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Chiton
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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.
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Encyclopedia
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.
Distribution
Chitons live worldwide, in cold water and in the tropics.
Morphology
General anatomy
Chitons lack a differentiated head; their nervous system resembles a dispersed ladder.
Senses
Chitons see with their aesthetes.
Some also have ocellus-like eyes. There is a relatively good fossil record of chiton shells, but ocelli are only present in those dating to or younger; this would make the ocelli, whose precise function is unclear, the most recent eyes to evolve.
Shell
from the beachdrift on Nevis, West Indies. Head plates at the top, tail plates at the bottom]]
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.
The front seven shells develop simultaneously, with the rear shell being added later in the developmental process.
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.)
Habitat
on a rock at high tide level in Guadeloupe]]
These mollusks 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 and 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.
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, lobster 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.
Evolutionary origins
Chitons have a relatively good fossil record, stretching back to the Devonian. Before this, some organisms have been interpreted (tentatively) as stem-group polyplacophora; the record of polyplacophora stretches back to the Ordovician.
Kimberella and Wiwaxia of the Precambrian and Cambrian may be related to ancestral polyplacophora. Matthevia is a Late Cambrian polyplacophoran preserved as individual pointed valves, and sometimes considered to be a chiton. Based on this and co-occuring fossils, one plausible hypothesis for the origin of polyplacophora has that they formed when an aberrant monoplacophoran was born with multiple centres of calcification, rather than the usual one. Selection quicly acted on the resultant conical shells to form them to overlap into protective armour; their original cones are homologous to the tips of the plates of modern chitons.
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.
Etymology
The English name "chiton" originates from the Latin word chiton, which means "mollusk", and in turn is derived from the Greek word "khiton", meaning tunic (which also is the source of the word chitin). The Greek word "khiton" 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.
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.
- Class Polyplacophora Gray, 1821
- Subclass Paleoloricata Bergenhayn, 1955
- Order Chelodida Bergenhayn, 1943
- Family Chelodidae Bergenhayn, 1943
- Chelodes Davidson et King, 1874
- Euchelodes Marek, 1962
- Calceochiton Flower, 1968
- Order Septemchitonida Bergenhayn, 1955
- Family Gotlandochitonidae Bergenhayn, 1955
- Gotlandochiton Bergenhayn, 1955
- Family Helminthochitonidae Van Belle, 1975
- Kindbladochiton Van Belle, 1975
- Diadelochiton Hoare, 2000
- Helminthochiton Salter in Griffith et M'Coy, 1846
- Echinochiton Pojeta, Eernisse, Hoare et Henderson, 2003
- Family Septemchitonidae Bergenhayn, 1955
- Septemchiton Bergenhayn, 1955
- Paleochiton A. G. Smith, 1964
- Thairoplax Cherns, 1998
Subclass Loricata Shumacher, 1817- 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
- Cymatochiton Dall, 1882
- Compsochiton Hoare et Cook, 2000
- Family Gryphochitonidae Pilsbry, 1900
- Family Lekiskochitonidae Smith et Hoare, 1987
- Lekiskochiton Hoare et Smith, 1984
- Family Permochitonidae Sirenko et Starobogatov, 1977
- Suborder Lepidopleurina Thiele, 1910
- Family Ferreiraellidae Dell’ Angelo et Palazzi, 1991
- Glaphurochiton Raymond, 1910
- ?Pyknochiton Hoare, 2000
- ?Hadrochiton Hoare, 2000
- Ferreiraella Sirenko, 1988
- Family Glyptochitonidae Starobogatov et Sirenko, 1975
- Glyptochiton Konninck, 1883
- 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
- Camptochiton DeBrock, Hoare et Mapes, 1984
- Pedanochiton DeBrock, Hoare et Mapes, 1984
- Euleptochiton Hoare et Mapes, 1985
- Pileochiton DeBrock, Hoare et Mapes, 1984
- Chauliochiton Hoare et Smith, 1984
- Stegochiton Hoare et Smith, 1984
- Family Nierstraszellidae Sirenko, 1992
- Nierstraszella Sirenko, 1992
- Family Mesochitonidae Dell’ Angelo et Palazzi, 1989
- Mesochiton Van Belle, 1975
- Pterygochiton Rochebrune, 1883
- Family Protochitonidae Ashby, 1925
- Protochiton Ashby, 1925
- Deshayesiella (Carpenter MS) Dall, 1879
- Oldroydia Dall, 1894
- Family Hanleyidae Bergenhayn, 1955
- Hanleya Gray, 1857
- Hemiarthrum Dall, 1876
- Order Chitonida Thiele, 1910
- Suborder Chitonina Thiele, 1910
- Superfamily Chitonoidea Rafinesque, 1815
- Family Ochmazochitonidae Hoare et Smith, 1984
- Ochmazochiton Hoare et Smith, 1984
- Family Ischnochitonidae Dall, 1889
- Ischnochiton Gray, 1847
- Stenochiton H. Adams et Angas, 1864
- Stenoplax (Carpenter MS) Dall, 1879
- Lepidozona Pilsbry, 1892
- Stenosemus Middendorff, 1847
- Subterenochiton Iredale et Hull, 1924
- Thermochiton Saito et Okutani, 1990
- Connexochiton Kaas, 1979
- Tonicina Thiele, 1906
- Family Callistoplacidae Pilsbry, 1893
- Ischnoplax Dall, 1879
- Callistochiton (Carpenter MS) Dall, 1879
- Callistoplax Dall, 1882
- Ceratozona Dall, 1882
- Calloplax Thiele, 1909
- Family Chaetopleuridae Plate, 1899
- Chaetopleura Shuttleworth, 1853
- Dinoplax (Carpenter MS) Dall, 1879
- Family Loricidae Iredale et Hull, 1923
- Lorica H. et A. Adams, 1852
- Loricella Pilsbry, 1893
- Oochiton Ashby, 1929
- Family Callochitonidae Plate, 1901
- Callochiton Gray, 1847
- Eudoxochiton Shuttleworth, 1853
- Vermichiton Kaas, 1979
- 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
- Incissiochiton Van Belle, 1985
- Schizochiton Gray, 1847
- Suborder Acanthochitonina Bergenhayn, 1930
- Family Mopalioidea Dall, 1889
- Family Tonicellidae Simroth, 1894
- Subfamily Tonicellinae Simroth, 1894
- Lepidochitona Gray, 1821
- Particulazona Kaas, 1993
- Boreochiton Sars, 1878
- Tonicella Carpenter, 1873
- Nuttallina (Carpenter MS) Dall, 1871
- Spongioradsia Pilsbry, 1894
- Oligochiton Berry, 1922
- Subfamily Juvenichitoninae Sirenko, 1975
- Juvenichiton Sirenko, 1975
- Micichiton Sirenko, 1975
- Nanichiton Sirenko, 1975
- Family Schizoplacidae Bergenhayn, 1955
- Family Mopaliidae Dall, 1889
- Subfamily Heterochitoninae Van Belle, 1978
- Heterochiton Fucini, 1912
- Allochiton Fucini, 1912
- 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
- Cryptochiton Middendorff, 1847
- Family Hemiarthridae Sirenko, 1997
- Hemiarthrum Carpenter in Dall, 1876
- Weedingia Kaas, 1988
- Family Choriplacidae Ashby, 1928
- Family Cryptoplacidae H. et A. Adams, 1858
- Cryptoplax de Blainville, 1818
Incertae sedis- Family Scanochitonidae Bergenhayn, 1955
- Scanochiton Bergenhayn, 1955
- Family Olingechitonidae Starobogatov et Sirenko, 1977
- Olingechiton Bergenhayn, 1943
- Family Haeggochitonidae Sirenko et Starobogatov, 1977
- Haeggochiton Bergenhayn, 1955
- Family Ivoechitonidae Sirenko et Starobogatov, 1977
- Ivoechiton Bergenhayn, 1955
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