ChitonsPolyplacophora

A chiton grazes rock, and it wins. The teeth on its radula are capped with magnetite, an iron mineral, and they are the hardest biomineral any animal is known to make, harder than the limestone the animal is scraping. A chiton feeding on a coastal boulder cuts into the stone itself, and on shores where they are common they count among the animals that wear a rocky coast away.

The rest of the animal is built like a small tank. A chiton is a mollusc with a low oval body carrying eight overlapping shell plates set in a leathery ring of muscle called the girdle. The plates articulate, so the armor bends: a chiton can flatten itself onto an uneven rock with no gap anywhere around the edge for a beak or a claw to get into, and if something does work it loose, it rolls into a ball with the plates outward, the same trick a pillbug uses.

The plates are sense organs as well as armor. Thousands of fine channels run up through the shell and end in windows on its surface, and in some species those windows hold lenses of aragonite that form real images. It is an animal looking out through its own shell, with eyes by the hundred made of the same material they are set in. What they are good at is exactly what matters here: a shadow moving overhead, and the clamp comes down before the shadow arrives.

On this coast they are a shore animal more than a diving one. Five species are listed for the 🌊 Red Sea, on rock and dead coral from the splash zone into shallow water, grazing at night and coming back afterward to the same patch, where years of return traffic wear a shallow scar in the rock that fits one animal exactly.