The Greenland shark (Somniosus microcephalus) can be described as the Methuselah of the sea. With a lifespan estimated at up to 400 years or more, these Arctic and North Atlantic giants have captured human’s attention for their remarkable longevity. But until recently, one mystery lingered: can these ancient sharks actually see!? After all, these predators inhabit deep, cold, dark waters — some as frigid as 30°F (-1.1°C) and as deep as 9,800 feet (3,000 meters) — and many have parasitic copepods attached to their corneas and sometimes even their butts. Add in centuries of aging and you might assume that heir eyes have long since lost their function. In fact, their infamous eye copepod (scientifically known as Ommatokoita elongata) is thought to destroy the corneal tissue, resulting in eventual partial blindness.
But new research combining genomic sequencing, retinal imaging, and molecular analysis tells a different story: Greenland sharks can still see, and their vision is surprisingly well-preserved despite being centuries old.
Unlike animals that evolve in total darkness, like cavefish, and lose their vision altogether, Greenland sharks retain a full suite of retinal structures. The researchers, led by Lily Fogg at the University of Basel, Switzerland, found densely packed, elongated rods in their eyes (structures specialized for low-light vision). Fogg and her colleagues found that the rods are not only present… they are fully functioning! Histological examination showed no signs of retinal degeneration, even in individuals likely several hundreds of years old. That’s notable because in most long-lived species, including humans, age-related eye deterioration is expected. But Greenland sharks seem to defy this biological rule.
So what’s their secret? Well, it may lie in the type of vision they rely on. Like many deep-sea creatures, Greenland sharks have a rod-dominated retina. Rods are the photoreceptors responsible for scotopic, or low-light, vision. They’re highly sensitive and can detect minimal light,which is ideal for life in the dim, murky deep sea. In contrast, cones — photoreceptors that handle bright-light and color vision — are either rare or nonfunctional in this species. The genetic analysis confirmed this. Nearly all cone-specific vision genes were either missing or had turned into pseudogenes, while the rod-specific genes were intact and actively expressed. It’s a clear case of evolutionary prioritization: keep what’s useful, discard what’s not.
One of the biggest surprises for the team came from looking at how well these rod-based systems are maintained over time. Using retinal RNA-sequencing, they found that not only are the essential vision genes still active, but the cells themselves (the rods, ganglion cells, bipolar and amacrine cells, and supportive Müller glia) are all present and structurally sound! The architecture of the retina remained completely intact across multiple specimens. For an animal potentially older than the United States… that’s an astonishing biological achievement.
How, exactly, is this possible? One clue may lie in the shark’s ability to maintain genomic stability over centuries, as the study found signs of robust DNA repair mechanisms at play in the retina. While efficient DNA repair is a hallmark of longevity in several organisms, in the Greenland shark it may be especially important for preserving retinal health in such extreme conditions. Cold water could also play a role by slowing metabolic processes and reducing cellular wear and tear, but it’s likely a combination of both genetics and environment that allows their eyes to remain functional for so long.
While Greenland sharks may still rely on vision to some extent, it’s not perfect. Hard to be when some individuals have parasites attaches to their corneas! But Ommatokoita elongata doesn’t appear to fully blind the sharks. Their optic tectum (a brain region involved in processing visual information) is still comparable in size to that of other sharks, which suggests that sight remains behaviorally relevant. The sharks also retain a tapetum lucidum, a reflective layer behind the retina that boosts their ability to capture light. You might’ve seen this in the eyeshine that cats, dogs and nocturnal animals sport when you shine a light near or on their eyes in the dark.
Altogether, this new study pushes back against the idea that longevity and vision are incompatible in deep-sea environments. And unlike species that evolved to shed unnecessary traits, the Greenland shark has kept its ability to see intact, even if it is modified for its shadowy world. “Together, these findings highlight the extraordinary adaptability of vertebrate sensory systems in extreme environments and the remarkable preservation of organ function over hundreds of years,” the team concludes.
