Sharks exhibit a remarkable trait: their eyes sometimes appear to turn black. This striking change, observed during specific behaviors, has long intrigued scientists and enthusiasts alike, hinting at sophisticated protective mechanisms at play beneath the waves.
Understanding the “Black Eye” Phenomenon
When observing sharks, people occasionally notice their eyes seem to vanish or become dark, giving the impression of a “black eye.” This visual change does not mean the shark’s iris or pupil literally shifts color. Instead, it results from a physical covering or movement of the eyeball itself. This phenomenon typically occurs during moments of heightened activity, such as when a shark is feeding, actively attacking prey, or encountering a potential threat.
The Role of the Nictitating Membrane
One primary way many sharks achieve this eye protection is through a structure known as the nictitating membrane. This is a specialized third eyelid, often described as being translucent or opaque, that sweeps horizontally across the eyeball. Some nictitating membranes are even reinforced with dermal denticles, which are small, tooth-like scales, providing added durability. This protective shield is present in many shark species, particularly those belonging to the Carcharhiniformes order, which includes well-known sharks like tiger sharks, bull sharks, reef sharks, and hammerhead sharks. When activated, the membrane effectively covers the eye, leading to its temporary dark or “blacked out” appearance.
This membrane acts as a physical barrier, safeguarding the delicate eye surface from potential injury. It deploys rapidly, allowing the shark to continue its actions while shielding its vision. The temporary visual obstruction caused by the membrane suggests that during the final moments of an attack, these sharks may rely more heavily on other senses, such as smell and electroreception, to pinpoint their target.
When Sharks Rotate Their Eyes
Not all shark species possess a nictitating membrane; some employ an alternative method for eye protection. Sharks like the formidable great white shark and the fast-moving mako shark, which lack this third eyelid, protect their eyes by rolling them backward into their heads. This action, often referred to as ocular rotation, tucks the sensitive eyeball deep into the socket.
When a shark performs ocular rotation, it exposes the tough, white sclera—the white outer layer of the eyeball—or the surrounding protective cartilage. This dramatic movement is typically executed at the last second, just before impact with prey, ensuring the eye is shielded during the most violent phase of an encounter.
The Purpose of Eye Protection
Regardless of the specific mechanism employed, whether it is the nictitating membrane or ocular rotation, the underlying purpose is consistent: to protect the shark’s eyes from physical harm. During aggressive feeding behaviors or high-speed attacks, a shark’s eyes are vulnerable to damage from struggling prey, which may possess sharp claws, teeth, or powerful thrashing movements. These protective adaptations also guard against bone fragments or collisions with underwater structures like coral reefs. Preserving the integrity of their eyes, even at the cost of momentary blindness, is a crucial survival strategy for these apex predators.