ET Cook

Chiton

[kahyt-n, kahy-ton]

Chitons are small to large, primitive marine mollusks in the class Polyplacophora. There are 900 to 1,000 extant species of chitons in the class, which was formerly known as Amphineura).

These mollusks are also sometimes commonly known as sea cradles or "coat-of-mail shells". They are also sometimes referred to more formally as loricates, polyplacophorans, and rarely as polyplacophores.

Chitons have a shell which is composed of eight separate shell plates or valves. These plates overlap somewhat at the front and back edges, and yet the plates articulate well with one another. Because of this, although the plates provide good protection for impacts from above, they nonetheless permit the chiton to flex upward when needed for locomotion over uneven surfaces, and also the animal can slowly curl up into a ball when it is dislodged from the underlying surface. The shell plates are surrounded by a structure known as a girdle.

The English name "chiton" originates from the Latin word chitōn, which means "mollusk", and in turn is derived from the Greek word "khitōn", meaning tunic (which also is the source of the word chitin). The Greek word "khitōn" can be traced to the Central Semitic word "*kittan", which is from the Akkadian words "kitû" or "kita’um", meaning flax or linen, and originally the Sumerian word "gada" or "gida".[2]

The Greek-derived name Polyplacophora comes from the words poly- (many), plako- (tablet), and -phoros (bearing), a reference to the chiton's eight shell plates.

Distribution

Chitons live worldwide, in cold water and in the tropics.

Habitat

Chitons live on hard surfaces such as on or under rocks, or in rock crevices. Some species live quite high in the intertidal zone and are exposed to the air and light for long periods. Others live subtidally. A few species live in deep water, as deep as 6,000 m (about 20,000 ft).

It is worth pointing out that chitons as a molluscan class are exclusively and fully marine. This is in contrast to the bivalves which were able to adapt to brackish water as well as freshwater, and the gastropods which were able to make successful transitions to freshwater and terrestrial environments.

Culinary uses

Chitons are eaten in Tobago and were eaten by native Americans of the Pacific coasts of both North and South America. The foot of the chiton is prepared in a manner similar to abalone.

Shell description

Chitons have shells made up of eight overlapping calcareous valves held together and surrounded by a girdle. In many species the surface of the girdle is covered in, or decorated with, scales, hair-like protrusions, or glassy bristles.

After a chiton dies, the individual valves which make up the 8-part shell come apart because the girdle is no longer holding them together, and then the plates sometimes wash up in beach drift. The individual shelly plates from a chiton are sometimes known as "butterfly shells" because of their shape.

The most anterior plate is crescent shaped, and is known as the cephalic plate (or head plate, although chitons don't have a head). The most posterior plate is known as the anal plate (or the tail plate, although chitons don't have a tail.)

Life habits

A chiton creeps along slowly on a muscular foot, and can cling to rocks very powerfully, like a limpet.

Chitons eat algae, bryozoans, diatoms and sometimes bacteria by scraping the rocky substrate with their well-developed radula.

A few species of chitons are predatory, such as the small western Pacific species Placiphorella velata. These predatory chitons have an enlarged anterior girdle. They catch other small invertebrates, such as shrimp and possibly even small fish, by holding the girdle up off the surface and then clamping down on the unsuspecting, shelter-seeking prey.

Some chitons exhibit homing behavior, returning to the same spot for the daylight hours and roaming around at night to feed.

Predators

Animals which prey on chitons include seagulls, seastars, crabs, and fish

The largest species

The largest chiton (up to 33 cm in length) is the brick-red gumboot chiton of the Pacific Northwest. In this species the valves are completely internal.

Anatomy

The calcareous valves that chitons carry dorsally are protective, made wholly of aragonite, and variously colored, patterned, smooth or sculptured. The shell is divided into eight articulating valves embedded in the tough muscular girdle that surrounds the chiton's body. This arrangement allows chitons to roll into a protective ball when dislodged and to cling tightly to even irregular surfaces.

The girdle is often ornamented with spicules, bristles, hairy tufts, spikes, or snake-like scales. The majority of the body is a snail-like foot, but no head or other soft-parts beyond the girdle are visible from the dorsal side.

Between the body and the girdle, there is a mantle cavity, connected to the outside by two water channels. The one on the side is the incurrent water channel. The one attached to the anus is the excurrent water channel.

The gills hang down into the mantle cavity, usually near the anus. An anterior head has a mouth containing a tongue-like structure called a radula, which has numerous rows of usually 17 teeth each. The teeth are coated with magnetite, a ferric/ferrous oxide mineral that hardens the teeth. The radula is used to scrape microscopic algae off the substratum.

Fossil ancestors of chitons

A possible Pre-Cambrian ancestor of chitons is Kimberella. Chitons may also share a connection to Wiwaxia of the Cambrian. A Late Cambrian polyplacophoran preserved as individual pointed valves is Matthevia.

History of the scientific investigation of chitons

Chitons were first studied by Carolus Linnaeus in 1758. Since his description of the first four species, chitons have been variously classified. They were called Cyclobranchians ("round arm") in the early 19th century, and then grouped with the aplacophorans in the subphylum Amphineura in 1876. The class Polyplacophora was named by J. E. Gray in 1821.

Taxonomy

Most classification schemes in use today are based, at least in part, on Pilsbry's Manual of Conchology (1892-1894), extended and revised by Kaas and Van Belle (1985-1990).

Since chitons were first described by Linnaeus (1758) there have been extensive taxonomic studies at the species level. However, the taxonomic classification at higher levels in the group has remained somewhat unsettled.

The most recent classification (Sirenko 2006) is based not only on shell morphology, as usual, but also other important features including aesthetes, girdle, radula, gills, glands, egg hull projections and spermatozoids. It includes all the living and extinct genera of chitons.

This system is now generally accepted.