The animal kingdom features diverse visual adaptations. While many creatures navigate with two eyes, others possess numerous visual organs. This diversity highlights how evolution tailors perception to specific survival needs and prompts curiosity about which animal has the most eyes.
The Record Holder for Eyes
The scallop, a marine bivalve, is recognized for having the most individual, image-forming eyes. These mollusks can possess up to 200 small, blue eyes, each about one millimeter in diameter, arranged along the edge of their mantle, the soft tissue lining their shells. Unlike most animals that use lenses, scallop eyes employ a unique mirror-based system. Light enters the eye, passes through a lens and two retinas, then reflects off a concave mirror made of guanine crystals at the back of the eye, focusing the image onto the retinas.
Each scallop eye functions like a reflecting telescope, forming images. These eyes feature a double-layered retina, capturing both peripheral and central fields of view simultaneously. The mirror is composed of millions of tiny, square-shaped guanine crystals arranged in a mosaic, which minimizes surface defects for a clearer picture. This intricate optical design allows scallops to achieve spatial vision, discerning shapes and changes in their environment.
Purpose of Multiple Eyes
The scallop’s numerous eyes provide functional advantages for its life on the seabed. Scallops are relatively stationary, making them vulnerable to predators like sea stars and crabs. Their many eyes offer an almost 360-degree field of view, serving as an early warning system. This broad visual coverage allows them to detect approaching threats from nearly any direction.
The two retinas within each eye play specialized roles, enhancing their ability to perceive their surroundings. One retina is more sensitive to dim light and peripheral vision, while the other excels at capturing movement in brighter conditions. This dual sensitivity allows scallops to react quickly to shadows or movements, either by snapping their shells shut or propelling themselves away by rapidly clapping their valves. Their eyes can also detect polarized light, aiding in navigation and identifying other marine life.
Beyond the Record: Other Many-Eyed Creatures
While scallops are notable for their numerous functional eyes, other animals also exhibit multi-eyed visual systems. Chitons, marine mollusks that cling to rocks, can possess up to 1,000 eyes embedded directly into their protective shells. These “shell eyes” have lenses made of aragonite and can form images, though many are simpler eyespots primarily detecting light.
Spiders typically have eight eyes, though some species may have fewer or more, occasionally up to 12. These simple eyes often specialize for different functions, such as high-acuity vision for hunting or broad peripheral vision for motion detection.
Flies, including common house flies, possess five eyes: two large compound eyes and three smaller ocelli on top of their head. Each compound eye is composed of thousands of individual visual units called ommatidia; fruit flies have around 700 per eye, and house flies around 3,500, creating a mosaic-like perception. Other examples include box jellyfish with 24 eyes and horseshoe crabs with 10 eyes, showcasing the diverse evolutionary paths of vision.