The Greenland shark, Somniosus microcephalus, is known for its incredible longevity, living in the deep, cold waters of the Arctic and North Atlantic Oceans. Scientists estimate this species can live for at least 272 years, possibly exceeding 500 years, making it the longest-living vertebrate known on Earth. Living at depths up to 2,800 meters, where sunlight does not penetrate, the shark requires specialized adaptations. While the shark possesses eyes, its vision is so severely impaired that it is considered functionally blind, a condition rooted in its unique ecological niche and a persistent parasite.
The State of Greenland Shark Vision
The Greenland shark is not anatomically blind; it possesses the necessary structures for sight, but its visual acuity is extremely limited. The eyes are small relative to the shark’s massive size and are adapted for low-light vision, featuring a rod-dominated retina. This adaptation includes the tapetum lucidum, a reflective layer behind the retina that boosts light-gathering capability. However, this ability is heavily compromised in most adult individuals.
The shark’s habitat, ranging from 100 to over 1,200 meters deep, requires little sharp, high-resolution vision. In the total darkness of the deep ocean, detecting movement and changes in light intensity is more important than discerning shapes and colors. The visual impairment found in nearly all adult Greenland sharks is not an evolutionary loss of the eye structure, but a direct result of chronic external damage.
The Mechanism: Ocular Parasitism
The main reason for the Greenland shark’s poor sight is the copepod Ommatokoita elongata, a crustacean that acts as an ectoparasite on the shark’s cornea. This parasite is highly prevalent, infecting at least one eye in a vast majority of the adult population. The female copepod attaches firmly to the eye surface using specialized hooks and a bulbous anchor structure called a bulla.
The parasite feeds on the corneal tissue, causing significant damage, including physical lesions, ulceration, and inflammation. This continuous irritation results in the formation of opaque scar tissue on the cornea, which physically blocks light passage and severely impairs vision. The copepod transforms the shark’s clear lens into an opaque, cloudy surface, rendering the eye functionally useless for sharp vision. Although some speculation suggests the parasite may be bioluminescent and act as a lure for prey, this hypothesis remains unverified.
Reliance on Non-Visual Senses
The shark’s ability to thrive despite severe visual impairment relies on sensory compensation, where other senses handle hunting and navigation. Since the deep-sea environment is characterized by darkness, the Greenland shark has evolved highly acute non-visual systems. This sensory suite allows it to locate food, including fish, seals, and carrion, without relying on sight.
Chemoreception, or the sense of smell, is acutely developed and serves as the primary long-range detection system. The shark effectively tracks chemical plumes and scent trails in the water, which is useful for locating carrion over vast distances. This strong sense of smell guides the animal toward potential food sources in the cold, deep environment.
For close-range detection, the shark employs electroreception using specialized organs called the Ampullae of Lorenzini. These minute pores are scattered across the shark’s head and filled with a conductive gel. The ampullae detect the weak bioelectric fields generated by the muscle contractions of living organisms. This capability is critical for the final moments of a strike, allowing the functionally blind predator to precisely locate and capture prey in total darkness.
Slow Metabolism and Deep-Sea Lifestyle
The ecological context of the Greenland shark’s life explains why functional blindness is not a barrier to its survival as an apex predator. The shark inhabits frigid Arctic and North Atlantic waters, where temperatures are near freezing. This cold environment contributes to an extremely slow metabolism, giving the species the lowest recorded swimming speed and tail-beat frequency of all fish. The average cruising speed is only about 0.34 meters per second.
This sluggish, energy-efficient movement is perfectly suited for a lifestyle that prioritizes scavenging and stealth. The shark is believed to primarily scavenge for carrion or engage in ambush predation, possibly sneaking up on sleeping seals. This strategy relies far more on patience and non-visual senses than on high-speed pursuit. The shark’s longevity and slow growth rate are direct consequences of this low-energy, deep-sea existence, demonstrating a highly successful adaptation where keen vision is not a prerequisite for survival.