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'''Chitons''' ({{IPAc-en|ˈ|k|aɪ|t|ɒ|n|z|,_|ˈ|k|aɪ|t|ən|z}}) are [[marine (ocean)|marine]] [[mollusc]]s of varying size in the [[class (biology)|class]] '''Polyplacophora''' ({{IPAc-en|ˌ|p|ɒ|l|i|p|l|ə|ˈ|k|ɒ|f|ər|ə}} {{respell|POL|ee|plə|KOF|ər|ə}}),<ref name=EB1911>{{cite EB1911|wstitle=Chiton|volume=6|pages=247–251}}</ref> formerly known as '''Amphineura'''.<ref>{{ITIS |id=78807 |taxon=Polyplacophora }}</ref> About 940<ref>{{cite journal |last1=Schwabe |first1=E |year=2005 |title=A catalogue of recent and fossil chitons (Mollusca: Polyplacophora) addenda |journal=Novapex |volume=6 |pages=89–105}}</ref><ref>{{cite journal |last1=Stebbins |first1=T.D. |last2=Eernisse |first2=D.J. |year=2009 |title=Chitons (Mollusca: Polyplacophora) known from benthic monitoring programs in the Southern California Bight |journal=The Festivus |volume=41 |pages=53–100}}</ref> [[Extant taxon|extant]] and 430<ref>{{cite journal |last1=Puchalski |first1=S. |last2=Eernisse |first2=D.J. |last3=Johnson |first3=C.C. |year=2008 |title=The effect of sampling bias on the fossil record of chitons (Mollusca, Polyplacophora) |journal=American Malacological Bulletin |volume=25 |pages=87–95 |doi=10.4003/0740-2783-25.1.87 |s2cid=59485784 |url=https://www.biodiversitylibrary.org/part/241820 |access-date=4 October 2021 |archive-date=26 July 2020 |archive-url=https://web.archive.org/web/20200726031855/https://www.biodiversitylibrary.org/part/241820 |url-status=live }}</ref> [[fossil]] species are recognized.
They are also sometimes known as
Chitons have a shell composed of eight separate shell plates or [[Valve (mollusc)|valves]].<ref name=EB1911/> 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.<ref>{{cite journal |last1=Connors |first1=M.J. |last2=Ehrlich |first2=H. |last3=Hog |first3=M. |last4=Godeffroy |first4=C. |last5=Araya |first5=S. |last6=Kallai |first6=I. |last7=Gazit |first7=D. |last8=Boyce |first8=M. |last9=Ortiz |first9=C. |year=2012 |title=Three-Dimensional Structure of the Shell Plate Assembly of the Chiton Tonicella Marmorea and Its Biomechanical Consequences |journal=Journal of Structural Biology |volume=177 |issue=2 |pages=314–328 |doi=10.1016/j.jsb.2011.12.019 |pmid=22248452}}</ref> The shell plates are encircled by a skirt known as a [[girdle (chiton)|girdle]].
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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 {{cvt|6000|m|ft}}.<ref>{{cite journal |last1=Lindberg |first1=David R. |title=Monoplacophorans and the Origin and Relationships of Mollusks |journal=Evolution: Education and Outreach |date=1 June 2009 |volume=2 |issue=2 |pages=191–203 |doi=10.1007/s12052-009-0125-4 |s2cid=26108547
Chitons are exclusively and fully marine, in contrast to the [[Bivalvia|bivalves]], which were able to adapt to [[brackish water]] and fresh water, and the [[Gastropoda|gastropods]] which were able to make successful transitions to freshwater and terrestrial environments.
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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 [[polysaccharides]] make up the bulk of the matrix.<ref name=Treves2003a/> The girdle spines often bear length-parallel striations.<ref name=Treves2003a/>
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.<ref name=Treves2003a/> Certainly species such as some members of the genus ''[[Acanthochitona garnoti|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.<ref name=2oceans>{{cite book|last1=Branch|first1=G. M.|last2=Griffiths|first2=C. L.|last3=Branch|first3=M. L.|last4=Beckley|first4=L. E. |name-list-style=amp |year=2010|title=Two Oceans: a Guide to the Marine Life of Southern Africa|publisher=Struik Nature|location=Cape Town}}</ref>
Spicules are secreted by cells that do not express [[engrailed (gene)|engrailed]], but these cells are surrounded by engrailed-expressing cells.<ref name=Jacobs2000>{{Cite journal |last2=Wray |last3=Wedeen |doi=10.1046/j.1525-142x.2000.00077.x |year=2000 |pages=340–347 |last4=Kostriken |last5=Desalle |pmid=11256378 |last8=Lindberg |last7=Gates |last6=Staton |issue=6 |volume=2 |first5=R. |first4=R. |first3=C. J. |first2=C. G. |last1=Jacobs |first6=J. L. |journal=Evolution & Development |title=Molluscan engrailed expression, serial organization, and shell evolution |first8=D. R. |first7=R. D. |first1=D. K. |s2cid=25274057}}</ref> 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.<ref name=Haas1981>{{cite journal |author=Haas, W |pages=403–418 |year=1981 |journal=Malacologia |title=Evolution of calcareous hard parts in primitive molluscs malacologia |volume=21}}</ref>
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{{Further|Aesthete (chiton)}}
The primary sense organs of chitons are the [[subradular organ]] and a large number of unique organs called [[aesthete (chiton)|aesthete]]s. The aesthetes consist of light-sensitive cells just below the surface of the shell, although they are not capable of true vision. In some cases, however, they are modified to form [[ocelli]], with a cluster of individual photoreceptor cells lying beneath a small [[aragonite]]-based [[lens (eye)|lens]].<ref name=Speiser2011>{{Cite journal |last1=Speiser |first1=D. I. |last2=Eernisse |first2=D. J. |last3=Johnsen |first3=S. N. |doi=10.1016/j.cub.2011.03.033 |title=A Chiton Uses Aragonite Lenses to Form Images |journal=Current Biology |volume=21 |issue=8 |pages=665–670 |year=2011 |pmid=21497091 |s2cid=10261602 |doi-access=free|bibcode=2011CBio...21..665S }}</ref> Each lens can form clear images, and is composed of relatively large, highly crystallographically
A relatively good fossil record of chiton shells exists, but ocelli are only present in those dating to {{Ma|10}} or younger; this would make the ocelli, whose precise function is unclear, likely the most recent eyes to evolve.<ref name=Serb2008>{{cite journal |first1=J. M. |first2=D. J. |last2=Eernisse |title=Charting Evolution's Trajectory: Using Molluscan Eye Diversity to Understand Parallel and Convergent Evolution |last1=Serb |journal=Evolution: Education and Outreach |volume=1 |issue=4 |pages=439–447 |year=2008 |doi=10.1007/s12052-008-0084-1 |doi-access=free}}</ref>
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Although chitons lack [[osphradium|osphradia]], [[statocyst]]s, 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.<ref>{{Cite journal |pmc=3916795 |year=2014 |last1=Sigwart |first1=J. D. |title=A new sensory organ in primitive molluscs (Polyplacophora: Lepidopleurida), and its context in the nervous system of chitons |journal=Frontiers in Zoology |volume=11 |issue=1 |pages=7 |last2=Sumner-Rooney |first2=L. H. |last3=Schwabe |first3=E. |last4=Heß |first4=M. |last5=Brennan |first5=G. P. |last6=Schrödl |first6=M. |pmid=24447393 |doi=10.1186/1742-9994-11-7 |doi-access=free }}</ref> Its function remains largely unknown, and has been suggested to be related to that of a larval eye.<ref name="sumner2015">{{cite journal | title = Is the Schwabe Organ a Retained Larval Eye? Anatomical and Behavioural Studies of a Novel Sense Organ in Adult ''Leptochiton asellus'' (Mollusca, Polyplacophora) Indicate Links to Larval Photoreceptors | last1 = Sumner-Rooney | first1 = L.H. | last2 = Sigwart | first2 = J.D. | journal = PLOS ONE | date = 2015 | doi = 10.1371/journal.pone.0137119 | volume = 10 | number = 9 | page = e0137119| pmid = 26366861 | pmc = 4569177 | bibcode = 2015PLoSO..1037119S | doi-access = free }}</ref>
However, chitons lack a cerebral ganglion.<ref>(Thorne. J. M, 1968; Moroz. L, et al., 1993).</ref>
==Homing ability==
Similar to many species of saltwater [[Patellogastropoda|limpets]], several [[species]] of chiton are known to exhibit [[homing (biology)|homing]] behaviours, journeying to feed and then returning to the exact spot they previously inhabited.<ref>{{cite journal |last1=Chelazzi |display-authors=et al |year=1983 |title=A comparative study on the movement pattern of two sympatric tropical chitons, Mollusca: Polyplacophora |journal=Marine Biology |volume=74 |issue=2 |pages=115–125 |doi=10.1007/bf00413914|title-link=Mollusca |bibcode=1983MarBi..74..115C |s2cid=56141764}}; {{cite journal |last1=Chelazzi |first1=G |display-authors=et al |year=1990 |title=The role of trail following in the homing of intertidal chitons: a comparison between three Acanthopleura spp |journal=Marine Biology |volume=105 |issue=3 |pages=445–450 |doi=10.1007/bf01316316 |bibcode=1990MarBi.105..445C |s2cid=83889350}}</ref> 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.<ref name=ref_>(Chelazzi, G. et al., 1987; Thorne, J. M., 1968).</ref>
The [[sea snail]] ''[[Nerita textilis]]'' (like all [[gastropod]]s) deposits a mucus trail as it moves, which a chemoreceptive organ is able to detect and guide the [[snail]] back to its home site.<ref>(Chelazzi, G. et al., 1985).</ref> 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.<ref name=ref_ />
The radular teeth of chitons are made of [[magnetite]], and the iron crystals within these may be involved in [[
==Culinary uses==
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.
==Life habits==
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==Evolutionary origins==
Chitons have a relatively good fossil record, stretching back to the Cambrian,<ref name=Runnegar1974 /><ref name="LesleyCherns2007">{{cite journal |last1=Cherns |first1=Lesley |date=2 January 2007 |title=Early Palaeozoic diversification of chitons (Polyplacophora, Mollusca) based on new data from the Silurian of Gotland, Sweden |url=https://onlinelibrary.wiley.com/doi/10.1080/00241160410002180 |journal=[[Lethaia]] |volume=37 |issue=4 |pages=445–456 |doi=10.1080/00241160410002180 |access-date=25 November 2022}}</ref> with the genus ''Preacanthochiton'', known from fossils found in Late Cambrian deposits in [[Missouri]], being classified as the earliest known polyplacophoran. However, the exact phylogenetic position of supposed Cambrian chitons is highly controversial, and some authors have instead argued that the earliest confirmed polyplacophorans date back to the [[Early Ordovician]].<ref name=Sigwart2007>{{cite journal |last1=Sigwart |first1=J.D. |last2=Sutton |first2=M.D. |date=October 2007 |title=Deep molluscan phylogeny: Synthesis of palaeontological and neontological data |journal=Proceedings of the Royal Society B: Biological Sciences |issue=1624 |volume=274 |pages=2413–2419 |pmid=17652065 |pmc=2274978 |doi=10.1098/rspb.2007.0701}} For a summary, see {{cite web |title=The Mollusca |publisher=University of California Museum of Paleontology |url=http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php |access-date=2 October 2008 |archive-date=15 December 2012 |archive-url=https://www.webcitation.org/6CvnfcV0o?url=http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php |url-status=live }}</ref> ''[[Kimberella]]'' and ''[[Wiwaxia]]'' of the Precambrian and Cambrian may be related to ancestral polyplacophorans. ''[[Matthevia]]'' is a Late Cambrian polyplacophoran preserved as individual pointed valves, and sometimes considered to be a chiton,<ref name=Runnegar1974>{{cite journal |first1=B. |last1=Runnegar | first2=J. Jr. |last2=Pojeta |date=October 1974 |title=Molluscan phylogeny: The paleontological viewpoint |volume=186 |journal=Science |issue=4161 |pages=311–317 |jstor=1739764 |pmid=17839855 |doi=10.1126/science.186.4161.311 |bibcode=1974Sci...186..311R |s2cid=46429653}}</ref> although at the closest, it can only be a stem-group member of the group.<ref name=Vendrasco>{{cite journal |last1=Vendrasco |first1=M.J. |last2=Wood |first2=T.E. |last3=Runnegar |first3=B.N. |year=2004 |title=Articulated Palaeozoic fossil with 17 plates greatly expands disparity of early chitons |journal=Nature |volume=429 |issue=6989 |pages=288–291 |pmid=15152250 |doi=10.1038/nature02548 |bibcode=2004Natur.429..288V |s2cid=4428441}}</ref>
[[File:Matthevia.JPG|thumb|right|Separate plates from ''Matthevia'', a Late [[Cambrian]] polyplacophoran from the Hellnmaria Member of the Notch Peak Limestone, Steamboat Pass, southern House Range, Utah are shown with a [[cent (United States coin)|US one-cent coin]] (19 mm in diameter).]]
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This system is now generally accepted.
* Class '''Polyplacophora''' <small>de Blainville, 1816</small>
** {{Extinct}}Subclass [[Paleoloricata]] <small>Bergenhayn, 1955</small>
*** {{Extinct}}Order [[Chelodida]] <small>Bergenhayn, 1943</small>
*** {{Extinct}}Order [[Septemchitonida]] <small>Bergenhayn, 1955</small>
***** {{Extinct}}Family [[Helminthochitonidae]] <small>Van Belle, 1975</small>
****** {{Extinct}}''[[Kindbladochiton]]'' <small>Van Belle, 1975</small>
****** {{Extinct}}''[[Diadelochiton]]'' <small>Hoare, 2000</small>
****** {{Extinct}}''[[Helminthochiton]]'' <small>Salter in Griffith & M'Coy, 1846</small>
****** {{Extinct}}''[[Echinochiton]]'' <small>Pojeta, Eernisse, Hoare & Henderson, 2003</small>
***** {{Extinct}}Family [[Septemchitonidae]] <small>Bergenhayn, 1955</small>
****** {{Extinct}}''[[Septemchiton]]'' <small>Bergenhayn, 1955</small>
****** {{Extinct}}''[[Paleochiton]]'' <small>A. G. Smith, 1964</small>
****** {{Extinct}}''[[Thairoplax]]'' <small>Cherns, 1998</small>
** Subclass
*** Order [[Lepidopleurida]] <small>Thiele, 1910</small>
**** Suborder [[Cymatochitonina]] <small>Sirenko & Starobogatov, 1977</small>
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****** ''[[Asketochiton]]'' <small>Hoare & Sabattini, 2000</small>
***** Family †[[Cymatochitonidae]] <small>Sirenko & Starobogatov, 1977</small>
****** {{Extinct}}''[[Cymatochiton]]'' <small>[[W. H. Dall|Dall]], 1882</small>
****** {{Extinct}}''[[Compsochiton]]'' <small>Hoare & Cook, 2000</small>
***** Family [[Gryphochitonidae]] <small>[[Henry Augustus Pilsbry|Pilsbry]], 1900</small>
****** ''[[Gryphochiton]]'' <small>Gray, 1847</small>
***** Family [[Lekiskochitonidae]] <small>Smith & Hoare, 1987</small>
****** ''[[Lekiskochiton]]'' <small>Hoare & Smith, 1984</small>
***** {{Extinct}}Family [[Permochitonidae]] <small>Sirenko & Starobogatov, 1977</small>
****** {{Extinct}}''[[Permochiton]]'' <small>[[Tom Iredale|Iredale]] & Hull, 1926</small>
**** Suborder [[Lepidopleurina]] <small>Thiele, 1910</small>
***** Family ''[[Abyssochitonidae]]'' (synonym: Ferreiraellidae) <small>Dell' Angelo & Palazzi, 1991</small>
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****** ?''[[Hadrochiton]]'' <small>Hoare, 2000</small>
****** ''[[Ferreiraella]]'' <small>Sirenko, 1988</small>
***** {{Extinct}}Family [[Glyptochitonidae]] <small>Starobogatov & Sirenko, 1975</small>
****** {{Extinct}}''[[Glyptochiton]]'' <small>Konninck, 1883</small>
***** Family [[Leptochitonidae]] <small>[[W. H. Dall|Dall]], 1889</small>
****** ''[[Colapterochiton]]'' <small>Hoare & Mapes, 1985</small>
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******* ''[[Cryptoplax]]'' <small>[[de Blainville]], 1818</small>
** ''[[Incertae sedis]]''
==References==
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