Invertebrates
Chiton
Polyplacophora · also called chitons, coat-of-mail shells
A chiton wears its armour in eight overlapping plates instead of one solid shell, letting it flex around uneven rock and curl into a protective ball if it is ever knocked loose. Its teeth are capped with a mineral that makes them harder than human tooth enamel, and in some species those same shell plates hide hundreds of tiny eyes made from the same material as the armour itself.
Overview
Chitons, class Polyplacophora, are armoured marine mollusks found clinging to rocky shores worldwide. A muscular foot grips the rock with remarkable force while a hard, magnetite-capped radula scrapes algae directly off the surface.
Armour, eyes, and a favourite spot
The segmented eight-plate shell flexes around uneven rock and lets a dislodged chiton curl into a ball for protection. Some species even carry true image-forming eyes built from aragonite embedded in the shell plates, and many show a strong homing instinct, always returning to graze from and rest in the exact same worn spot on the rock.
Status
Least Concern — common and widespread across rocky coastlines worldwide, secure thanks to strong armour and an abundant algal food supply.
Chiton: common questions
Why does a chiton have eight separate shell plates instead of one solid shell?
The segmented, eight-plate shell arrangement gives a chiton considerably more flexibility than a single rigid shell would allow, letting the animal bend and conform closely to an uneven or curved rock surface while still remaining continuously armoured, and it also allows the chiton to curl into a protective ball if dislodged. This combination of flexibility and full-body protection is a distinctive solution not seen in the great majority of other mollusks, most of which rely on a single, non-segmented shell.
Are chiton teeth really harder than human teeth?
Yes, the teeth on a chiton's radula, its rasping feeding structure, are capped with the mineral magnetite, and measurements of this material have found it to be among the hardest biological substances known, exceeding the hardness of human tooth enamel. This exceptional hardness allows a chiton to scrape algae directly and repeatedly off hard, abrasive rock surfaces without its feeding teeth wearing down too quickly to remain effective.
Can a chiton actually see with eyes built into its shell?
In a number of chiton species, yes, in a genuinely unusual arrangement, hundreds of microscopic light-sensing structures called aesthetes are embedded directly within the shell plates themselves, and in some species these have evolved into true, image-forming eyes constructed from aragonite, the same mineral that makes up the shell. This means certain chitons effectively have eyes made of the same material as their armour, an arrangement that is considered highly unusual among animals generally.
Why is a chiton so hard to pull off a rock?
A chiton's broad, muscular foot can create very strong suction and grip against a rock surface, and a firmly attached, undisturbed chiton can resist a surprising amount of force before letting go, a defensive strategy that helps protect it both from predators attempting to pry it loose and from being swept away by strong wave action in its often turbulent intertidal habitat. Attempting to pull one off forcefully can injure the animal, which is why marine biologists generally recommend allowing a chiton to detach naturally rather than prying it free.