involved with vision were still intact and functioning.
The team also looked at cross sections of the sharks’ eyes to see if the structure of the tissue was degraded. “We found that it’s actually beautifully intact,” Fogg said.
Furthermore, the researchers found the structure of the eyes is consistent with what they expected for an animal that spends most of its life in very dim environments.
Human eyes, for example, have two kinds of photoreceptors: cones, which function well in bright light, and rods, which are sensitive to low light and help us see at night. Greenland sharks only have rods, the study found. “They didn’t have any cones at all,” Fogg said.
While the findings suggest Greenland sharks can see, they likely don’t see well, Fogg said. “It’s not really sharp … and that’s normal for species that live in dim habitats.”
Fogg added the sharks may use their vision to tip them off to the location of prey, which they then hunt with other senses.
The Greenland sharks in the study were all between 100 and 150 years old, barely reaching sexual maturity and considered young for the species. Fogg said it would be interesting to test some older sharks to see if their vision still holds up centuries later.
Still, being able to preserve vision for more than a century is no easy feat. Fogg said the sharks appear to have a robust system for DNA repair, which might play a role and could have implications for biomedical research aimed at preserving long-term vision in humans.
