Sharks sometimes appear to roll their eyes back or turn them opaque white. This behavior is a protective reflex, shielding their vulnerable eyes from potential harm. This temporary change is a natural adaptation, allowing sharks to navigate dangerous situations without compromising their vision long-term. This mechanism is a testament to the evolutionary strategies sharks employ for survival.
Understanding the Nictitating Membrane
The primary mechanism behind a shark’s eye appearing white is the deployment of its nictitating membrane. This structure is a translucent, protective “third eyelid” located in the lower part of the eye socket. It moves horizontally across the eyeball, from the bottom or inner corner outwards, effectively covering the eye. This membrane acts as a shield, capable of being drawn across the eye to safeguard it.
The nictitating membrane is not a true eyelid like those found in humans but rather a robust, cartilaginous layer. Its outer surface is often covered with dermal denticles, which are small, tooth-like scales similar to those found on a shark’s skin, enhancing protection. When activated, this opaque layer covers the eye, giving it the characteristic white appearance. Many shark species, particularly those in the Carcharhiniformes order, such as requiem sharks (including bull sharks and tiger sharks), possess this specialized membrane.
Situations Requiring Eye Protection
Sharks deploy their eye protection mechanisms in various high-risk scenarios to prevent injury. A common situation is during predatory attacks, especially when engaging with struggling prey. As a shark bites down and its prey thrashes, there is a significant risk of the eyes being scratched, punctured, or otherwise damaged by claws, teeth, or sharp scales. The nictitating membrane provides a barrier against such potential harm, allowing the shark to complete its attack.
Defensive maneuvers also prompt this protective reflex. If a shark feels threatened or is involved in close-quarters interactions, it will activate its eye covering to shield itself. This reflex can also occur during accidental collisions with underwater objects or during investigative behaviors where the shark approaches something closely. During these moments, physical protection is prioritized over vision.
Diverse Methods of Eye Protection in Sharks
While many shark species, including tiger sharks, bull sharks, and hammerhead sharks, utilize the nictitating membrane, not all sharks possess this adaptation. For instance, species belonging to the Lamniformes order, such as great white sharks, mako sharks, and thresher sharks, do not have a nictitating membrane. These sharks have evolved alternative methods to protect their eyes during potentially damaging encounters.
Instead, these species employ a strategy known as ocular rotation. Just before impact with prey or during a close encounter, they roll their eyeballs backward into their sockets. This action exposes a tough, cartilaginous surface, effectively shielding the delicate eye from injury. This eye-rolling mechanism is particularly noticeable in great white sharks during a strike, where their eyes appear to disappear into their heads. This temporary eye protection is a reflex, allowing these powerful predators to avoid damage.