Chitons see through thousands of tiny eyes built into their shells
A chiton looks like a slow, armoured oval clamped to a rock. Its shell is made of eight overlapping plates, and in some species it is studded with thousands of tiny eyes whose lenses are made of mineral. They can tell the shadow of a predator from a passing cloud, and may be the most recently evolved eyes on Earth.
Around 940 living species are known, along with 430 fossil ones, all strictly marine. Most cling to rocks between the tides, though a few live as deep as 6,000 metres. The eight plates, held together by a tough muscular skirt called the girdle, let a chiton hug uneven surfaces and roll into a ball if knocked loose. When one dies the girdle lets go, and the separate plates, nicknamed butterfly shells, sometimes wash up on beaches.
Beneath the shell surface sit light-sensitive organs called aesthetes. In some species these form ocelli, each with a lens of aligned aragonite crystals that can focus a clear image. Fossils show these eyes only in shells 10 million years old or younger. There is a price: the bigger and more complex the eyes, the weaker the shell, and vice versa.
The feeding tongue, or radula, carries rows of 17 teeth coated in magnetite, a hard iron oxide, used to scrape algae off rock. The iron may do double duty, since experiments suggest chitons can detect magnetic fields. Several species also show homing: they wander off to graze and return to exactly the same spot, possibly by memorising the rock's contours or following chemical traces, though no one yet knows for sure.
The girdle can bristle with spines; in some Acanthochitona species these break off in careless fingers and stay lodged as an irritant. The largest species, the gumboot chiton of the North Pacific, grows to 36 centimetres. People eat chitons from the Caribbean and the Philippines to the Pacific coasts of the Americas, preparing the foot much like abalone.
Source: Chiton
