Many animals are covered in protective armor—shells, scales, plates, and frills that protect their soft bodies. But one group of little-known sea creatures called chitons have evolved armor that's unlike anything else in nature.

Because it contains eyes. Hundreds of eyes.

With lenses made from rock, that erode as the animals age and have to be continuously replaced.

Chitons are mollusks, related to snails, clams, and octopuses. Their oval bodies are covered by a hard shell consisting of eight overlapping plates, which makes them look a bit like a woodlouse with a skirt, or perhaps like the forehead of a Klingon. In many species, these plates are dotted with hundreds of tiny beads, each less than a tenth of a millimeter across. These are eyes. Each contains a lens, a light-sensitive retina, and a layer of black pigment.

Scientists have known about these eyes since the 19th century, but for most of that time, no one knew whether chitons could actually see out of them, or much about that anatomy. That all changed a few years ago, almost by accident.

Daniel Speiser, then a graduate student at the University of California, Santa Barbara, had dissected the lenses from the eyes of a West Indian fuzzy chiton, and dunked them into an acid bath to clean them. But lenses didn't become cleaner. They just disappeared.  

Speiser soon discovered why. Unlike almost all other animal lenses, which are made from organic proteins that would have resisted the acid bath, chiton lenses are made from a mineral called aragonite. That's a form of calcium carbonate or limestone, which dissolves easily in acid. These animals peer at the world through lenses made of rock.

They can certainly detect light. When Speiser flashed shadows over idling chitons, the creatures would hunker down and flatten their armor against whatever they were resting upon. He also calculated that their eyes ought to be able to form images, although with a thousand times poorer resolution than human eyes. But why have hundreds of them? Is each one essentially its own pixel, like the facets of an insect's compound eye? Or does the animal combine the images from all its eyes into a single view of the world?

To find out, Speiser teamed up with Ling Li and Matthew Connors, two graduate students from the Massachusetts Institute of Technology. They placed a fuzzy chiton in an extremely powerful x-ray scanner to study the structure of its eyes.

The team found that the grains of aragonite in the lenses are much bigger than those in other parts of the chiton's armor, and strongly aligned. There's a reason for that. Every time light passes through the boundaries of different grains, it risks being scattered; by minimizing those boundaries, the chiton's lenses become better light-collectors.

Li and Connors tested their abilities by projecting objects through them to see if they genuinely can form images as Speiser had calculated. They can—blurry and heavily pixellated images, yes, but images nonetheless. Each eye could, for example, detect the shape of a 20-centimeter fish from a few meters away.

They could achieve higher resolutions if they weren't so very small. They can only pack so many light-sensitive cells beneath each lens, which limits the number of pixels in the image they can see. So, why do chitons have hundreds of tiny low-resolution eyes rather than just a few high-resolution ones—like us, or flies, or octopuses, or eagles, or jumping spiders?

The team suspects that the answer lies in the eyes' location—not on some obvious head, but actually embedded within the chiton's armor. They may help the animal to see threats, but they also compromise its defenses. Each eye consists of a large pear-shaped chamber beneath the lens, and these cavities, full of soft sensory tissues, create weaknesses in the chiton armour. The same aligned grains that help the lenses to collect more light also make them uniquely fragile. Li and Connors found that they collapse under forces that barely dent the rest of the plates.

If the eyes were any bigger, the chiton's shell would get even weaker. Their small size, Li thinks, represents a compromise between two different functions—vision and defense—that exist in the same suit of armor.

That explains why the eyes are small. But why so numerous? There are a few possibilities. Having aragonite lenses means that a chiton's eyes, unlike ours, will slowly erode with time. That's why it constantly replaces them much like a shark continuously grows new teeth; and perhaps having hundreds of eyes provides even more back-ups. They also allow the animal to detect threats coming from many different directions, which is important because the eyes can't move and chitons take several minutes to turn around.

But Sonke Johnsen from Duke University, who was Speiser's graduate advisor, is still perplexed at how good the chiton's eyes are. “They're forming decent images in an animal that, to be really blunt, is not that smart,” he says. “A chiton doesn't have many ... behavioral outputs. It can wander around, graze, and glue down onto a rock. That's about it. Why something that only really needs to tell if a predator is round is building all these really beautiful crystal lenses, and paying the price of making their armor not as strong, is a really good question.”

“It goes against the general paradigm that the world is divided into dumb animals without heads and not a lot of sensory capabilities, and smart animals with heads that move around and have sharp sense,” he adds. “There are all these animals that don't fit these categories. They open our eyes up to new ways of solving the same problems.”