Most people associate blinking eyelids with keeping eyes moist and protected, a common feature across many land-dwelling creatures. However, the aquatic world operates under different principles, and the ocular structures of fish reflect their unique environment. The vast majority of fish do not possess eyelids in the way humans do, highlighting their specialized aquatic existence.
Why Eyelids Are Rare in Fish
The absence of traditional eyelids in most fish stems directly from their aquatic habitat. For terrestrial animals, eyelids serve the primary functions of moistening the eye surface and clearing away airborne particles like dust. Since fish are constantly immersed in water, their eyes are continuously washed clean, eliminating the need for tear glands or blinking to maintain moisture. The surrounding water acts as a consistent cleansing agent and a buffer against potential irritants.
The eyes of most fish do not require the same range of movement or protection from external elements as land animals’ eyes. Fish typically lack the muscular structures necessary for extensive eye movement, often relying on head movements to adjust their field of vision. This difference means that the protective and lubricating mechanisms of eyelids are largely unnecessary for fish. Most fish sleep with their eyes open, as they lack the anatomical structures to close them.
The Fishy Exceptions and Their Unique Adaptations
While most fish lack eyelids, certain species have evolved specialized structures that serve similar protective or adaptive purposes. One prominent example is the nictitating membrane found in many sharks. This translucent, and sometimes opaque, “third eyelid” can be drawn across the eye for protection, particularly during feeding or when encountering abrasive objects. The outer surface of this membrane can even be covered with dermal denticles, enhancing its protective capabilities.
Another distinct adaptation is the adipose eyelid, a transparent, fatty tissue that partially or fully covers the eyes of various bony fish, including milkfish, mackerel, herring, and mullets. These structures are not movable in the same way as human eyelids. Adipose eyelids may act as a lens to improve visual focus, aid in seeing polarized light, or block harmful ultraviolet light. They also provide a physical barrier against foreign objects in the water, and in some cases, can filter out light with wavelengths shorter than 305 nanometers.
Beyond these, some deep-sea fish possess tubular eyes equipped with a transparent cornea that functions as a built-in lens cover, offering protection in their extreme environments. Certain fish, like the parrotfish, secrete a mucus cocoon that envelops their bodies, including their eyes, providing protection from predators and parasites while they rest. Uniquely, the mudskipper, an amphibious fish, is known to blink, a rare exception among fish that highlights its adaptation to spending time both in and out of water.