'Chitons' are
mollusks of the
class Polyplacophora 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 'Polyplacophora', which was formerly called 'Amphineura'
[1]). 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".
The largest chiton (up to 33 cm in length) is the brick-red
gumboot chiton of the
Pacific Northwest.
Structure
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, made wholly of
aragonite, and 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 ferric/ferrous oxide mineral that hardens the teeth. The radula is used to scrape microscopic algae off the substratum.
Name
A chiton shell on display (''Acanthopleura spinosa'').
The English name "chiton" originates from the
Latin word ''
chitÅn'', which means "
mollusk", and in turn is derived from the
Greek word "
khitÅn", meaning "tunic" (which also is the source of the word
chitin). The Greek word "khitÅn" 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 Polyplacophoran comes from the words ''poly-'' (many), ''plako-'' (tablet), and ''-phoros'' (bearing).
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.
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,
seastars,
crab,
fish, and
sea anemones.
.
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 6,000 meters.
Lined chitons range from Northern
Japan around the
Aleutians to
California. It is found in both the
Intertidal and Subtidal zones, commonly on rocky surfaces. Chitons are particularly diverse and abundant on the shores of South Australia.
A possible Pre-Cambrian ancestor is
Kimberella. It may also share a connection to
Wiwaxia.
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).
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
★
★
★
★
★
★ ''
Gryphochiton''
Gray, 1847
★
★
★
★
★ Family
Lekiskochitonidae Smith et Hoare, 1987
★
★
★
★
★
★ ''
Lekiskochiton''
Hoare et Smith, 1984
★
★
★
★
★ Family
Permochitonidae Sirenko et Starobogatov, 1977
★
★
★
★
★
★ ''
Permochiton''
Iredale et Hull, 1926
★
★
★
★ Suborder
Lepidopleurina Thiell, 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 Iredall 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
★
★
★
★
★
★ ''
Schizoplax''
Dall, 1878
★
★
★
★
★ 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
★
★
★
★
★
★ ''
Acanthochitona''
Gray, 1921
★
★
★
★
★
★ ''
Craspedochiton''
Shuttleworth, 1853
★
★
★
★
★
★ ''
Spongiochiton'' (Carpenter MS)
Dall, 1882
★
★
★
★
★
★ ''
Notoplax''
H. Adams, 1861
★
★
★
★
★
★ ''
Pseudotonicia''
Ashby, 1928
★
★
★
★
★
★ ''
Bassethullia''
Pilsbry, 1928
★
★
★
★
★
★ ''
Americhiton''
Watters, 1990
★
★
★
★
★
★ ''
Choneplax'' (Carpenter MS)
Dall, 1882
★
★
★
★
★
★ ''
Cryptoconchus'' (De Blainville MS)
Burrow, 1815
★
★
★
★
★ Subfamily
Cryptochitoninae Pilsbry, 1893
★
★
★
★
★
★ ''
Cryptochiton''
Middendorff, 1847
★
★
★
★
★ Family
Hemiarthridae Sirenko, 1997
★
★
★
★
★
★ ''
Hemiarthrum''
Carpenter in Dall, 1876
★
★
★
★
★
★ ''
Weedingia''
Kaas, 1988
★
★
★
★
★ Family
Choriplacidae Ashby, 1928
★
★
★
★
★
★ ''
Choriplax''
Pilsbry, 1894
★
★
★
★
★ 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
References
1.
2. American Heritage dictionary of the English Language: Fourth Edition. 2000. entry for chiton
★ Sirenko BI. New outlook on the system of chitons (Mollusca: Polyplacophora). Venus, 65 (1-2): 27-49, 2006
External links
★
Extensive list of species, classified by families
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