Greenland Sharks Keep Sharp Eyes for Centuries in the Dark

Scientists have found that Greenland sharks, which can live more than 400 years in the cold, dark waters of the Arctic and North Atlantic, keep their vision working well for centuries. These large fish, often covered in eye parasites and thought to be blind, actually have eyes built for low light with no signs of breakdown even in old age. The work comes from teams at the University of California, Irvine and the University of Basel in Switzerland, who looked at shark eyes from specimens around 100 to 150 years old.

Background

Greenland sharks swim in waters 200 to 1,000 meters deep, where almost no light reaches. For years, people thought these sharks could not see because of the darkness and parasites stuck to their corneas. These parasites cloud the eyes, and the sharks move slowly, adding to the idea they rely on smell or touch to hunt seals, fish, and squid.

But recent studies changed that view. Researchers took eyes from sharks caught in ongoing projects. They checked the tissue under microscopes, read the genes, and tested how light hits the cells. What they saw surprised them: the retinas looked perfect, with long rod cells packed tight for catching faint blue light that filters down from above.

These sharks grow slow and reach sexual maturity late, around 150 years. One shark dated back to the 1600s through eye lens tests. Living so long in harsh cold, with low oxygen and pressure, should wear down their eyes like it does in humans and other animals. Yet the sharks show no wear.

Key Details

The shark eyes have rods that stretch long and thin, perfect for dim settings. These cells hold a pigment shifted to grab blue light better than sharks from shallow seas. Cone cells, used for color and bright light, do not work; their genes are broken or silent. That makes sense for a life in blackness.

DNA Repair Holds Eyes Together

The big find sits in the genes for fixing DNA damage. Shark retinas pump out high levels of repair tools, like the ERCC1-XPF complex. This setup stops the slow loss of light-sensing cells that happens in aging humans. In people, rods drop by 0.2% to 0.6% each year. Over 400 years, that would leave almost no vision. But shark eyes from century-old fish look young and strong.

Tissue slices show all key cell types: rods, support glia, and nerve cells to send signals to the brain. No dead zones or scars mark the retina. Parasites hurt the front of the eye but leave the back unharmed.

"The absence of obvious signs of retinal degeneration is remarkable given that even in healthy aging, vertebrate retinas undergo progressive photoreceptor loss and DNA damage over time." – Dorota Skowronska-Krawczyk, UC Irvine

Researchers tested opsin proteins, the light catchers in rods. They work in lab dishes just like in fresh-caught sharks. The full genome shows vision genes active, not lost or junked.

Sharks in the study were young adults by their standards. Older ones might hold up even better, or show limits. Still, keeping sight sharp past 100 years beats most animals.

What This Means

This work points to ways eyes fight time. The DNA repair in sharks could guide treatments for human eye woes like macular degeneration or glaucoma. Those hit millions as rods and nerves fade with age. If we learn how sharks keep cells safe, drugs or gene tweaks might slow human vision loss.

The findings also shift how we see deep-sea life. Sharks use sight as one tool among senses to spot prey in faint glows. Evolution tuned their eyes for endurance over speed.

Scientists now hunt more shark eyes from old fish to check if repair holds for 300 or 400 years. Lab tests on the repair genes in mice or human cells could speed medical uses. Funding stays key since shark work needs boats, labs, and rare samples.

"We can learn so much about vision and longevity from long-lived species like the Greenland shark, so having the funds to do research like this is very important." – Study coauthor

Broader questions rise on aging. Why do some animals outlast others? How does cold slow damage? Sharks offer a live lab for answers that might help people live healthier longer, not just see better.

Teams plan to map more genes and test repair paths in other long-livers like whales or clams. Each step builds on shark eyes to unlock human health secrets hidden in ocean giants.