Chitons: Polyplacophora

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CHITONS: Polyplacophora



Chitons (KI-tons) are flattened mollusks that are egg-shaped in outline. They have eight distinct and overlapping shell plates, or valves, across their backs. The valves are layered, with each layer made up of mostly calcium carbonate. Each valve is usually shaped like a butterfly. A ring of tissue surrounds or sometimes covers the entire body. This tissue is the margin of the mantle. As in other mollusks, the mantle produces the valves. Because the chiton mantle is stiff and surrounds the body, it is referred to as a girdle. Depending on the species, the surface of the chiton's body may be covered with scales, bristles, or small spines.

The underside of the body is made up of a broad, muscular foot. On the sides of the body, the foot and girdle are separated by a special groove. Inside the groove are gills that help the chiton to breathe underwater. Oxygen-carrying water enters the grooves near the head, flows through the gills, and exits at the rear of the body. As the water exits this system, it carries away waste products released from the anus. The anus is the opening at the end of the digestive tract through which solid waste leaves the body. The water also flushes away liquid waste produced by the chiton's two large kidneylike organs. These kidney-like organs remove waste from the blood found inside the body cavity.

The adult chiton head is not distinct and does not have any eyes or tentacles. The mouthparts are made up of the radula. The radula is a long, beltlike structure with seventeen bands of curved teeth. The teeth are very hard. In some species, the teeth are covered with a material that contains iron.


Chitons are found along seashores worldwide but are most common in cooler waters.


Chitons live on hard surfaces, especially rocks. They are found from the seashore to depths of more than 13,123 feet (4,000 meters).


Chitons eat many kinds of algae (AL-jee). Placiphorella velata preys on worms and small crustaceans.


Most chitons feed by scraping food off rocks with their radula. Others eat large species of algae, such as kelp. Predatory species use a special flap on the mantle near their head to capture small animals. Most species usually feed at night. Species living along the shore remain in one place when exposed to the air by low tides.

Chitons have few defenses. If pulled off their rocks, some species will roll up like a pillbug. This motion also helps them to right themselves. Some species will return to the same spot on a rock after they forage for food, while others continue to move on as they feed.


Roughly half of the approximately one thousand species of chitons live near coastlines. And, more kinds of chitons live along the eastern shores of the Pacific Ocean than anywhere else. About one-fifth of all the world's species live on or near the coastline that runs from Alaska south to Southern California.

Both male and female chitons are usually required for reproduction. Males always release their sperm into the sea. The sperm is carried on the ocean currents to the eggs. Depending on the species, females either release their eggs singly or in strings into the water or keep them inside the special groove that separates the girdle and muscular foot. Eggs fertilized in the water usually develop into free-swimming, unsegmented larvae (LAR-vee) covered with tiny, hair-like structures called cilia (SIH-lee-uh). Eggs that develop inside the groove remain with the adult female until they become well-developed young chitons.


Native Americans living along the Pacific Coast of North America used to eat the gumboot chiton, Cryptochiton stelleri. The remains of their valves are commonly found in their ancient trash heaps.


No species of chiton is considered threatened or endangered.


Physical characteristics: This is the largest chiton in the world, measuring up to 13 inches (330 millimeters) in length and 5 inches (130 millimeters) across. It resembles a brick-red meatloaf. Its large size and color distinguish this species from other chitons. The leathery and reddish mantle wraps around the entire body and hides all of the valves. It is covered with bundles of tiny spines.

Geographic range: Gumboot chitons are found from the Aleutian Islands in Alaska, south to San Miguel and San Nicolas Islands off the coast of southern California. They are also found in northern Hokkaido Island, Japan, and the Kurile Islands in Kamchatka, Russia.

Habitat: This species lives on rocky shorelines and soft bottoms in protected habitats near deep channels at depths of 70 feet (21.3 meters).

Diet: They eat several kinds of algae, including giant kelp, sea lettuce, and red algae.

Behavior and reproduction: This species grows very slowly and lives as long as twenty years. They have a weak grip and often fall from their rocks at low tide. They do not live in groups and move very slowly. Captured individuals have remained within 65.6 feet (20 meters) of their release point, even after two years.

California populations reproduce between March and May. The reddish eggs are laid in jelly-like spiral strings measuring up to 3.3 feet (1 meter) in length. The strings are broken up into smaller pieces by ocean waves. The eggs hatch about five days after they are fertilized. The free-swimming larval stage lasts about twenty hours. Then they settle to the bottom and develop into young chitons.

Gumboot chitons and people: This species was eaten by Native Americans living along the Pacific Coast of North America.

Conservation status: Gumboot chitons are not considered threatened or endangered. ∎


Physical characteristics: This species is distinguished by the girdle around the margins of shell valves, which is greatly expanded toward the front to form a "head flap." The girdle is red and green underneath. The body is up to 1.9 inches (5 centimeters) long, with brown to red shell valves that are short and wide and variously mottled and streaked with green, beige, white, and brown.

Geographic range: Although uncommon, the veiled chiton is found from Forrester Island (Alaska) to Isla Cedros (Baja California) and the upper Gulf of California.

Habitat: This species lives in rocky depressions and crevices or under stones. They are found from the lower levels of shorelines affected by tidal action to a depth of 50 feet (15.2 meters).

Diet: They scrape algae off rocks or capture small worms and crustaceans.

Behavior and reproduction: This species belongs to the only group of predatory chitons. Like other chitons, this species moves very slowly, but it uses a special flap on the girdle near the head to capture small animals.

California populations reproduce in September.

Veiled chitons and people: This species is not known to impact people or their activities.

Conservation status: Veiled chitons are not considered threatened or endangered. ∎



Meinkoth, N. A. The Audubon Society Field Guide to North American Seashore Creatures. New York: Alfred A. Knopf, 1992.

Pearse, V., J. Pearse, M. Buchsbaum, and R. Buchsbaum. Living Invertebrates. Boston, MA: Blackwell Scientific Publications, 1997.

Slieker, F. J. Chitons of the World. Ancona, Italy: L'Informatore, 2000.

Web sites:

Worldwide Chitons! (accessed on April 27, 2005).

Welcome to (accessed on April 27, 2005).