Chitons (Class Polyplacophora)

Chitons are molluscs and are related to slugs and snails (gastropods), mussels and oysters (bivalves), and squids and octopuses (cephalopods). Within the phylum Mollusca, chitons belong to a distinct group known as the Polyplacophora. This name refers to the eight separate shell plates, which are embedded in a thick girdle surrounding the body and cover the dorsal surface. Many chiton species are found in the intertidal zone, in shaded depressions or crevices or under rocks on open coast, where they are active at night or at high tide, but they also occur to depths of 1000 m or more.

Chitons do not have cephalic eyes or tentacles, and the head is located on the underside of the body, anterior to the broad muscular foot. The body is oval in outline and generally fairly flat, but the many species exhibit a wide diversity of colour and ornamentation, and may be smooth, scaly, spiny, granulose, and/or set with setae, spicules, or needles.

Chitons move by creeping slowly using the muscular foot for locomotion and adhesion, and their separate, articulating valves allow them to move over and cling tightly to sharply curved or irregular surfaces. When disturbed, chitons clamp down tightly against the substrate, and then lift the inner margin of the girdle to create a vacuum and hold the animal firmly in place. If dislodged, they will roll into a tight “‘amadillo” shaped ball for protection.

Most chitons are herbivorous and feed on algae and other sedentary organisms, including sponges, bryozoans, and coelenterates, which they scrape off the substrate or devour using a tongue-like organ, set with teeth (radula), which can be protruded and then retracted. Chitons in the genus Placiphorella however, have adopted a unique carnivorous feeding method by using the expanded front of the girdle to trap small crustaceans and other invertebrates. In these chitons, the dorsal surface of the girdle has long scaly hairs or setae, while the expanded anterior flange has setae dorsally, bristles on the side, and ventral papillae with several long, fleshy tentacles anterior and lateral to the head. The anterior girdle or head flap is held high above the substrate like a veil and the chiton traps small crustaceans and worms under the veil by lowering it rapidly when they crawl underneath.  Smaller crustaceans (2 mm and below) are swallowed whole, while larger prey including crabs up to 1 cm across are torn up by the radula before being eaten.