This Bizarre Rock Creature Has Minerals for Eyes

An alien in our midst, in the form of a Caribbean chiton.

A living chiton, Acanthopleura granulate, also known as the West Indian fuzzy chiton, on a rock in Guadeloupe in the Caribbean.

What amazing biological adaptations might we discover when we finally find extraterrestrial life? Even within Earth’s biosphere, we’ve identified one incredible species after the other. One especially intriguing example is a chiton—a type of mollusk—that uses minerals as eyes.

The chiton Acanthopleura granulate looks like a fossil that became rock, but is actually a living organism that thrives in the Caribbean and uses heavy armor to protect itself from predatory fish. Previous work already hinted that smoothly aligned aragonite, a type of calcium carbonate mineral, could function in the chiton as an eye. Each eye lens is less than a tenth of a millimeter in size, and the animal has hundreds of them. What was unclear was whether this set-up, using aragonite instead of proteins for “eyes,” could create an actual image.

Now Ling Li and coauthors have found that it can. They determined that images could be formed of objects such as a predatory fish by individual mineral lenses in water. The rock lenses form clear images of a 20-centimeter (5-inch) long fish from a distance of 30 cm (7.5 inches). The images have enough resolution to allow the chiton to react to the fish by clamping down to the rock substrate so that it is not easily dislodged.

The shells of chitons have thus evolved to satisfy two conflicting environmental requirements: they’re good for protection and they can be used to sense the organism’s surroundings. The mineral eyes are located in the valleys of the protruding armor, which protects them from blunt impacts in the same way that our eyes are protected by eye sockets.

Still unknown is how this relatively simple organism integrates the visual information from hundreds of eyes. But nature seems to be very inventive with regard to vision, since even some single-celled organisms have eyes, and there are many different evolutionary designs for eyes. All of which makes us wonder: What solution would an extraterrestrial organism employ to sense its surroundings? Would it be something found on Earth, or something completely different?

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