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.