The question of whether fish close their eyes to sleep is a common one. Most fish appear perpetually awake, maintaining an unblinking gaze day and night. This observation raises the question of how an animal can rest without closing its eyes. The answer lies in the physical structure of the fish eye and redefining what sleep means for a creature living entirely underwater.
Why Fish Cannot Close Their Eyes
The primary reason fish cannot close their eyes is a straightforward anatomical one: the vast majority of species simply do not possess eyelids. Eyelids in terrestrial animals serve two main purposes: protecting the eye from injury and distributing a fluid layer, like tears, across the cornea for lubrication. Fish live in an environment where water provides constant moisture and acts as a natural cleaning agent, making the necessity for blinking obsolete. This continuous immersion means that their eyes are already protected from drying out and are continuously washed free of small debris.
While most fish lack true movable eyelids, some species have developed specialized structures. Certain sharks, for example, possess a nictitating membrane, a transparent or translucent third eyelid that slides horizontally across the eye. This membrane is primarily used for protection, shielding the eye from struggling prey during a feeding strike. Additionally, some fast-swimming species like mackerel and certain jacks have a fixed, transparent adipose eyelid. This structure is not movable but is thought to reduce drag and potentially improve visual focus while swimming at high speeds.
Defining Sleep and Rest in Fish
Since closing their eyes is not an option, the definition of sleep in fish relies on distinct behavioral and physiological markers. Researchers identify a resting state as a period of reduced metabolic rate, decreased responsiveness to external stimuli, and minimized movement. During this time, the fish’s breathing rate often slows, and its overall vigilance is lowered. They maintain enough awareness, however, to react to a sudden threat. This state is sometimes referred to as a “low power mode,” allowing for necessary restoration without complete unconsciousness.
Many species follow a diurnal or nocturnal rhythm, resting at predictable times, often near the bottom or within a sheltered area. Studies on fish like the zebrafish show they experience periods of brain activity that resemble the slow-wave and paradoxical sleep stages seen in mammals. This suggests that the restorative functions of sleep are achieved through changes in brain and body chemistry, rather than the physical act of eye closure. The duration of these rest periods varies widely, but the underlying purpose remains the same: conserving energy and allowing for physiological recovery.
Specialized Resting Behaviors
The way fish rest is highly diverse, reflecting the vast array of environments they inhabit and the constant need for predator avoidance. Reef fish, such as the parrotfish, employ a remarkable strategy by secreting a transparent, mucous cocoon around their bodies before settling down for the night. This slimy envelope is believed to mask the fish’s scent, hiding it from nocturnal predators like moray eels, and may also offer protection against parasites. The parrotfish remains stationary within this cocoon until daylight.
In contrast, highly active pelagic species, including certain sharks and tuna, must maintain continuous movement to breathe, a process known as ram ventilation. For these swimmers, stopping completely would result in suffocation, so their resting state involves a form of unihemispheric sleep. This allows one half of the brain to rest while the other half remains alert and controls the necessary swimming movements. Other fish, such as the Pacific sand lance and various wrasses, actively burrow into the sand or mud to conceal themselves while they rest. This behavior provides a physical barrier against predators and protects them from strong currents, allowing for a deeper, more secure period of reduced activity.