Chiton
Chiton Temporal range:
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A live lined chiton, Tonicella lineata photographed in situ: The anterior end of the animal is to the right. | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Mollusca |
Class: | Polyplacophora Blainville, 1816 |
Subgroups | |
Chitons ( species are recognized.
They are also sometimes known as sea cradles or coat-of-mail shells or suck-rocks, or more formally as loricates, polyplacophorans, and occasionally as polyplacophores.
Chitons have a shell composed of nine separate shell plates or valves.[3] These plates overlap slightly at the front and back edges, and yet articulate well with one another. Because of this, the shell provides protection at the same time as permitting the chiton to flex upward when needed for locomotion over uneven surfaces, and even allows the animal to curl up into a ball when dislodged from rocks.[8] The shell plates are encircled by a skirt known as a girdle.
Habitat
Chitons live worldwide, from cold waters through to the tropics. They 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. Most species inhabit intertidal or subtidal zones, and do not extend beyond the photic zone, but a few species live in deep water, as deep as 6,000 m (20,000 ft).[9]
Chitons are exclusively and fully marine, in contrast to the bivalves, which were able to adapt to brackish water and fresh water, and the gastropods which were able to make successful transitions to freshwater and terrestrial environments.
Morphology
Shell
All chitons bear a protective
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The most anterior plate is crescent-shaped, and is known as the cephalic plate (sometimes called a head plate, despite the absence of a complete head). The most posterior plate is known as the anal plate (sometimes called the tail plate, although chitons do not have tails.)
The inner layer of each of the six intermediate plates is produced anteriorly as an articulating flange, called the articulamentum. This inner layer may also be produced laterally in the form of notched insertion plates. These function as an attachment of the valve plates to the soft body. A similar series of insertion plates may be attached to the convex anterior border of the cephalic plate or the convex posterior border of the anal plate.[12]
The sculpture of the valves is one of the taxonomic characteristics, along with the granulation or spinulation of the girdle.[12]
After a chiton dies, the individual valves which make up the eight-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 shell plates from a chiton are sometimes known as butterfly shells due to their shape.
Girdle ornament
The girdle may be ornamented with scales or spicules which, like the shell plates, are mineralized with aragonite — although a different mineralization process operates in the spicules to that in the teeth or shells (implying an independent evolutionary innovation).[11] This process seems quite simple in comparison to other shell tissue; in some taxa, the crystal structure of the deposited minerals closely resembles the disordered nature of crystals that form inorganically, although more order is visible in other taxa.[11]
The protein component of the scales and sclerites is minuscule in comparison with other biomineralized structures, whereas the total proportion of matrix is 'higher' than in mollusc shells. This implies that
The wide form of girdle ornament suggests it serves a secondary role; chitons can survive perfectly well without them. Camouflage or defence are two likely functions.[11] Certainly species such as some members of the genus Acanthochitona bear conspicuous paired tufts of spicules on the girdle. The spicules are sharp, and if carelessly handled, easily penetrate the human skin, where they detach and remain as a painful irritant.[13]
Spicules are secreted by cells that do not express engrailed, but these cells are surrounded by engrailed-expressing cells.[14] These neighbouring cells secrete an organic pellicle on the outside of the developing spicule, whose aragonite is deposited by the central cell; subsequent division of this central cell allows larger spines to be secreted in certain taxa.[15] The organic pellicule is found in most polyplacophora (but not basal chitons, such as Hanleya)[15] but is unusual in aplacophora.[16] Developmentally, sclerite-secreting cells arise from pretrochal and postrochal cells: the 1a, 1d, 2a, 2c, 3c and 3d cells.[16] The shell plates arise primarily from the 2d micromere, although 2a, 2b, 2c and sometimes 3c cells also participate in its secretion.[16]
Internal anatomy
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. The mantle cavity consists of a narrow channel on each side, lying between the body and the girdle. Water enters the cavity through openings in either side of the mouth, then flows along the channel to a second, exhalant, opening close to the anus.[17] Multiple gills hang down into the mantle cavity along part or all of the lateral pallial groove, each consisting of a central axis with a number of flattened filaments through which oxygen can be absorbed.[18]
The three-chambered heart is located towards the animal's hind end. Each of the two auricles collects blood from the gills on one side, while the muscular ventricle pumps blood through the aorta and round the body.
The excretory system consists of two nephridia, which connect to the pericardial cavity around the heart, and remove excreta through a pore that opens near the rear of the mantle cavity. The single gonad is located in front of the heart, and releases gametes through a pair of pores just in front of those used for excretion.[18]
The
Chitons lack a clearly demarcated head; their nervous system resembles a dispersed ladder.[19] No true ganglia are present, as in other molluscs, although a ring of dense neural tissue occurs around the oesophagus. From this ring, nerves branch forwards to innervate the mouth and subradula, while two pairs of main nerve cords run back through the body. One pair, the pedal cords, innervate the foot, while the palliovisceral cords innervate the mantle and remaining internal organs.[18]
Some species bear an array of tentacles in front of the head.[20]
Senses
The primary sense organs of chitons are the
A relatively good fossil record of chiton shells exists, but ocelli are only present in those dating to 10 million years ago or younger; this would make the ocelli, whose precise function is unclear, likely the most recent eyes to evolve.[19]
Although chitons lack osphradia, statocysts, and other sensory organs common to other molluscs, they do have numerous tactile nerve endings, especially on the girdle and within the mantle cavity.
The order Lepidopleurida also have a pigmented sensory organ called the Schwabe organ.[26] Its function remains largely unknown, and has been suggested to be related to that of a larval eye.[27]
However, chitons lack a cerebral ganglion.[28]
Homing ability
Similar to many species of saltwater limpets, several species of chiton are known to exhibit homing behaviours, journeying to feed and then returning to the exact spot they previously inhabited.[29] The method they use to perform such behaviors has been investigated to some extent, but remains unknown. One theory has the chitons remembering the topographic profile of the region, thus being able to guide themselves back to their home scar by a physical knowledge of the rocks and visual input from their numerous primitive eyespots.[30] The
The radular teeth of chitons are made of magnetite, and the iron crystals within these may be involved in magnetoreception,[32] the ability to sense the polarity and the inclination of the Earth's magnetic field. Experimental work has suggested that chitons can detect and respond to magnetism.[33]
Culinary uses
Chitons are eaten in several parts of the world. This includes islands in the Caribbean, such as
Life habits
A chiton creeps along slowly on a muscular foot. It has considerable power of adhesion and can cling to rocks very powerfully, like a limpet.
Chitons are generally herbivorous grazers, though some are omnivorous and some carnivorous..
A few species of chitons are
Reproduction and life cycle
Chitons have separate sexes, and fertilization is usually
The egg has a tough spiny coat, and usually hatches to release a free-swimming trochophore larva, typical of many other mollusc groups. In a few cases, the trochophore remains within the egg (and is then called lecithotrophic – deriving nutrition from yolk), which hatches to produce a miniature adult. Unlike most other molluscs, there is no intermediate stage, or veliger, between the trochophore and the adult. Instead, a segmented shell gland forms on one side of the larva, and a foot forms on the opposite side. When the larva is ready to become an adult, the body elongates, and the shell gland secretes the plates of the shell. Unlike the fully grown adult, the larva has a pair of simple eyes, although these may remain for some time in the immature adult.[18]
Predators
Animals which prey on chitons include humans, seagulls, sea stars, crabs, lobsters and fish.[citation needed]
Evolutionary origins
Chitons have a relatively good fossil record, stretching back to the Cambrian,
Based on this and co-occurring 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 quickly 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.[1]
The chitons evolved from multiplacophora during the Palaeozoic, with their relatively conserved modern-day body plan being fixed by the Mesozoic.[38]
The earliest fossil evidence of aesthetes in chitons comes from around 400 Ma, during the Early Devonian.[19]
History of scientific investigation
Chitons were first studied by Carl Linnaeus in his 1758 10th edition of Systema Naturae. 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 de Blainville 1816.
Etymology
The name chiton is Neo-Latin derived from the Ancient Greek word khitōn, meaning tunic (which also is the source of the word chitin). The Ancient 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.[citation needed]
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), extensive taxonomic studies at the species level have been made. However, the taxonomic classification at higher levels in the group has remained somewhat unsettled.
The most recent classification, by Sirenko (2006),[39] 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.
Further resolution within the Chitonida has been recovered through molecular analysis.[40]
This system is now generally accepted.
- Class Polyplacophora de Blainville, 1816
- †Subclass Paleoloricata Bergenhayn, 1955
- †Order Chelodida Bergenhayn, 1943
- †Family Chelodidae Bergenhayn, 1943
- †Chelodes Davidson & King, 1874
- †Euchelodes Marek, 1962
- †Calceochiton Flower, 1968
- †Family Chelodidae Bergenhayn, 1943
- †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 & M'Coy, 1846
- †Echinochiton Pojeta, Eernisse, Hoare & Henderson, 2003
- †Family Septemchitonidae Bergenhayn, 1955
- †Septemchiton Bergenhayn, 1955
- †Paleochiton A. G. Smith, 1964
- †Thairoplax Cherns, 1998
- †Family Gotlandochitonidae Bergenhayn, 1955
- †Order Chelodida Bergenhayn, 1943
- Subclass Loricata Shumacher, 1817
- Order Lepidopleurida Thiele, 1910
- Suborder Cymatochitonina Sirenko & Starobogatov, 1977
- Family Acutichitonidae Hoare, Mapes & Atwater, 1983
- Acutichiton Hoare, Sturgeon & Hoare, 1972
- Elachychiton Hoare, Sturgeon & Hoare, 1972
- Harpidochiton Hoare & Cook, 2000
- Arcochiton Hoare, Sturgeon & Hoare, 1972
- Kraterochiton Hoare, 2000
- Soleachiton Hoare, Sturgeon & Hoare, 1972
- Asketochiton Hoare & Sabattini, 2000
- Family †Cymatochitonidae Sirenko & Starobogatov, 1977
- †Dall, 1882
- †Compsochiton Hoare & Cook, 2000
- †
- Family Gryphochitonidae Pilsbry, 1900
- Gryphochiton Gray, 1847
- Family Lekiskochitonidae Smith & Hoare, 1987
- Lekiskochiton Hoare & Smith, 1984
- †Family PermochitonidaeSirenko & Starobogatov, 1977
- †Permochiton Iredale & Hull, 1926
- Family Acutichitonidae Hoare, Mapes & Atwater, 1983
- Suborder Lepidopleurina Thiele, 1910
- Family Abyssochitonidae (synonym: Ferreiraellidae) Dell' Angelo & Palazzi, 1991
- Glaphurochiton Raymond, 1910
- ?Pyknochiton Hoare, 2000
- ?Hadrochiton Hoare, 2000
- Ferreiraella Sirenko, 1988
- †Family Glyptochitonidae Starobogatov & Sirenko, 1975
- †Glyptochiton Konninck, 1883
- Family Dall, 1889
- Colapterochiton Hoare & Mapes, 1985
- Coryssochiton DeBrock, Hoare & Mapes, 1984
- Proleptochiton Sirenko & Starobogatov, 1977
- Schematochiton Hoare, 2002
- Dall, 1882
- LeptochitonGray, 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 & Mapes, 1984
- Pedanochiton DeBrock, Hoare & Mapes, 1984
- Euleptochiton Hoare & Mapes, 1985
- Pileochiton DeBrock, Hoare & Mapes, 1984
- Chauliochiton Hoare & Smith, 1984
- Stegochiton Hoare & Smith, 1984
- Family Nierstraszellidae Sirenko, 1992
- Nierstraszella Sirenko, 1992
- Family Mesochitonidae Dell' Angelo & Palazzi, 1989
- Mesochiton Van Belle, 1975
- Pterygochiton Rochebrune, 1883
- Family ProtochitonidaeAshby, 1925
- Protochiton Ashby, 1925
- Dall, 1879
- Dall, 1894
- Family HanleyidaeBergenhayn, 1955
- Hanleya Gray, 1857
- Dall, 1876
- Family Abyssochitonidae (synonym: Ferreiraellidae) Dell' Angelo & Palazzi, 1991
- Suborder Cymatochitonina Sirenko & Starobogatov, 1977
- Order Chitonida Thiele, 1910
- Suborder Chitonina Thiele, 1910
- Superfamily Rafinesque, 1815
- Family Ochmazochitonidae Hoare & Smith, 1984
- Ochmazochiton Hoare & Smith, 1984
- Family Dall, 1889
- Ischnochiton Gray, 1847
- Stenochiton H. Adams & Angas, 1864
- Dall, 1879
- Lepidozona Pilsbry, 1892
- Stenosemus Middendorff, 1847
- Subterenochiton Iredale & Hull, 1924
- Thermochiton Saito & Okutani, 1990
- Connexochiton Kaas, 1979
- Tonicina Thiele, 1906
- Family Callistoplacidae Pilsbry, 1893
- Dall, 1879
- Dall, 1879
- Dall, 1882
- Dall, 1882
- Calloplax Thiele, 1909
- Family Chaetopleuridae Plate, 1899
- Chaetopleura Shuttleworth, 1853
- Dall, 1882[41]
- Family Loricidae Iredale & Hull, 1923
- Family Callochitonidae Plate, 1901
- Callochiton Gray, 1847
- Eudoxochiton Shuttleworth, 1853
- Vermichiton Kaas, 1979
- Family Rafinesque, 1815
- Subfamily 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 Dall, 1889
- Acanthopleura Guilding, 1829
- Liolophura Pilsbry, 1893
- Enoplochiton Gray, 1847
- Squamopleura Nierstrasz, 1905
- Subfamily
- Family Ochmazochitonidae Hoare & Smith, 1984
- Superfamily Dall, 1889
- Family Dall, 1889
- Incissiochiton Van Belle, 1985
- Schizochiton Gray, 1847
- Family
- Superfamily
- Suborder Acanthochitonina Bergenhayn, 1930
- Superfamily Dall, 1889
- Family Tonicellidae Simroth, 1894
- Subfamily Tonicellinae Simroth, 1894
- Lepidochitona Gray, 1821
- Particulazona Kaas, 1993
- Boreochiton Sars, 1878
- Carpenter, 1873
- CarpenterMS) Dall, 1871
- Spongioradsia Pilsbry, 1894
- Oligochiton Berry, 1922
- Subfamily Juvenichitoninae Sirenko, 1975
- Juvenichiton Sirenko, 1975
- Micichiton Sirenko, 1975
- Nanichiton Sirenko, 1975
- Subfamily Tonicellinae Simroth, 1894
- Family Schizoplacidae Bergenhayn, 1955
- Dall, 1878
- Family Dall, 1889
- Subfamily Heterochitoninae Van Belle, 1978
- Heterochiton Fucini, 1912
- Allochiton Fucini, 1912
- Subfamily Dall, 1889
- AerilammaHull, 1924
- Guildingia Pilsbry, 1893
- FrembleyaH. Adams, 1866
- Diaphoroplax Iredale, 1914
- Plaxiphora Gray, 1847
- Placiphorina Kaas & Van Belle, 1994
- Nuttallochiton Plate, 1899
- Mopalia Gray, 1847
- Maorichiton Iredale, 1914
- Dall, 1879
- Katharina Gray, 1847
- Amicula Gray, 1847
- Subfamily Heterochitoninae Van Belle, 1978
- Family Tonicellidae Simroth, 1894
- Superfamily Cryptoplacoidea H. & A. Adams, 1858
- Family Acanthochitonidae Pilsbry, 1893
- Subfamily Acanthochitoninae Pilsbry, 1893
- Acanthochitona Gray, 1921
- Craspedochiton Shuttleworth, 1853
- Dall, 1882
- Notoplax H. Adams, 1861
- Pseudotonicia Ashby, 1928
- Bassethullia Pilsbry, 1928
- Americhiton Watters, 1990
- Dall, 1882
- de BlainvilleMS) Burrow, 1815
- Subfamily Cryptochitoninae Pilsbry, 1893
- CryptochitonMiddendorff, 1847
- Subfamily Acanthochitoninae Pilsbry, 1893
- Family Hemiarthridae Sirenko, 1997
- Dall, 1876
- Weedingia Kaas, 1988
- Family Choriplacidae Ashby, 1928
- Family Cryptoplacidae H. & A. Adams, 1858
- de Blainville, 1818
- Family Acanthochitonidae Pilsbry, 1893
- Superfamily
- Suborder Chitonina Thiele, 1910
- Order Lepidopleurida Thiele, 1910
- Incertae sedis
- Family Scanochitonidae Bergenhayn, 1955
- Scanochiton Bergenhayn, 1955
- Family Olingechitonidae Starobogatov & Sirenko, 1977
- Olingechiton Bergenhayn, 1943
- Family Haeggochitonidae Sirenko & Starobogatov, 1977
- Haeggochiton Bergenhayn, 1955
- Family Ivoechitonidae Sirenko & Starobogatov, 1977
- Ivoechiton Bergenhayn, 1955
- Family Scanochitonidae Bergenhayn, 1955
- †Subclass Paleoloricata Bergenhayn, 1955
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