The Greenland shark isn't blind after all, despite spending centuries in dark water and having severe eye parasites. In fact, its retina doesn't seem to degrade at all

“You see it move its eye,” says Dorota Skowronska-Krawczyk, an associate professor at UC Irvine. “The shark is tracking the light — it’s fascinating.”

The Greenland shark is the longest-living vertebrate we know of, with a lifespan of over 400 years. It spends those years in the freezing, pitch-black depths of the Arctic, where vision is rarely useful. Furthermore, a Greenland shark often has a grotesque, pinkish parasite called Ommatokoita elongata dangling from its cornea. For decades, scientists looked at this scarred swimmer and assumed that it must be blind.

Now, a new study published in Nature Communications says that’s not the case. By mapping the genome and analyzing the eye tissue of these centenarians, researchers have discovered that not only can these sharks see, but their eyes are marvels of biological engineering that show almost no signs of aging.

Built for the Dark

To understand why this is such a big surprise, you have to know where this creature lives. They patrol the Arctic and North Atlantic at depths approaching 3,000 meters. That’s well in the “aphotic” zone, where there’s virtually no light around, and they rarely come to the surface. But that’s not the only weird thing about them.

Greenland sharks can measure up to 6.4 m (21 ft) and weigh more than 1,000 kg (2,200 lb). They reach sexual maturity around 150 years of age and give birth after pregnancies lasting 8 to 18 years. They’ll eat pretty much whatever they can get. As the researchers found after analyzing their eyes, their vision is also unusual.

In most vertebrates, vision is a trade-off between two types of photoreceptor cells: cones (for color and bright light) and rods (for dim light).

The research team, led by scientists including Lily G. Fogg and Dorota Skowronska-Krawczyk, found that the Greenland shark has gone all-in on the rods. Their retina only has rods, no cones. They simply don’t need them.

Instead, their eyes are packed densely with elongated rods designed to catch every stray photon in the twilight zone. The visual pigment in these rods, rhodopsin, has evolved a “blue-shift,” meaning it is tuned perfectly to the specific wavelength of blue light that penetrates the deep ocean. They can see in what is absolute darkness for most creatures, and that’s how they keep their eyes active enough. Furthermore, they have a DNA repair mechanism that enables them to maintain their vision over centuries with no sign of retina degradation.

But What About the Parasite?

Skowronska-Krawczyk was inspired to study Greenland sharks after reading a 2016 research paper published in the journal Science.

“One of my takeaway conclusions from the Science paper was that many Greenland sharks have parasites attached to their eyes — which could impair their vision,” she says. “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.”

The researchers tested this by measuring light transmission through infected corneas and compared them to clear human corneas. Even with the parasite, the shark’s cornea allows plenty of light to reach the retina. The shark can’t fend off the parasite, but it’s adapted to it.

But perhaps the most startling finding is how long these sharks keep seeing.

In the animal kingdom, eyes are usually the first thing to go. Retinal degeneration is a hallmark of aging. If a human lived to be 400 years old, calculations suggest they would have lost over 90% of their rod photoreceptors due to natural decay. We lose roughly 0.2% to 0.6% of these cells every year. The Greenland shark, however, ignores this rule. When the researchers examined the retinas of sharks estimated to be over a century old, they found no obvious signs of degeneration.

In addition to all this, the shark has one more ace up its sleeve: grease.

A Triumph of Biochemistry

Living in sub-zero water usually turns cellular membranes into stiff, brittle sheets. Think of how butter becomes rigid in the fridge. For eyes to work, the membranes in the retina need to be fluid, so proteins can move around and transmit signals.

The sharks’ retinas are loaded with Very-Long-Chain Polyunsaturated Fatty Acids (VLC-PUFAs), specifically DHA (docosahexaenoic acid). The Greenland shark’s retina contains 41% DHA, compared to just 26% in a cow’s retina. These unique fats act as a molecular antifreeze, keeping the membranes fluid and flexible despite the crushing cold. It is a perfect biochemical adaptation that neatly resolves a challenging issue.

Of course, you’re bound to get some notable adaptations when you live for so long in such unfriendly environments. But the shark’s vision is so impressive it even challenges our fundamental understanding of biological aging.

We often view aging as an inevitable accumulation of damage: cells break, DNA frays, and systems fail. But the Greenland shark suggests that with the right genetic toolkit, this decay is not mandatory.

“Not a lot of people are working on sharks, especially shark vision,” says Emily Tom, a UC Irvine Ph.D. student and physician-scientist in training who works in Skowronska-Krawczyk’s lab. “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.”

The study was published in Nature Communications.