As a fish swims over the ocean floor, it’s being watched by hundreds of rocks. The rocks are actually the eyes of a chiton, an armoured relative of snails and other molluscs. Perhaps uniquely among living animals, it sees the world through lenses of limestone, and its eyes literally erode as it gets older.

Chitons are protected by a shell consisting of eight plates. The plates are dotted with hundreds of small eyes called ocelli. Each one contains a layer of pigment, a retina and a lens. People have known about the ocelli for years, but no one knew what they were made from or how much the chitons could actually see with them.

Daniel Speiser from the University of California, Santa Barbara has solved the mystery by studying the charmingly named West Indian fuzzy chiton. It all started with a surprising bath. Speiser had removed the lenses from a chiton and dipped them in a mildly acidic liquid, which was meant to clean them. Instead, it quickly dissolved them!

The vast majority of animal lenses are made of proteins, which should be unharmed by weak acid. The chiton lenses were ...