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".
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.
Chitons live worldwide, in cold water and in the tropics.
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.
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.
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.)
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.
Animals which prey on chitons include seagulls
, 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.
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.
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.
Subclass Loricata Shumacher, 1817
- Class Polyplacophora Gray, 1821
- Order Lepidopleurida Thiele, 1910
- Suborder Cymatochitonina Sirenko et Starobogatov, 1977
- Family Acutichitonidae Hoare, Mapes et Atwater, 1983
- Acutichiton Hoare, Sturgeon et Hoare, 1972
- Elachychiton Hoare, Sturgeon et Hoare, 1972
- Harpidochiton Hoare et Cook, 2000
- Arcochiton Hoare, Sturgeon et Hoare, 1972
- Kraterochiton Hoare, 2000
- Soleachiton Hoare, Sturgeon et Hoare, 1972
- Asketochiton Hoare et Sabattini, 2000
- Family Cymatochitonidae Sirenko et Starobogatov, 1977
- Family Gryphochitonidae Pilsbry, 1900
- Family Lekiskochitonidae Smith et Hoare, 1987
- Family Permochitonidae Sirenko et Starobogatov, 1977
- Suborder Lepidopleurina Thiele, 1910
- Family Ferreiraellidae Dell’ Angelo et Palazzi, 1991
- Family Glyptochitonidae Starobogatov et Sirenko, 1975
- Family Leptochitonidae Dall, 1889
- Colapterochiton Hoare et Mapes, 1985
- Coryssochiton DeBrock, Hoare et Mapes, 1984
- Proleptochiton Sirenko et Starobogatov, 1977
- Schematochiton Hoare, 2002
- Pterochiton (Carpenter MS) Dall, 1882
- Leptochiton Gray, 1847
- Parachiton Thiele, 1909
- Terenochiton Iredale, 1914
- Trachypleura Jaeckel, 1900
- Pseudoischnochiton Ashby, 1930
- Lepidopleurus Risso, 1826
- Hanleyella Sirenko, 1973
- Family Camptochitonidae Sirenko, 1997
- Family Nierstraszellidae Sirenko, 1992
- Family Mesochitonidae Dell’ Angelo et Palazzi, 1989
- Family Protochitonidae Ashby, 1925
- Family Hanleyidae Bergenhayn, 1955
- Order Chitonida Thiele, 1910
- Suborder Chitonina Thiele, 1910
- Superfamily Chitonoidea Rafinesque, 1815
- Family Ochmazochitonidae Hoare et Smith, 1984
- Family Ischnochitonidae Dall, 1889
- Family Callistoplacidae Pilsbry, 1893
- Family Chaetopleuridae Plate, 1899
- Family Loricidae Iredale et Hull, 1923
- Family Callochitonidae Plate, 1901
- Family Chitonidae Rafinesque, 1815
- Subfamily Chitoninae Rafinesque, 1815
- Chiton Linnaeus, 1758
- Amaurochiton Thiele, 1893
- Radsia Gray, 1847
- Sypharochiton Thiele, 1893
- Nodiplax Beu, 1967
- Rhyssoplax Thiele, 1893
- Teguloaplax Iredale & Hull, 1926
- Mucrosquama Iredale, 1893
- Subfamily Toniciinae Pilsbry, 1893
- Tonicia Gray, 1847
- Onithochiton Gray, 1847
- Subfamily Acanthopleurinae Dall, 1889
- Acanthopleura Guilding, 1829
- Liolophura Pilsbry, 1893
- Enoplochiton Gray, 1847
- Squamopleura Nierstrasz, 1905
- Superfamily Schizochitonoidea Dall, 1889
- Family Schizochitonidae Dall, 1889
- Suborder Acanthochitonina Bergenhayn, 1930
- Family Mopalioidea Dall, 1889
- Family Tonicellidae Simroth, 1894
- Subfamily Tonicellinae Simroth, 1894
- Subfamily Juvenichitoninae Sirenko, 1975
- Family Schizoplacidae Bergenhayn, 1955
- Family Mopaliidae Dall, 1889
- Subfamily Heterochitoninae Van Belle, 1978
- Subfamily Mopaliinae Dall, 1889
- Aerilamma Hull, 1924
- Guildingia Pilsbry, 1893
- Frembleya H. Adams, 1866
- Diaphoroplax Iredale, 1914
- Plaxiphora Gray, 1847
- Placiphorina Kaas & Van Belle, 1994
- Nuttallochiton Plate, 1899
- Mopalia Gray, 1847
- Maorichiton Iredale, 1914
- Placiphorella (Carpenter MS) Dall, 1879
- Katharina Gray, 1847
- Amicula Gray, 1847
- Superfamily Cryptoplacoidea H. et A. Adams, 1858
- Family Acanthochitonidae Pilsbry, 1893
- Subfamily Acanthochitoninae Pilsbry, 1893
- Subfamily Cryptochitoninae Pilsbry, 1893
- Family Hemiarthridae Sirenko, 1997
- Family Choriplacidae Ashby, 1928
- Family Cryptoplacidae H. et A. Adams, 1858
- Sirenko BI. New outlook on the system of chitons (Mollusca: Polyplacophora). Venus, 65 (1-2): 27-49, 2006