Chiton
Chitons (/ˈkaɪtɒnz, ˈkaɪtənz/) are marine molluscs of varying size in the class Polyplacophora (/ˌpɒlipləˈkɒfərə/ POL-ee-plə-KOF-ər-ə),[3] formerly known as Amphineura.[4] About 940[5][6] extant and 430[7] fossil species are recognized.
This article is about the mollusc class. For the mollusc genus, see Chiton (genus). For other uses, see Chiton (disambiguation).
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.
Morphology[edit]
Shell[edit]
All chitons bear a protective dorsal shell that is divided into eight articulating aragonite valves embedded in the tough muscular girdle that surrounds the chiton's body. Compared with the single or two-piece shells of other molluscs, this arrangement allows chitons to roll into a protective ball when dislodged and to cling tightly to irregular surfaces. In some species the valves are reduced or covered by the girdle tissue.[10][11] The valves are variously colored, patterned, smooth, or sculptured.
Homing ability[edit]
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 sea snail Nerita textilis (like all gastropods) deposits a mucus trail as it moves, which a chemoreceptive organ is able to detect and guide the snail back to its home site.[31] It is unclear if chiton homing functions in the same way, but they may leave chemical cues along the rock surface and at the home scar which their olfactory senses can detect and home in on. Furthermore, older trails may also be detected, providing further stimulus for the chiton to find its home.[30]
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[edit]
Chitons are eaten in several parts of the world. This includes islands in the Caribbean, such as Trinidad, Tobago, The Bahamas, St. Maarten, Aruba, Bonaire, Anguilla and Barbados, as well as in Bermuda. They are also traditionally eaten in certain parts of the Philippines, where it is called kibet if raw and chiton if fried. Native Americans of the Pacific coasts of North America eat chitons. They are a common food on the Pacific coast of South America and in the Galápagos. The foot of the chiton is prepared in a manner similar to abalone. Some islanders living in South Korea also eat chiton, slightly boiled and mixed with vegetables and hot sauce. Aboriginal people in Australia also eat chiton; for example they are recorded in the Narungga Nation Traditional Fishing Agreement.
History of scientific investigation[edit]
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[edit]
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.
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.
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.