Chiton

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This article is about the mollusk. For the ancient Greek article of dress (the creature's namesake) see chiton (costume).

Chiton
Fossil range: Cambrian - Recent

Lined Chiton (Tonicella lineata)

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Scientific classification

Kingdom:	Animalia
Subkingdom:	Metazoa
Phylum:	Mollusca
Class:	Polyplacophora J.E. Gray, 1821

Families

See text.

Chitons are mollusks that live near the edge of the ocean in most of the world, but some species have been found in deep water. They creep along slowly on their muscular feet and cling to rocks. Chitons have shells made up of eight overlapping calcareous plates.

Chitons are also called sea cradles, loricates, coat-of-mail shells, polyplacophorans, Chitonidae and rarely polyplacophores. There are 900-1000 extant species of molluscs of the Class Amphineura or Polyplacophora. Chitons, unlike some other molluscs, are entirely marine; most species are found in the intertidal zone (the "littoral" zone), on or in rocks, but some species have been found as deep as 6000 meters (about 20,000 feet). Individual plates are sometimes called "butterfly shells."

1 Structure
2 Name
3 Scientific Investigation
4 Activities
5 Classification
6 Habitat
7 Miscellaneous
8 References
9 External links

[edit] Structure

The underside of Crytochiton stelleri (The gumboot chiton), showing the foot, in the center, surrounded by the gills and mantle. The mouth is visible above and to the left of the foot.

Ontogeny of Polyplacophora: First image is trochophore, second is stadium in metamorphosis, third is juvenile Polyplacophora.

The calcareous armour shell that chitons carry dorsally are protective, are made wholly of aragonite, and are variously colored, patterned, smooth or sculptured. The shell is divided into eight articulating plates (also called 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. [1] 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 ferrous mineral that hardens the teeth. The radula is used to scrape microscopic algae off the substratum.

[edit] Name

Black Chiton (Katharina tunicata) on a stone with a starfish

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".^[1]

The Greek-derived name Polyplacophoran comes from the words poly- (many), plako- (tablet), and -phoros (bearing).

[edit] Scientific Investigation

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.

[edit] Activities

Chitons eat algae, bryozoans, diatoms and sometimes bacteria by scraping the rocky substrate with their well-developed radula. Some species have an enlarged anterior girdle by which they feed on 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 during the day and coming out at night to feed.

Predators include seagulls, starfish, crab, fish, and sea anemones.

[edit] Classification

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).

Classification method I [2]:

Class Amphineura/Polyplacophora
- Order Neoloricata
  - Family Leptochitonidae
    
    include genus Leptochiton
  - Family Afossochitonidae
  - Family Ischnochitonidae
    
    include genera Callistochiton, Callochiton, Chaetopleura, Ischnochiton, Lepidochitona, and Lepidozona
  - Family Mopaliidae
  - Family Chitonidae
    
    include genus Chiton
  - Family Cryptoplacidae
  - Family Acanthochitonidae
    
    include genera Acanthochitona, Notoplax
Class Monoplacophora
- Order Tryblidiida
  - Family Neopilinidae
  - Family Vemidae
  - Family Monoplacophoridae
  - Family Laevipilinidae

The following classification is Parker, S. P. [3]:

Class Amphineura
- Sub-class Polyplacophora
  - Order Neoloricata
    - Suborder Lepidopleurina
      - Family Hanleyidae
      - Family Choriplacidae
      - Family Lepidopleuridae
    - Suborder Ischnochitonina
      - Family Subterenochitonidae
      - Family Ischnochitonidae
      - Family Schizoplacidae
      - Family Callochitonidae
      - Family Callistoplacidae
      - Family Chaetopleuridae
      - Family Mopaliidae
      - Family Schizochitonidae
      - Family Chitonidae
    - Suborder Acanthochitonina
      - Family Acanthochitonidae

Chitons were divided into Chismobranchians and Polyplaxiphores in the early nineteenth century.

[edit] Habitat

Chitons are found in the littoral zone at the edge of the ocean throughout most of the world. Some species are found at depths of 6000 meters.

Lined Chitons range from Northern Japan around the Aluetians to California. It is found in both the Intertidal and Subtidal zones, commonly on rocky surfaces. Chitons are particularly diverse and abundant in the shores of South Australia.