Chiton (Polyplacophora): Marine molluscs with eight-plated shells

Anatomy and distinctive features

Chitons have eight articulating dorsal shell plates (valves) set into a tough, often ornamented girdle that surrounds the body. Underneath is a broad, muscular foot used for strong adhesion to substrate and slow locomotion. The mouth contains a rasping radula used to scrape algae and biofilm; in many species the radular teeth are reinforced with iron minerals (including magnetite) to withstand abrasion. The mantle and shell bear numerous tiny sensory organs called aesthetes; in some species these form arrays of hundreds of microscopic eyes with mineral lenses that detect light and shadows.

Ecology and behavior

Most chitons are herbivorous grazers feeding on microalgae, diatoms and encrusting organisms. They are well adapted to high-energy shorelines and can cling tightly to rock to resist waves and predators. Many intertidal species show homing behavior, returning to a preferred scar on the rock when the tide recedes. Some deep-water chitons are detritivores or have specialized diets associated with chemosynthetic habitats.

Reproduction and life cycle

Reproduction is typically by external fertilization: males and females release gametes into the water column. Fertilized eggs develop into planktonic larvae that disperse before settling and metamorphosing into juvenile chitons, during which the characteristic plates form gradually.

Evolution and fossil record

Chitons have a long evolutionary history, documented by fossil forms that indicate the group has persisted for hundreds of millions of years. The basic multi-plated body plan has remained relatively conservative, although fossil species show a range of plate shapes and girdle structures that illustrate past diversity.

Human relevance and identification

Chitons are not of major commercial importance but are ecologically significant on rocky coasts and of scientific interest for studies of biomineralization, sensory systems and intertidal ecology. At a glance they might be mistaken for limpets or other shelled animals, but the combination of eight overlapping plates and a broad creeping foot distinguishes them readily.

Key characteristics

• Class: Polyplacophora
• Eight dorsal plates (valves) embedded in a flexible girdle
• Broad muscular foot used for adhesion and movement
• Rasping radula; some teeth mineralized with iron (magnetite)
• Sensory aesthetes, in some species forming numerous tiny eyes
• Worldwide distribution from intertidal rocks to deep sea