The powerful image of a shark lunging for prey often includes a striking detail: its eyes appearing to roll back, revealing a stark white surface. This popular belief suggests a moment of heightened aggression or even blindness during an attack.
The Protective Shield: Understanding the Nictitating Membrane
Sharks do not actually “turn white” during an attack; instead, many species possess a specialized internal eyelid called a nictitating membrane. This membrane is a thin, tough, and often translucent layer that sweeps across the eye. Its primary purpose is to safeguard the shark’s eye from potential harm.
Not all shark species have this third eyelid, however. Sharks such as the great white, mako, thresher, and whale sharks do not possess a nictitating membrane.
For these species, an alternative protective mechanism is used: they roll their eyes backward into their heads just before impact. This ocular rotation exposes a tough cartilaginous area, effectively shielding the vulnerable eye.
When and Why Sharks Use This Protection
Sharks deploy their nictitating membrane in specific scenarios where their eyes are at risk of injury. This includes moments of feeding or attacking prey, especially when the prey is thrashing or attempting to defend itself. They also use it when bumping into objects, other animals, or during defensive encounters.
The eyes are an important sensory organ for sharks, essential for hunting and navigating their environment. Protecting them is a crucial evolutionary adaptation, as damage could impair a shark’s ability to locate food or avoid threats.
This protective behavior underscores the importance of eye integrity for these predators. Whether it is the nictitating membrane or the eye-rolling action, the goal remains the same: to shield their sensitive eyes during the chaotic and potentially damaging moments of interaction.
Beyond the Eyes: Shark Attack Dynamics
While eye protection is a notable aspect, shark attack dynamics involve a complex interplay of sensory inputs. Sharks use a sophisticated array of senses to detect prey beyond vision.
They possess a lateral line system, a network of specialized cells that detect pressure changes and vibrations in the water, alerting them to nearby movement. They also employ electroreception through organs called the ampullae of Lorenzini, which can detect weak electrical fields generated by living organisms, even those hidden from sight.
This allows sharks to pinpoint prey during the final stages of an approach, especially when their eyes might be obscured by the protective membrane or rolled back. A shark’s predatory or defensive response involves a quick approach and bite. The activation of the nictitating membrane or eye-rolling is one component of this instinctual process, highlighting the shark’s adaptation for survival in its marine habitat.