The question of whether a shark closes its eyes is complex because different species use distinct methods for protecting their vision. Unlike humans who blink to lubricate their eyes, sharks live in a constantly moist environment, so their “blinking” is purely a defensive action. When a shark prepares to strike prey or encounters debris, it must shield its sensitive eyes. It employs one of two specialized biological mechanisms, depending entirely on the shark’s family, which reflects evolutionary adaptations for survival during a predatory strike.
The Nictitating Membrane: A True Eyelid
Many common shark species possess a physical shield known as the nictitating membrane, often called the “third eyelid.” This structure is a tough, translucent layer drawn horizontally across the eyeball from the lower lid. Its primary function is to provide instant physical protection against injury, particularly from struggling prey or contact with objects in the water.
This feature is common in the order Carcharhiniformes, which includes most requiem sharks like the Bull, Blacktip, and Grey Reef sharks. The membrane deploys reflexively just before the moment of impact when the shark lunges to bite its target. Although the membrane temporarily obscures vision, the protection it offers is a necessary trade-off against potential damage.
The Alternative Defense: Ocular Rotation
Not all sharks use the nictitating membrane; species like the Great White, Mako, and Hammerhead sharks employ a different method of eye protection. These sharks lack a full membrane and instead utilize ocular rotation to defend their eyes immediately before a strike. Ocular rotation involves the shark actively rolling its eyeball backward into the socket as it opens its jaws to bite its prey.
This action effectively turns the eye inward, exposing the tough connective tissue and cartilage surrounding the orbit. This defense is necessary because prey, such as seals or sea lions, can fight back with sharp claws and teeth, posing a significant threat. The consequence of this eye-rolling is that the shark is completely blind for the final moments of its attack. It must rely on other finely tuned senses, like electroreception, to guide the final lunge, which is why rotation is performed only in the last fraction of a second before contact.
Eye Function During Rest and Activity
When sharks are not actively feeding, their eyes generally remain open for constant awareness of the surroundings. Many pelagic species, like the Great White shark, are obligate ram ventilators. This means they must swim continuously to force oxygen-rich water over their gills for respiration. This requirement prevents them from entering a deep, immobile sleep state, so their eyes must stay open to monitor their movement and environment.
Some bottom-dwelling sharks, such as Nurse or Draughtsboard sharks, use buccal pumping to draw water over their gills while stationary. These species can enter periods of inactivity that resemble sleep. While they may sometimes be observed with their eyes closed, this is not a consistent indicator of rest. Studies have shown that some inactive sharks still have their eyes open a significant portion of the time, suggesting that eye closure is more likely a response to external factors like light rather than a standard part of their resting behavior. Regardless of the species, many sharks possess a reflective layer called the tapetum lucidum to enhance vision in the dim ocean depths.