Polyplacophora – Chitons
The Polyplacophora are a class in the phylum of the Mollusca, which are often called chitons in the literature. Chiton, however, is only one of many genera. Polyplacophora, depending on sources, contain 750 to 1000 species.
The body of the Chitons is always elongated, oval. Viewed from above, eight aragonite valves are surrounded by the muscular girdle. Only in rare cases, the plates are partially or completely covered by the girdle. Their plates contain a mechanical sense organ, the so-called aesthetes. This allows them to experience water movements. Moreover, inside or outside of the aesthetes, there are ocelli, aragonite-based eyes. This allows them to perceive bright and dark without detaching the body from the ground.
On the underside are the foot, the radula, the subradula and the gills of the Polyplacophora.
With their strong feet, they are able to stay attached on the rocks in case of a strong water movement.
Magnetite is stored into the teeth of the radula, which makes them very hard. Chaetopleura apiculata has the hardest teeth known in nature!
The subradula organ shows the chitons, where there is edible material in its environment.
Their size varies between 0.5cm and 33cm.
Habitat and Distribution
Polyplacophora are marine animals which occur at least from the tropics to temperate climate. Some species are still found at a depth of 5000m. The majority, however, live close to the coast, on a solid ground, in the surf zone.
Most of them are herbivores that graze on rocks or mussels. Only a small part has passed to a predatory way of life. They lift the front part of their girdle and wait until small process in the mantle registers prey. They then press themselves firmly back against the rock, which means that the prey has no longer an escape route.
Polyplacophora have predominantly separated sexes. The fertilization takes place externally. A young chiton develops threw a free-flowing larvae stage.
Only in a few species an internal fertilization taking place, and some even breed the juveniles in their mantle.
Species of Chitons on the Canary Islands