Scallops Have Eyes, and Each One Builds a Beautiful Living Mirror

The structures are so complex that they almost defy belief.

Scallop eyes
Scallop eyes. They feel the magic between you and I. (Dan-Eric Nilsson, Lund University)

In 2019, if everything goes according to plan, the much-delayed James Webb Space Telescope will finally launch into orbit. Once assembled, it will use an array of 18 hexagonal mirrors to collect and focus the light from distant galaxies. This segmented-mirror design was developed in the 1980s, and it has been so successful that it will feature in almost all the large telescopes to be built in the near future.

But as always, nature got there first. For millions of years, scallops have been gazing at the world using dozens of eyes, each of which has a segmented mirror that’s uncannily similar to those in our grandest telescopes. And scientists have just gotten a good look at one for the first time.

Yes, those scallops—the pan-seared pucks of white flesh that grace our dinner plates. Those pucks are just the muscles that the animals use to close their beautiful shells. Look at a full, living scallop, and you’ll see a very different animal. And that animal will be looking right back at you, using dozens of eyes that line the fleshy mantle on the inner edges of its shell. Some species have up to 200 eyes. Others have electric-blue ones.

Inside the eyes, the weirdness deepens. When light enters a human eye, it passes through a lens, which focuses it onto the retina—a layer of light-sensitive cells. When light enters a scallop eye, it passes through a lenslike structure, which ... doesn’t seem to do anything. It then passes through two retinas, layered on top of each other. Finally, it hits a curved mirror at the back of the eye, which reflects it back onto the retinas. It’s this mirror, and not the lens, which focuses the incoming light, in much the same way that those in segmented telescopes do.

Michael Land from the University of Sussex discovered much of this in the 1960s, by carefully eyeballing the eyes under a microscope, and tracing the path that light must take within them. He identified the mirror, he showed that it consists of layered crystals, and he suggested that the crystals are made of guanine—one of the building blocks of DNA. “It’s very impressive how Land was right about pretty much everything from some pretty simple approaches,” says Daniel Speiser from the University of South Carolina, who also studies scallop eyes. “But no one has gotten a good look at an intact mirror before.”

The problem is that powerful microscopes tend to dehydrate samples in the process of analyzing them, and that would ruin the placement of the mirror’s crystals. Now, Lia Addadi from the Weizmann Institute of Science has found a way around this problem. Her team, including Benjamin Palmer and Gavin Taylor, used a microscope that rapidly freezes samples, so everything within stays in the right place. They’ve finally reconstructed the structure of the mirror in glorious detail, confirming many of Land’s ideas, and fleshing others out.

The mirror consists of flat, square guanine crystals, each a millionth of a meter wide. They tessellate together into a chessboard-like grid. Between 20 and 30 of these grids then stack on top of each other, with a liquid-filled gap between them. And the layers are arranged so that the squares in each one lie directly beneath the squares in the one above. The crystals and the gaps between them are respectively 74 and 86 billionths of a meter thick, and these exacting distances mean that the mirror as a whole is great at reflecting blue-green light—the color that dominates the scallop’s underwater habitat.

Guanine crystals in a scallop’s eye. Credit: Lia Addadi

The whole structure is a master class in precision engineering. “When there is an elegant physical solution, the evolutionary process is very good at finding it,” says Alison Sweeney, a physicist at the University of Pennsylvania who studies animal vision.

This precision is all the more remarkable because guanine crystals don’t naturally form into thin squares. If you grow them in the lab, you get a chunky prism. Clearly, the scallop actively controls the growth of these crystals, shaping them as they form. Guanine crystals grow in layers, and Addadi thinks that the scallop somehow shifts the orientation of each layer by 90 degrees relative to the ones above and below it. As the layers grow outward, they do so in only four directions, creating a square. How it does that is a mystery, as is everything else about the way the mirrors form.

Also, the mirror is not an inanimate structure within the eye. It’s a living thing. The square crystals grow inside the cells of the scallop’s eye, filling them up. It’s the cells that then tessellate together to form the layers. “The cells can’t be dead,” Addadi says, “or the whole thing would break apart.” So not only must the cells control the growth of the crystals inside them, but they also have to communicate with each other to arrange themselves just so. “How do they do that? I really don’t know,” she adds.

Whatever their trick, it clearly produces results. Scallop vision isn’t going to rival ours anytime soon, but it’s far sharper than you might expect for an animal that’s basically a fancy clam. Speiser demonstrated this a decade ago by putting scallops in little seats and playing movies of drifting food particles. Even when the particles were just 1.5 millimeters wide, the scallops would open their shells, ready to feed. “The idea that these animals are forming really nice images with their eyes feels very solid to me,” Speiser says.

Addadi’s team also noticed that the scallop’s mirror is slightly tilted relative to its retinas. As a result, the mirror focuses light from the center of the animal’s visual field onto the upper retina, and light from the periphery onto the lower one. Perhaps that’s why the creature has two retinas: They allow it to focus on different parts of its surroundings at the same time.

“It’s a really amazing study,” says Jeanne Serb from Iowa State University, who has also studied scallop eyes. It helps to solve the mystery of the double retinas—something that scientists have long tried to address, with no success.

But Speiser isn’t completely convinced. He says that the eyes get easily deformed when they’re dissected, and even a gentle squish could change the orientation of the mirror and retinas. Still, he doesn’t have a better explanation, despite testing several possible ideas over the last 12 years. “Nothing checked out, and this is as good a hypothesis as any,” he says.

The next big goal for scallop aficionados, he adds, is to work out why scallops have quite so many eyes. They probably allow it to scan a wide area, but does it consider the information from each eye separately, or combine them all into a single image? After centuries of study, scientists finally know how each individual eye sees. But “we still have no idea what the animal as a whole is perceiving,” he says.