The Greenland shark, a creature of the deep, cold Arctic and North Atlantic waters, is known for its exceptional longevity, with some individuals estimated to live for centuries. This remarkable lifespan often leads to curiosity about their sensory capabilities, particularly their vision, given the perpetually dark environment they inhabit. Are Greenland sharks truly blind? The answer involves unique adaptations and a surprising biological interaction that profoundly impacts their sight.
The Truth About Their Vision
While not completely sightless, the vision of Greenland sharks is severely compromised, often rendering them functionally blind. This impairment is primarily due to a specific parasitic copepod, Ommatokoita elongata, which commonly attaches to their corneas. This small, pinkish-white crustacean, up to 30 millimeters (1.2 inches) long, is frequently found permanently affixed to the eyes of these sharks.
The Ommatokoita elongata parasite directly damages the shark’s corneal tissue as it feeds, potentially destroying photoreceptor cells. The parasite uses hooked antennae and sharp appendages to anchor itself to the eye’s surface, and its feeding activities can lead to significant corneal opacity, scarring, and erosion. Multiple parasites can entirely cover the cornea, creating a dense layer that significantly impairs light detection and blurs vision. Studies indicate that over 90% of Greenland sharks are infected with this parasite, with some populations showing 100% infection rates.
Adapting to Low Light and Impaired Vision
The Greenland shark’s natural habitat in deep, cold waters (100 to 1,200 meters) requires adaptations to low light conditions. Even without the parasitic burden, visual hunting would be challenging in these depths where sunlight barely penetrates. Their physiology is well-suited to this extreme environment, characterized by near-freezing temperatures.
The species exhibits a remarkably slow metabolism, allowing them to conserve energy and survive for extended periods on minimal food. This sluggish pace (about 0.34 meters per second) aligns with an ambush predator strategy. They are thought to hunt by stealthily approaching sleeping or injured prey, or by scavenging carrion, making keen eyesight less necessary for survival. Their adaptations to cold waters also include specialized compounds in their tissues that prevent freezing and maintain osmotic balance.
Relying on Other Senses
Despite their limited vision, Greenland sharks possess highly developed non-visual senses that enable them to navigate, locate prey, and survive in their dark environment. Their sense of smell, or olfaction, is particularly acute, allowing them to detect prey and carrion over long distances. Their large nostrils connect to highly folded sensory organs called olfactory rosettes, which enhance their ability to process chemical cues in the water.
Greenland sharks also utilize electroreception through specialized organs called Ampullae of Lorenzini. These jelly-filled pores, concentrated around their snout, detect the faint electrical fields generated by other marine life, even those hidden in sand. This allows them to pinpoint prey in complete darkness, acting as a close-range detection system. The lateral line system, a series of sensory organs along their body, detects movement, vibration, and pressure changes in the surrounding water. This system helps them perceive nearby objects, water currents, and the movements of potential prey or predators.