It sounds like the start of a grade school joke, but it’s not. There’s this shark in Greenland that lives a really long time – like centuries. And it does so in almost total darkness. It doesn’t help that parasitic copepods often make their home on the sharks corneas. So, for decades, scientists figured that this species – Somniosus microcephalus – could give Mr. Magoo a run for his money.
Well, the punchline might need a rewrite.
In research published in Nature Communications, an international team of scientists reveals that this odd little predator boasts a fully intact, operational visual system. And it appears that evolution has tweaked it to perform in extreme low light while remaining remarkably resistant to age-related degeneration.
Obviously, this revelation challenges the conventional wisdom about sensory decline in extreme environments. It also sparks new questions about how some vertebrates preserve neural function across extraordinary lifespans.
A Retina Built for Darkness – and Endurance
The Greenland shark is the longest-living vertebrate that we know of. Radiocarbon dating of its eye tissue hints at lifespans that flirt with around 400 years. It lives its life in the coldest, deepest waters on the planet. It’s a desolate place that sunlight barely touches. And it’s an environment that takes its toll on other species.
But when researchers took a closer look at the retinal tissue of adult Greenland sharks, they found no signs of the retinal degeneration typical of aging vertebrates.
Histological analyses revealed a retina composed entirely of rod photoreceptors. These rods were densely packed and elongated, a configuration typical of deep-sea species that have to make the most of photon capture.
What’s more? All of the major retinal layers remained intact, including those responsible for transmitting visual signals to the brain. And tests designed to identify DNA fragmentation uncovered no evidence of ongoing retinal cell death.
“In humans, rods are lost steadily with age,” the authors note. “At known rates of decline, a human living for several centuries would lose the majority of their photoreceptors.”
The Greenland shark, in defiance of that logic, appears to maintain retinal integrity.
Methodology
To better understand this durability, the team combined whole-genome sequencing with retinal RNA analysis. The results exposed a streamlined visual system – not a degraded one. The researchers noted that all the genes that the sharks need for rod-based phototransduction were present and actively expressed. These included the rhodopsin gene (rh1) and its supporting molecular machinery, expressed at levels comparable to other adult shark species.
In stark contrast, most genes associated with cone-based (bright-light and color) vision were either pseudogenized or not expressed at all. This genetic pattern, the researchers argue, strongly suggests that Greenland sharks rely exclusively on scotopic, or low-light, vision. It represents an efficient specialization instead of a loss of function.
The study also found that the shark’s rod opsin is unusually blue-shifted, absorbing light most efficiently at shorter wavelengths than those of shallow-water sharks. This shift matches the spectral qualities of light that penetrate Arctic waters at depth, where blue wavelengths dominate.
Seeing Despite Parasites
One of the most persistent arguments against Greenland shark vision has been the presence of parasitic copepods attached to the cornea. This, scientists have long argued, must mean that these sharks must be functionally blind.
To put that belief to the test, researchers measured how much light passes through parasitized shark corneas. The answer came as a shock.
Despite the parasites, corneal transmission ranged from roughly 70% to nearly 100%, including in the blue wavelengths most relevant to the shark’s visual pigment. Simply put, light still reaches the retina at levels consistent with functional vision.
A Molecular Clue
But the study’s most provocative insight might lie in what it reveals about aging itself.
The Greenland shark retina showed elevated expression of genes involved in DNA repair – particularly components of the ERCC1-XPF repair complex, known to protect against retinal degeneration in other species. Defects in this pathway cause premature vision loss in humans and mice.
The authors suggest that robust DNA repair may help preserve retinal neurons for hundreds of years, presenting a rare example of long-term maintenance of central nervous system tissue in a vertebrate.
This research doesn’t just show us a functional visual system. It reveals a uniquely adapted one that evolution’s stripped down to the basics and preserved. Over time.
This work challenges the argument that harsh environments and long lifespans ultimately drive sensory decline. Instead, it suggests that evolution can favor durability just as strongly as efficiency.
For scientists who spend their careers trying to find out more about aging, neurodegeneration, and sensory biology, the Greenland shark might be more than an odd tale of longevity. It might just provide a living model of how neurons – and vision itself – can last much longer than we thought.
Further Reading
At Least 1 in 5 Dementia Cases Linked to Vision Loss
