Chitons (Polyplacophora) are a class of flattened, oval-shaped marine mollusks found worldwide. This ancient lineage includes over 900 living species recognized today. Often called “coat-of-mail shells” or “sea cradles,” the chiton’s most distinguishing feature is its unique dorsal armor. This protective covering allows them to thrive in harsh environments, making them significant members of the ocean’s coastal ecosystems.
Defining Physical Characteristics
The chiton’s protective armor is not a single shell but a series of eight separate, overlapping plates called valves, embedded along the dorsal side of the body. This eight-part structure, composed of the mineral aragonite, provides robust defense and flexibility. The plates articulate well, allowing the chiton to flex its body to conform tightly to irregular rock surfaces or curl into a protective ball when dislodged.
Encircling these plates is a tough, muscular ring of tissue known as the girdle. The girdle secures the plates and is often covered in structures like spicules, bristles, or scales. On the ventral side, the chiton possesses a large, flat, muscular foot. This foot, combined with the girdle, creates a powerful suction that allows the animal to cling tenaciously to hard substrates and serves as its primary means of movement.
Habitat and Movement
Chitons are exclusively marine organisms that inhabit hard bottoms, ranging from the rocky intertidal zone to the deep sea, though most species are found in shallow water. They are most commonly observed in the high-energy intertidal zone, living on or under rocks and in crevices. Their flat shape and powerful adhesion are adaptations that help them withstand intense wave action and the risk of desiccation during low tide.
Locomotion is accomplished through a slow, creeping movement of the large foot, similar to a snail. Many intertidal species exhibit homing behavior, moving out to graze during high tide. They return to a specific, sheltered spot, often a crevice, during low tide.
Feeding Mechanism and Unique Biology
The chiton is primarily a herbivorous grazer, using a specialized feeding organ called a radula to scrape microalgae and diatoms off hard rock surfaces. The radula is a ribbon-like structure lined with numerous teeth, which are continually replaced as they wear down. This constant scraping against abrasive rock has led to the evolution of one of the hardest known biological materials in the animal kingdom.
The cusps of the chiton’s largest teeth are reinforced with an iron-based mineral called magnetite. This formation involves a complex biomineralization process where the chiton biologically incorporates iron into the teeth caps. New teeth are produced along a conveyor-belt-like structure, and as they mature, specialized cells modulate the pH to favor the precipitation of iron oxide. This results in a dark, ultra-hard cap, making the teeth strong enough to grind against the hardest substrates without breaking.
Life Cycle and Ecological Role
Chitons have separate sexes, and reproduction generally involves external fertilization in the water column. The animals release eggs and sperm into the sea, where fertilization occurs. Following successful fertilization, the embryo develops into a free-swimming trochophore larva.
This planktonic larval stage is an important element of the marine plankton before the larva settles and metamorphoses into the flattened adult form. As adults, chitons function as primary consumers, grazing extensively on the thin layer of algae coating the rocks. Their grazing pressure is a significant factor in structuring intertidal communities by preventing the unchecked growth of certain algal species. Chitons also serve as a food source for various predators, including fish, shorebirds, and sea stars, playing a key role in the coastal food web.