The whale shark, the largest fish in the world, is known for its immense size and gentle filter-feeding nature, consuming plankton and small fish with its wide mouth. This enormous marine animal, reaching over 60 feet, navigates tropical and warm oceans, posing no threat to humans. An unusual characteristic of these docile giants is the presence of tiny, tooth-like structures covering their eyeballs. This discovery has sparked curiosity regarding their nature and purpose.
What Are Whale Shark Eye Denticles?
The structures on a whale shark’s eyes are not true teeth, but rather specialized dermal denticles. These are similar to the placoid scales covering the rest of a shark’s body, providing a rough, sandpaper-like texture, but are uniquely modified for the eyes. Dermal denticles consist of an inner core of pulp, a layer of dentine, and a hard enamel-like coating. On the whale shark’s eyeballs, these denticles are small, robust, and densely packed, particularly around the iris.
A single whale shark eyeball can possess over 3,000 of these denticles. Their morphology differs from the denticles found on the main body, indicating a specialized function. Under a microscope, these eye denticles have been described as having an “oak-leaf” shape with a central ridge and several branching sub-ridges, optimized for abrasion resistance. This adaptation has not been observed in other shark species, making it a distinguishing feature of the whale shark.
Why Whale Sharks Need Eye Protection
Whale sharks navigate with eyes relatively small compared to their massive bodies, and they lack eyelids. Their eyes protrude from the sides of their heads, increasing their exposure to potential harm. This makes the eye surface susceptible to mechanical damage from debris, contact with prey, or accidental abrasions during feeding and movement.
The eye denticles act as a robust, flexible shield against such injuries. Their specialized structure is designed for abrasion resistance, protecting the soft tissue of the eyeball. Beyond the physical armor of the denticles, whale sharks possess another protective mechanism: the ability to retract their eyeballs deep into their sockets. This retraction can pull the eye almost halfway into the skull and is often accompanied by a slight rotation.
The presence of these protected eye features, including denticles and eye retraction, suggests that vision plays a more significant role in the whale shark’s perception of its environment than previously assumed. Despite their seemingly small eyes and prior assumptions of low reliance on vision, these protective adaptations indicate the importance of preserving their visual capabilities in the challenging marine world.