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.