Chitons

A Chiton (Tonicella lokii) off the coast of California

Italo Calvino’s existential classic, Il cavaliere inesistente (“The Nonexistent Knight”) is a novel about Sir Agilulf, a medieval knight who follows the rules of chivalry with complete devotion. Unfortunately, Sir Agilulf is only an empty suit of armor: although the knight trains and stands vigils and fights in the manner of a person, there is actually nobody inside the shell. Calvino’s perplexing fable evocatively captures the workaday world where many (maybe most) people are empty “suits” who exist merely to keep a seat warm, however this predicament is nothing when compared to that of the chiton, a primitive marine mollusk covered by a row of aragonitic shells. Not only are the chiton’s rocky eyes a constituent part of its armor, the chiton itself is part of its armor!  Bands of dense muscles are interwoven through the plates and sensory cells are embedded within. When the chiton dies, the shell phalanx falls apart into perplexingly shaped hunks of calcium carbonate.   The remarkable segmented shells are composed of 8 plates and they afford substantial protection to the chitons which are capable of rolling themselves into armored balls. Chitons are classed as the Polyplacophora.  They take their common name “chiton” from the Latin word chitōn, which means “mollusc”. The Romans derived this word from the Greek word “khitōn”, meaning tunic.

Another Chiton (Tonicella marmoreal)

Cryptoconchus porosus

 

For locomotion the chiton relies on its rubbery “foot”, a large band of adhesive muscles with which it crawls along the sand and rocks of the ocean bed. A chiton’s foot can produce substantial adhesion, and the creatures are able to cling to rocks with amazing tenacity.  Most chitons are herbivorous grazers and eat algae, bryozoans, diatoms, and other microbes, all of which they scrape up with their sharp radula tongue–however, a few species of chiton have left the gentle lifestyle of herbivore behind and become predators. These hunting chitons trap their prey (usually small shrimp and fish) by making a box trap out of their enlarged, hood-like front end.  They hold this segment of their girdle above the ocean bottom and then clamp down on unsuspecting prey which thinks they are small cavern-like rocks.

The Gumboot Chiton (Cryptochiton stelleri), the world's largest chiton!

 

Chitons are ancient.  Fossils of stem-group chitons date back to the beginning of the Ordovician over 488 million years ago (and these lineages probably stretch back earlier into the Cambrian). Yet the Chiton’s rocky aragonite eye is comparatively recent, having evolved only ten million years ago. Chitons are hundreds of millions of years older than mammals but their eyes are much more contemporary than our own.

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