Wonder-Sighting on Planet Earth: The Space Telescope Eye of the Scallop
By Maria Popova
“The Eye altering alters all,” William Blake wrote not long before Darwin extolled the eye as the crown jewel of evolution — an organ of “such wonderful structure” and “inimitable perfection” that it magnetizes us to the mystery of life itself. In On the Origin of Species, he began a section titled “Organs of Extreme Perfection and Complication” with a love letter to the eye:
To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself first originated.
But marvelous as our own human eyes may be, they are far from the crowning curio of the animal kingdom. The honor might belong to a creature much lower on the evolutionary ladder of sentience. (“Never say higher or lower,” Darwin exhorted himself in the margins of a book. “Say more complicated.”)
The bay scallop (Argopecten irradians), older than Homo sapiens by some 200 million years, sees with 200 eyes the color of Uranus — bright blueberries bejeweling the ridges of its rippled shell, each equipped with not one but two retinae, containing threefold as many opsins — the light-sensitive proteins in photoreceptor cells, tasked with converting light into electrochemical signals — as ours.
An upper retina covers the scallop’s central field of view, allowing it to see silhouettes moving in the dark. A lower retina is tasked with the animal’s peripheral vision, helping it navigate — unlike other bivalves that live appended to the seafloor, rocks, or vegetation, scallops are fully mobile, moving by a kind of jet-propulsion, clapping their shells together with their powerful adductor muscles as they push water from one end of the shell to the other, swimming in a zig-zag motion.
But the most wondrous aspect of the scallop eye is its structure, more akin to that of a space telescope than to that of the human eye. Images are not projected through a lens that focuses light but reflected onto the retina by a tiny mosaic of mirrors in the back of each eye, tiled with millions of miniature crystals of a shape never seen elsewhere in nature: a flat square.
Although he lived long before powerful microscopes illuminated the wonder of the scallop eye, Darwin was awed by its uncommon beauty visible to the naked human eye — three scallop shells grace the coat of arms of the Darwin family.
Radiating from this alien marvel of nature is a shimmering reminder that there are as many ways of seeing as there are ways of being, and this dazzling difference is precisely what makes our planet a world.
Published June 21, 2023