The Greenland shark (Somniosus microcephalus) is well-known for its impressive lifespan. Marine biologists believe the world’s longest-living vertebrate often reaches over 400 years old, and possibly lives even longer. But while the shark isn’t known for its vision, a lot could be learned from the deep-sea predator’s eyes.
According to new research recently published in the journal Nature Communications, the Greenland shark retained its visual organs throughout millions of years of evolution for a reason. The explanations of why its eyes have stayed the same may help experts better understand and treat debilitating ocular diseases in humans.
Never mind the eyeball parasites
Greenland sharks are as fascinating as they are impressive. The fish can easily measure 20 feet long and weigh over 2,000 pounds, although it may take them a while to reach such a size. The shark doesn’t reach sexual maturity until around 150 years-old, and often lives for at least another three centuries. All that time also means that these fish are not in a huge rush to birth the next generation of Greenland sharks. Pups are only born after eight to 18 years of gestation, after which they’ll spend the rest of their lives swimming through the frigid North Atlantic and Arctic Ocean waters at depths as low as 3,900 feet.
Heightened vision isn’t as big of an issue in such a dim environment, which is why so many deep-sea creatures are functionally blind. For decades, many scientists also assumed this was the case for the Greenland shark. Although it still features baseball-sized eyeballs, they’re opaque and often obscured by a small parasitic crustacean called a copepod (Ommatokoita elongata) that attaches to the organs. But after reviewing both video footage of live specimens as well as examining harvested eyes in a lab, biologists now think there’s more to see in the Greenland shark, so to speak.
“You see it move its eye,” physiologist and study co-author Dorota Skowronska-Krawczyk recounted in an accompanying University of California, Irvine profile. “The shark is tracking the light—it’s fascinating.”
Skowronska-Krawczyk first became interested in the Greenland shark’s vision around six years ago after reading a study published in the journal Science.
“One of my takeaway conclusions…was that many Greenland sharks have parasites attached to their eyes—which could impair their vision,” she said. “Evolutionarily speaking, you don’t keep the organ that you don’t need. After watching many videos, I realized this animal is moving its eyeball toward the light.”
A 200-year-old eyeball
After organizing an international team of researchers, including University of Copenhagen marine biologists working off the coast of Greenland’s Disko Island, scientists caught Greenland shark specimens during multiple expeditions between 2020 and 2024. They then dissected and preserved individual eyeballs before shipping them to Skowronska-Krawczyk’s lab.
“I opened the package, and there was a giant, 200-year-old eyeball sitting on dry ice just staring back at me. We’re used to working with mouse eyeballs, which are the size of a papaya seed,” said study co-author and Ph.D. candidate Emily Tom.
After carefully thawing the eyes, Tom and colleagues carefully analyzed the organs. They noted typical deep-sea visual adaptations like the presence of densely packed, lengthier rods—photoreceptors designed to function better in lower light. However, they also found other unexpected features.
“[We] confirmed the presence of essential visual cell types: rods, Müller glia, and bipolar, amacrine, and ganglion cells,” the team wrote in their study. “Moreover, despite being over a century old, the examined specimens showed no obvious signs of retinal degeneration.”
Seeing in shark vision
Although early in their research, Skowronska-Krawczyk and her team believe that the Greenland shark has evolved a DNA repair mechanism to guard their vision over hundreds of years. They also believe a better understanding of the complexities of the shark’s eyes could lead to new treatments for age-related vision problems in humans. For example, discovering how a Greenland shark maintains its healthy ocular cells and tissues for centuries may help develop therapies for diseases including glaucoma and macular degeneration that often develop later in life. The information could also inform our wider understanding of vision evolution across the animal world.
“Not a lot of people are working on sharks, especially shark vision,” said Tom. “We can learn so much about vision and longevity from long-lived species like the Greenland shark.”
All that research generally requires federal funding—something that’s increasingly difficult to secure in the current cultural climate.
“Having the funds to do research like this is very important,” added Tom.
Despite the uncertainties, Skowronska-Krawczyk said that, like the Greenland shark, it’s important to see the bigger picture and outlast day-to-day threats.
“We will prevail,” she promised.
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