MOLLUSCS OF BRITISH COLUMBIA

 Class Polyplachora (Chitons)

by

James A. Cosgrove

Click here for further information on chitons, and a key to the species of chitons by Aaron Baldwin

The Class Polyplacophora (poly = many, placo = tablet, phoros = bearing) (formerly known as the Class Amphineura) is characterized by animals having 8 overlapping plates that may or may not be clearly visible.  These animals, commonly called chitons, have a worldwide distribution in the intertidal zone or in relatively shallow water where sunlight is present and where plants grow.  However, some chitons live in depths to 6,000 meters and some hunt meat. 

Current literature suggests that slightly less that 1,000 species exist today.

The vast majority of these strictly marine animals are herbivores or plant eaters.  Chitons, like most members of the Phylum Mollusca, have a file-like tongue called a radula.  This tongue is pressed against the food and then the tongue is pulled back into the mouth.  The teeth on the radula carve off small bits of the plant.

A chiton has a single large foot for locomotion.  Along the edge of the foot and suspended in the mantle cavity are the gills for respiration.  On the outside edge of the mantle cavity is the fleshy girdle that is often covered in bristles or other structures.  The girdle is frequently colourful and may cover the plates completely.

When a chiton dies the calcareous plates often separate and may be found on the beach.  The individual plates, commonly called “sea butterflies” due to their shape, have nothing to do with terrestrial insects or with the pteropods (Class Gastropoda), which are also called “sea butterflies”. 

The world’s largest chiton, Cryptochiton stelleri, called the Gumboot Chiton is common in British Columbia.