Dissected Greenland shark eyeballs could help humans see forever
The Greenland shark (Somniosus microcephaly) is renowned for its impressive lifespan. Marine biologists believe that the world’s oldest vertebrate often reaches more than 400 years old, and perhaps even longer. But even though the shark isn’t known for its vision, the eyes of the deep-sea predator could teach us a lot.
According to new research recently published in the journal Natural communicationsThe Greenland shark has retained its visual organs throughout millions of years of evolution for a reason. The explanations for why his eyes stayed the same could help experts better understand and treat debilitating eye diseases in humans.
Never mind eyeball parasites
Greenland sharks are as fascinating as they are impressive. The fish can easily measure 20 feet long and weigh more than 2,000 pounds, although it may take some time to reach such a size. The shark does not reach sexual maturity until around 150 years old and often lives for at least three more centuries. All this time also means these fish aren’t in much of a hurry to give birth to the next generation of Greenland sharks. Pups are only born after eight to 18 years of gestation, after which they will spend the rest of their lives swimming in the frigid waters of the North Atlantic and Arctic Ocean at depths as low as 3,900 feet.
Increased vision is not as serious a problem in such a dark environment, which is why so many deep-sea creatures are functionally blind. For decades, many scientists also assumed that this was the case for the Greenland shark. Although it still features baseball-sized eyeballs, they are opaque and often obscured by a small parasitic crustacean called a copepod (Ommatokoita lying down) which attaches to the organs. But after viewing video footage of live specimens and examining lab-collected eyes, biologists now think there’s more to see in the Greenland shark, so to speak.
“You see him moving his eyes,” physiologist and study co-author Dorota Skowronska-Krawczyk recounted in an accompanying profile from the University of California, Irvine. “The shark follows the light, it’s fascinating.”
Skowronska-Krawczyk first became interested in the Greenland shark’s vision about six years ago after reading a study published in the journal Science.
“One of my takeaways…was that many Greenland sharks have parasites attached to their eyes, which could impair their vision,” she said. “From an evolutionary perspective, you don’t keep the organ you don’t need. After watching a lot of videos, I realized that this animal was moving its eyeball toward the light.”
A 200-year-old eyeball
After organizing an international team of researchers, including marine biologists from the University of Copenhagen working off Greenland’s Disko Island, the scientists captured Greenland shark specimens during several expeditions between 2020 and 2024. They then dissected and preserved individual eyeballs before shipping them to Skowronska-Krawczyk’s laboratory.
“I opened the package and there was a giant 200-year-old eyeball sitting on dry ice looking 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 his colleagues carefully analyzed the organs. They noted visual adaptations typical of deep water, such as the presence of longer, densely packed rods, photoreceptors designed to function better in low light conditions. However, they also discovered other unexpected features.
“[We] confirmed the presence of essential visual cell types: rods, MĂĽller glial cells, and bipolar, amacrine, and ganglion cells,” the team wrote in their study. “Furthermore, despite being over a century old, the specimens examined showed no obvious signs of retinal degeneration.”
See with shark vision
Although early in their research, Skowronska-Krawczyk and her team believe the Greenland shark evolved a DNA repair mechanism to protect its vision for hundreds of years. They also believe that a better understanding of the complexity 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 eye cells and tissues for centuries could help develop therapies for diseases such as glaucoma and macular degeneration that often develop later in life. This information could also inform our broader understanding of the evolution of vision in the animal world.
“Not many people work on sharks, especially their vision,” Tom said. “We can learn a lot about vision and longevity from long-lived species like the Greenland shark. »
All of this research typically requires federal funding, which is increasingly difficult to obtain in today’s cultural climate.
“Having the funds to conduct research like this is very important,” Tom added.
Despite the uncertainties, Skowronska-Krawczyk said that, as with the Greenland shark, it is important to see the big picture and survive daily threats.
“We will overcome,” she promised.
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