A Telescope in a Shell
Along the mantle of a single scallop, up to 200 tiny, bright blue eyes peer out into the aquatic world. These are not simple light-sensing pits. Each one-millimeter-wide eye is a fully functional reflecting telescope, parallel to the segmented mirror telescopes used in modern astronomy. While most animals, including humans, use a lens to refract and focus light, the scallop uses a concave mirror at the back of its eye to reflect light onto its retinas.
The mirror is a living structure. It is built from millions of perfectly square, flat crystals of guanine, the same molecule that forms one of the building blocks of DNA. The scallop exerts precise biological control to shape the guanine into squares—a form not typically found in nature—and tiles them together with minimal distortion. The crystal tiles are arranged in 20 to 30 layers, separated by cytoplasm, to create a highly reflective surface. The entire structure is tuned to reflect blue-green light around a wavelength of 500 nanometers, the light that best penetrates its seafloor habitat. The local Spiny Scallop (Chlamys hastata) and Weathervane Scallop (Patinopecten caurinus) found in these waters have this visual system.
Two Retinas, Two Views
The scallop eye has a dual-retina system. Light enters the eye, passes through a weak lens and the two retinal layers, and is then focused by the mirror back onto them. The mirror and retinas are slightly tilted, an arrangement that creates two separate areas of focus.
The upper (distal) retina receives a focused image from the center of the scallop's field of view. This retina is adept at detecting dark, moving objects, triggering the scallop's primary defense mechanism: clapping its shells together for jet-propelled swimming to escape predators like the Sunflower Sea Star (Pycnopodia helianthoides). The lower (proximal) retina receives light from the periphery of the mirror. Its exact function is less understood, but it is believed to provide information about the scallop's static surroundings, helping it find suitable habitats. The division of labor allows the scallop to simultaneously watch for danger while assessing its environment.