Chitons are oval shaped marine mollusks with flattened bodies.  Their dorsal shell is made up of eight separate plates or valves.  When the shell plates are lined up together, they resemble the overlapping metal plates of ancient armour and have been called “coat-of-mail shells”.  I personally think of these oval creatures as small prehistoric animals roaming over the rocky bottom.  They are in fact a very ancient lineage of mollusk and fossil evidence suggests that the chiton has changed little since they first appeared in the Late Cambrian period (about 500 million years ago).

Chitons move by the use of a large muscular foot on their underside, and this foot will cling tightly to the rocky surface it is resting on.   The eight protective shell plates are controlled by a complex muscle system, allowing the chiton to move over sharp edges or irregular surfaces.

Chitons, like snails, have a rasp tongue or radula that they use to scape food off of the rocky substrate, and feed mostly on algae and bryozoans.

These animals have separate sexes with fertilization taking place externally.  The male releases his sperm into the water, and the female releases her eggs, which are often in long strings.  Fertilization usually takes place in the surrounding water, but occasionally may take place within the mantle of the female.

The giant Pacific chiton of the Pacific Northwest has eight shell plates but they are all hidden under the animal’s girdle, and not exposed like the shell plates of other chitons.  This chiton also known as a gumboot is the largest chiton in the world, growing to 14 inches (36 cm) in length.  The gumboot is very long-lived and may roam over rocky surfaces for up to forty years, which I think is amazing.  I’m often surprised by how many gumboot chitons I’ll see on a dive, and sometimes by the occasional acrobatic ability of this giant chiton as it stretches out across a small abyss to reach toward another nearby rocky surface.