Inside the hinged shell of the scallop lurks a musclebound creature that’s best enjoyed seared in butter. But there’s much more to appreciate about this delicious entree: the scallop sees its world with hundreds of eyes in an array that mirrors microscopically a giant space telescope.
While some invertebrate eyes sense only light and dark, scientists have long suspected scallops can make out images, perhaps recognizing predators quickly enough to jet to safety. But scallop eyes, each the size of a poppy seed, are tiny and delicate and scientists have struggled to understand how they work.
Miniature mirrors made of millions of tiles
But recently a team of Israeli researchers reported in the journal Science that each scallop eye contains a miniature mirror made up of millions of square tiles. The mirror reflects incoming light onto two retinas, each of which can detect different parts of the scallop’s surroundings.
Our own eye has been likened to a camera using a lens to focus light on the retina. The new research suggests that scallop eyes are more akin to another kind of technology: the reflector telescope of the sort first invented by Newton and today, by astronomers gigantic reflector telescopes built to look in deep space. They also build their mirrors out of tiles.
Earlier studies had hinted the scallop eye was weirdly complex, each with a lens, a pair of retinas, and a mirrorlike structure at the back. Scientists suspected that light passed through the lenses and the retinas, which are mostly transparent, bounced off the mirror, and struck the retinas on the return.
But no one knew how the mirror works, or why scallops needed two retinas when other animals need only one.
Using new cryo-electron microscopy, Benjamin A. Palmer, a researcher at the Weizmann Institute of Science in Israel, and his colleagues froze slices of the eyes, making it possible to inspect the tissue down to its fine molecular details. (Last October three pioneers of cryo-electron microscopy won the 2017 Nobel Prize in Chemistry.)
Researchers have long known that the mirror in a scallop eye is made from a molecule called guanine. It’s best known as one of the main ingredients of DNA, but in some animals guanine is packed into crystals that reflect light.
Transparent, flat square guanine crystals
Some fish have a silvery tint to their scales thanks to guanine crystals. Chameleons use guanine crystals to help them change the colour of their skin. But no one knew how guanine helped scallops to see.
Dr Palmer and his colleagues discovered that scallops make a kind of guanine crystal never seen before in nature, a flat square. “We were amazed,” he said.
The researchers found that the mirrors are made of twenty to thirty layers of guanine, each containing millions of squares that fit together snugly like tiles on a wall.
Paradoxically, the guanine squares don’t reflect light on their own; they’re transparent. But their arrangement turns them into a collective mirror.
The model created by Dr. Palmer and his colleagues may also solve the mystery of the two retinas. The researchers found that each retina receives sharply focused light from different parts of the animal’s field of view.
One retina can create a sharp image of what’s right in front of the eye. The other retina gives a better view of the periphery. The retina that sees the central field of view might allow scallops to quickly recognize oncoming predators, allowing them to beat a hasty retreat by swimming away.
Scallops may pay attention to their peripheral vision instead when they’re searching for a spot on the sea floor where they can settle down to feed.
What’s more, the hundreds of eyes on a scallop all deliver signals to a single cluster of neurons, which may combine that information to create a rich picture of the outside world. Dr Palmer said that scallop eyes may provide inspirations for new inventions.
There’s certainly precedent: NASA has built X-ray detectors to study black holes that mimic lobster eyes. Perhaps an artificial scallop eye could take pictures in dim seawater.
But Dr Palmer is more excited by the prospect of creating materials new to engineering. His study shows that scallops have evolved a mastery over forming crystals, guiding them into shapes that researchers didn’t think possible.