The common image of sleep involves unconsciousness with eyes closed, but for sharks, this picture is misleading. A shark’s period of rest differs significantly from the restorative slumber experienced by mammals. Their unique physiology and the need for constant movement in some species dictate how they rest, which determines the state of their eyes. The question of whether a shark closes its eyes when resting is answered by looking at the specific anatomy of its eye and the mechanics of its breathing.
Understanding Shark Rest and Sleep
Sharks do not undergo the deep, cyclical sleep stages, like REM, that characterize mammalian sleep. Instead, they enter a state of decreased responsiveness and a lowered metabolic rate, which scientists refer to as rest or a sleep-like state. Recent research indicates that this restful state includes physiological evidence of sleep, such as a drop in metabolic activity and a relaxed body posture.
This period of neurological rest is often partial, with some scientists suggesting a form of unihemispheric slow-wave sleep (USWS) in active species, similar to what is seen in dolphins and whales. USWS allows one hemisphere of the brain to rest while the other remains active and vigilant. This is a necessary adaptation for an animal that must maintain continuous movement. This partial rest permits a level of environmental awareness, preventing the total unconsciousness associated with traditional sleep.
The Anatomy of Shark Eyes
The answer to whether a shark closes its eyes lies in its physical anatomy, as most species simply do not possess the movable eyelids found in humans. Sharks lack the muscle structure that would allow for a voluntary, complete closing of the eye for rest. For the majority of sharks, the eyes remain open or appear glazed over during periods of rest.
Some species, such as the Great White Shark, lack a nictitating membrane, which is a protective “third eyelid.” These sharks protect their eyes during a feeding strike by rotating the entire eyeball backward into the socket. Other species, including the Tiger Shark and Blue Shark, do possess a nictitating membrane. This membrane is typically drawn across the eye for protection during feeding or when sensing a potential threat, not as a normal part of their resting behavior. A few bottom-dwelling species have been documented closing their eyes, but this eye closure is not a universal trait among sharks.
Respiration Methods During Rest
A shark’s method of breathing is a major factor in how and where it rests, which influences its eye behavior. Sharks must constantly move water over their gills to extract oxygen. Oceanic species, like the Mako and Great White Sharks, are obligate ram ventilators, meaning they must swim continuously with their mouths slightly open to force water across their gills. This constant motion prevents them from settling into a stationary rest where eye closure might occur.
Bottom-dwelling species use a different method called buccal pumping. This involves actively drawing water into the mouth and pumping it over the gills using cheek and throat muscles, allowing the shark to remain motionless on the seafloor. Some stationary species also have spiracles, small openings behind the eyes that draw in oxygenated water, which is especially useful when resting on the bottom. While the ability to rest without continuous movement provides the opportunity for stationary rest, their eyes typically remain open even in this state.
Diversity in Shark Resting Behavior
The contrast in resting behavior is clearly illustrated by comparing different species. Obligate ram ventilators, like the Great White Shark and Mako Shark, never stop swimming, even when in a resting state. They rely on the constant forward motion to breathe, maintaining a perpetual low-level cruise. The appearance of their eyes remains open and functional due to the need for continuous vigilance while moving.
In contrast, bottom-dwelling species, which utilize buccal pumping, can be found completely stationary. Nurse sharks are a prime example, often seen resting in piles or caves during the day. While they are motionless and their metabolism is lowered, their eyes are usually still open, monitoring their surroundings. This stationary rest is the closest a shark comes to a full stop, yet the anatomical limitations and the need for awareness mean that for most species, the eyes remain open.