Chiton: The Animal with Hundreds of Teeny, Tiny Eyes in It’s Armor

Lined Chiton, Tonicella lineata
Lined Chiton, Tonicella lineata

The brightly colored sea creature above is a Tonicella lineata chiton, commonly known as a “lined chiton.” The picture was taken at about 50 feet depth on the west side of Whidbey Island, Washington. (Wikipedia)

The lined chiton is one of about 940 known extant species and 430 fossil species of the class Polyplacophora. This particular species grows to about two inches (5 cm) in length and has hundreds of tiny eyes, each less than a tenth of a millimeter across, embedded in the shell on its back. Each eye contains a lens, a light-sensitive retina, and a layer of pigment, just as your eyes and mine do. But they are much smaller, and they are scattered around the shell in all directions. No other armor in nature is known to be like this.

Closeup of chiton shell, showing black eyes
Closeup of chiton shell, showing black eyes

To imagine how very small these eyes are, first imagine the length of an inch. It’s very roughly the size of your second finger bone, just behind the tip. It takes 25 millimeters to make an inch, and each of this animal’s eyes is only a tenth of a millimeter wide. So you could fit 250 of them or so in a very thin line along your finger between the joints. I’d say those are tiny eyes! You could fit maybe a couple of hundred of them on the head of a straight pin, but they wouldn’t dance like angels are said to do.

The lenses are made from a mineral called aragonite, a crystalline form of calcium carbonate or limestone, which dissolves easily in acid. It is, in fact, the same mineral that pearls and abalone shells are made of. These animals literally peer at the world through lenses made of rock that erode as the animals age and have to be continuously replaced like shark’s teeth.

Armadillidium vulgare pill bug
Crustacean wood louse

Chitons are mollusks, related to snails, clams, and octopuses; but their oval bodies are covered by hard shells, each consisting of eight overlapping plates that give some of them the general appearance of wood lice.

Wood lice, also known as ball bugs, pill bugs, or roly-polies, are interesting in their own right, since they are the only group of crustaceans — think shrimp, crabs, lobsters, and barnacles — to have left the water and colonized the continents. It’s fascinating to me that this group of molluscs look so much like that group of crustaceans that we usually think of as filthy “bugs.”

But I digress. In fact, I’m often prone to digression. You may have noticed. Things are just so incredibly, wonderfully, marvelously fascinating it’s hard to stay focused on just one thing!

And people think science is dull? And boooring?!? Don’t ever believe it!

Two individuals of Acanthopleura granulata chitons on a rock at high tide level in Guadeloupe
Two less colorful mollusc chitons

Anyway, chitons live in salt water pretty much worldwide, but mostly in the tropics. The valves, or plates, of different species are variously colored, patterned, smooth, or sculptured; so some have been given colorful names like “sea cradles” and “coat-of-mail shells.”

Most chiton species live on hard surfaces, on or under rocks, or in rock crevices, in intertidal or subtidal zones. Some species actually live quite high in the intertidal zone and are exposed to air and light for long periods. Only a few species live in the deep ocean. Some of  the larger species can grow up to 13 inches (33 cm) long.

Daniel Speiser, then a graduate student at the University of California, Santa Barbara, dissected the lenses from the eyes of a West Indian fuzzy chiton, and dunked them into an acid bath to clean them. But they didn’t get clean. They just disappeared. That’s when he discovered they were mineral instead of organic, and had simply dissolved in his acid.

The underside of the gumboot chiton, Cryptochiton stelleri, showing the foot in the center, surrounded by the gills and mantle- The mouth is visible to the left in this image.
Underside of gumboot chiton

He later teamed up with Ling Li and Matthew Connors, two graduate students from MIT, who tested the individual eyes to see if they could actually form images. They can, but not very clear ones. Because each eye is too small to have very many photosensitive cells, they form blurry, heavily pixellated images; but each eye is capable of detecting the shape of an 8 inch (20 cm) fish from a few yards away. This should be good enough to help with predator avoidance. (Though just how the slow-moving animals can use the information to avoid predation is still in question.)

Chitons have a dorsal shell — i.e., a shell on their back — which is composed of eight separate plates. These plates overlap a little at the front and back edges, but articulate well. Because of this, although the plates provide good protection for impacts from above, they still permit the chiton to flex upward to move over uneven surfaces. They also allow the animal to curl up into a ball, like the “ball bug” mentioned above.

Many species are edible and are enjoyed by various peoples around the world. (However, don’t try eating the ball bugs some of them look like.)



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