The animal kingdom displays a remarkable range of visual organs, from simple light-sensing patches to complex camera-type eyes. Defining what constitutes an “eye” is the first step in answering which animal has the most, separating single light-sensitive cells from true organs of vision. Some animals possess simple photoreceptors that only distinguish between light and dark, while others have intricate structures capable of forming detailed images. The record holder for the highest number of individual, functional eyes is a marine mollusk with a unique visual adaptation.
Identifying the Record Holder
The title for the animal with the most individual eyes belongs to the scallop, a bivalve mollusk found in oceans worldwide. Scallops, particularly species from the genus Pecten, possess dozens of visual organs, with some individuals having up to 200 eyes. These small, iridescent blue eyes are approximately one millimeter in diameter. They are arranged in a row along the edge of the animal’s mantle, the fleshy tissue just inside the shell. This circular arrangement provides the scallop with nearly a 360-degree field of view.
These numerous eyes are complex structures capable of forming images, not merely primitive light sensors. They are positioned strategically to monitor the surrounding water for signs of danger. If a scallop loses an eye due to damage, it can regenerate a new one within a short period, typically around 40 days. This regenerative capacity ensures continuous visual surveillance against potential threats.
The Unique Visual System of the Champion
The scallop’s eyes are structurally distinct from the camera-like eyes of vertebrates, employing a unique optical system that functions like a reflecting telescope. Unlike human eyes, which use a lens to focus light onto a retina, the scallop’s eye uses a concave mirror located at the back of the eyeball. This mirror is constructed from precisely arranged, square-shaped crystals of guanine, a substance also found in DNA.
Light enters the eye through a cornea and a small lens before traveling to the back, where the guanine mirror reflects and focuses the light onto a double-layered retina. This reflective mechanism is efficient at gathering light in the dim underwater environment. The mirror’s layered structure is tuned to reflect the specific wavelengths of blue-green light that penetrate deep into the ocean water.
The scallop possesses two retinas, the distal and the proximal, situated at different distances from the mirror. This dual-retina system allows the scallop to image both the peripheral and central visual fields simultaneously. This spatial vision enables the scallop to detect movement and shadows. Detecting a threat allows the scallop to react by rapidly clapping its shells together to “swim” away in short, powerful bursts.
Other Animals Known for High Eye Counts
While the scallop holds the record for the most individual eyes, other creatures have complex visual arrays. The box jellyfish (class Cubozoa) has a total of 24 eyes clustered into four sensory structures called rhopalia. Each rhopalium contains six eyes of four distinct morphological types: four simple light-sensing pit and slit eyes, and two sophisticated lens-bearing eyes. These lens eyes allow the jellyfish to form images, which it uses for complex behaviors like navigating around obstacles and finding its preferred habitat.
Many arthropods appear to have a large number of eyes, but the definition of an eye is important for the count. For example, dragonflies have two large compound eyes, each containing up to 30,000 individual visual units called ommatidia. These ommatidia are considered lenses or facets of a single compound eye, not separate eyes themselves. In contrast, most spiders possess eight simple eyes, though they rely more on touch and vibration than on sharp vision.