Greenland shark (small-headed sleep bug) is known for its impressive service life. Marine biologists believe that the world’s longest-lived vertebrates often live to be more than 400 years old, and possibly even longer. While sharks are not known for their eyesight, we can learn a lot from the eyes of deep-sea predators.
According to new research recently published in the journal nature communicationsThere’s a reason Greenland sharks have retained their visual organs over millions of years of evolution. An explanation of why its eyes remain unchanged may help experts better understand and treat debilitating eye diseases in humans.
No need to worry about eye parasites
Greenland sharks are both fascinating and impressive. This fish can easily reach 20 feet in length and weigh over 2,000 pounds, although it can take them a while to reach these sizes. Sharks don’t reach sexual maturity until they are around 150 years old, and usually live for at least another three centuries. Such a long time also means the fish are in no rush to give birth to the next generation of Greenland sharks. The pups are born after a gestation period of eight to 18 years, after which they spend the rest of their lives in the frigid waters of the North Atlantic and Arctic oceans at depths of up to 3,900 feet.
Improved vision isn’t a big deal in such dim environments, which is why so many deep-sea creatures are functionally blind. For decades, many scientists also thought this was the case with Greenland sharks. While it still had baseball-sized eyeballs, they were opaque and often obscured by small parasitic crustaceans called copepods.Long-shaped stink bug) attached to organs. But after reviewing video footage of live specimens and examining harvested eyes in the lab, biologists now believe there’s arguably more to see about the Greenland shark.
“You would see it move the eyes,” physiologist and study co-author Dorota Skowronska-Krawczyk recalled in a profile at the University of California, Irvine. “The shark is following the light – it’s really fascinating.”
Skowronska-Krawczyk first became interested in Greenland shark vision about six years ago after reading a study published in the journal science.
“One conclusion I came to is that … many Greenland sharks have parasites attached to their eyes, which may impair their vision,” she said. “Evolutionarily speaking, you don’t keep organs that you don’t need. After watching a lot of videos, I realized that this animal was moving its eyeballs toward the light.”
200 year old eyeball
An international team of scientists, including marine biologists from the University of Copenhagen working off the coast of Greenland’s Disko Island, captured Greenland shark specimens during multiple expeditions between 2020 and 2024. They then dissected and preserved individual eyeballs before transporting them to Skowronska-Krawczyk’s lab.
“I opened the package and there was a giant, 200-year-old eyeball sitting on dry ice staring back at me. We’re used to using mouse eyeballs, and it was the size of a papaya seed,” said study co-author Dr. Candidate Emily Tom.
After carefully thawing the eyes, Tom and his colleagues carefully analyzed the organs. They noted typical deep-sea visual adaptations, such as the presence of dense, longer rods, photoreceptors designed to function better in lower light. However, they also discovered other unexpected features.
“[We] “The presence of important visual cell types was confirmed: rods, Müller glia, bipolar cells, amacrine cells, and ganglion cells,” the team wrote in the study. “Furthermore, despite being more than a century old, the specimens examined showed no obvious signs of retinal degeneration.”
shark vision
Although the research is in its infancy, Skowronska-Krawczyk and her team believe Greenland sharks have evolved a DNA repair mechanism to protect their vision for hundreds of years. They also believe that a better understanding of the complexities of shark eyes could lead to new treatments for age-related vision problems in humans. For example, understanding how Greenland sharks maintain healthy eye cells and tissue over centuries could help develop treatments for diseases like glaucoma and macular degeneration that often appear later in life. This information could also help us understand the evolution of vision more broadly across the animal world.
“Not many people are working on shark research, especially shark vision research,” Tom said. “We can learn a lot about vision and lifespan from long-lived species like the Greenland shark.”
All of this research typically requires federal funding — something that is increasingly difficult to obtain in the current cultural climate.
“Having the funding to conduct research like this is very important,” Tom added.
Despite the uncertainty, Skronska-Krawczyk said, like the Greenland shark, it’s important to see the bigger picture and defeat everyday threats.
“We will prevail,” she promised.
