The chiton is a rather timid sea creature that spends its time clinging to rocks and grazing. But one fascinating aspect of its anatomy is probably what keeps it alive. The chiton's body is covered with scaly armour that has, literally, hundreds of eyes.

Unlike human eyes, chiton eyes are made of aragonite. Argonite is a mineral made up of calcium carbonate, also known as limestone, are dissolvable in acid and slowly erode as the chiton goes about its business. The small mollusk grows new eyes on its scales to replace the old ones.

When scientists flashed bright lights on chitons, they found that the creatures would get into defensive positions by flattening themselves against whatever surface they were on. The scientists inferred from this behaviour that chitons do indeed see with their aragonite eyes. Under X-ray scans the argonite in plates acting as lenses appeared much bigger than in other parts – a mechanism to reduce scattering of inbound light.

On projecting objects through the argonite lenses, the research team saw blurry and heavily-pixelated images being formed. The kind of "sight" needed to spot a 20-centimeter fish from a distance of several meters.


Beneath the lenses are large patches of soft sensory tissues. The presence and size of the lenses shows a trade-off between the chiton's vision and its defence mechanism.

The scientists were surprised that an animal with very few behavioural functions still has such an extensive visual mechanism. Speaking to The Atlantic, Sonke Johnsen from Duke University said, “They're forming decent images in an animal that, to be really blunt, is not that smart."

The chiton's vision, however blurry, is providing new insights to the effect that animals need not be classified only as the smart ones with heads and the less smart ones without heads.