The Greenland shark (Somniosus microcephalus) is well-known for its impressive longevity. Marine biologists estimate the world’s longest-living vertebrate frequently exceeds 400 years of age, and perhaps survives even longer. But while the shark is not noted for its sight, a great deal could be gleaned from the deep-sea predator’s eyes.
Per new research recently published in the journal Nature Communications, the Greenland shark preserved its visual organs across millions of years of evolution for a tangible reason. The interpretations concerning why its eyes have remained the same may assist experts in better comprehending and treating debilitating visual ailments in people.
Never mind the eyeball parasites
Greenland sharks are as intriguing as they are formidable. The fish can readily measure 20 feet in length and weigh more than 2,000 pounds, though it may take them time to attain such dimensions. The shark does not attain sexual maturity until approximately 150 years old, and commonly dwells for at least another three centuries. All that time also means that these fish are not in a huge hurry to bring forth the next cohort of Greenland sharks. Pups are only born after an eight to 18 year gestation period, after which they’ll spend the remainder of their existence swimming through the frigid North Atlantic and Arctic Ocean depths at levels as low as 3,900 feet.
Heightened sight isn’t as vital in such a dim setting, which is why many deep-sea organisms are functionally sightless. For decades, many scientists also supposed this was the situation for the Greenland shark. Although it still possesses baseball-sized eyeballs, they’re murky and frequently obstructed by a small parasitic crustacean termed a copepod (Ommatokoita elongata) that fastens to the organs. But after reviewing both video footage of living specimens as well as examining extracted eyes in a laboratory, biologists now suspect there is more to discern in the Greenland shark, so to speak.
“You observe it shift its eye,” physiologist and study co-author Dorota Skowronska-Krawczyk stated in an accompanying University of California, Irvine profile. “The shark is following the light—it’s captivating.”
Skowronska-Krawczyk initially became intrigued by the Greenland shark’s vision around six years prior after reading a report published in the journal Science.
“One of my chief conclusions…was that numerous Greenland sharks possess parasites affixed to their eyes—which could diminish their vision,” she remarked. “Evolutionarily speaking, you do not retain the organ that you do not require. After viewing many videos, I noted this creature is moving its eyeball toward the illumination.”
A 200-year-old eyeball
After arranging an international collective of researchers, including University of Copenhagen marine biologists operating off the shore of Greenland’s Disko Island, scientists captured Greenland shark subjects during several voyages between 2020 and 2024. They then excised and kept individual eyeballs prior to dispatching them to Skowronska-Krawczyk’s facility.
“I unwrapped the container, and there was a massive, 200-year-old eyeball resting on dry ice just gazing back at me. We are accustomed to dealing with mouse eyeballs, which are the dimension of a papaya seed,” commented study co-author and Ph.D. candidate Emily Tom.
Subsequent to gently warming the eyes, Tom and her peers methodically inspected the organs. They observed typical deep-sea visual adjustments such as the presence of densely clustered, extended rods—photoreceptors engineered to work better in dimmer light. However, they also discovered other unforeseen characteristics.
“[We] verified the existence of crucial visual cell types: rods, Müller glia, and bipolar, amacrine, and ganglion cells,” the group penned in their research. “Furthermore, despite being well over a century in age, the studied examples exhibited no apparent indications of retinal deterioration.”
Seeing in shark vision
Although early in their investigation, Skowronska-Krawczyk and her group contend that the Greenland shark has developed a DNA restoration system to safeguard their sight across hundreds of years. They also posit that a deeper comprehension of the intricacies of the shark’s eyes could lead to novel treatments for age-related sight problems in people. For instance, elucidating how a Greenland shark preserves its healthy ocular components and tissues for centuries may aid in creating remedies for conditions including glaucoma and macular degeneration that frequently arise later in life. The data could additionally enhance our broader appreciation of vision development across the animal kingdom.
“Not many individuals are studying sharks, particularly shark sight,” stated Tom. “We are able to discover so much about vision and lifespan from long-lived species like the Greenland shark.”
All that investigation generally necessitates federal financing—something that is increasingly hard to obtain in the present societal atmosphere.
“Securing the resources to conduct research such as this is extremely important,” added Tom.
Despite the unknowns, Skowronska-Krawczyk mentioned that, much like the Greenland shark, it is crucial to perceive the larger perspective and endure day-to-day challenges.
About the Author
