View source for Sclerite

{{Short description|Hardened body part}}
[[File:Monaco.Musée océanographique089.jpg|thumb|right|The [[exoskeleton]] of a [[spiny lobster]] is made of a series of sclerites, connected by flexible joints.]]
A '''sclerite''' ([[Greek language|Greek]] {{lang|el|[[:wikt:σκληρός|σκληρός]]}}, ''{{lang|el|sklēros}}'', meaning "[[hardness|hard]]") is a hardened body part. In various branches of biology the term is applied to various structures, but not as a rule to vertebrate anatomical features such as bones and teeth. Instead it refers most commonly to the hardened parts of [[arthropod]] [[exoskeleton]]s and the internal [[wikt:spicule|spicule]]s of invertebrates such as certain [[sponges]] and [[Octocorallia|soft corals]]. In [[paleontology]], a '''scleritome''' is the complete set of sclerites of an organism, often all that is known from fossil invertebrates.

==Sclerites in combination==
Sclerites may occur practically isolated in an organism, such as the [[Stinger|sting]] of a [[Conus|cone shell]]. Also, they can be more or less scattered, such as tufts of defensive sharp, mineralised bristles as in many marine [[polychaete]]s. Or, they can occur as structured, but unconnected or loosely connected arrays, such as the mineral "teeth" in the [[radula]] of many [[Mollusca]], the [[Valve (mollusc)|valves]] of [[chiton]]s, the beak of a [[cephalopod]], or the articulated exoskeletons of Arthropoda.

When sclerites are organised into an unarticulated structure, that structure may be referred to as a scleritome, a term largely used in [[paleontology]].

==Arthropods==
[[File:Parasite170028-fig10 Illustrated guide of Phlebotominae (Diptera, Psychodidae).png|thumb|left|Sclerites of cervix and thorax of [[Phlebotominae]]]]
In [[Arthropoda]], the hardening that produces sclerites is accomplished either by the cross-linking of protein chains in the [[exocuticle]], a process called [[Sclerotin|sclerotization]], or by incorporation of minerals such as calcium carbonate into regions of the exoskeleton, or both. Thus, the arthropod exoskeleton is divided into numerous sclerites, joined by less-sclerotized, membranous regions or sutures.

[[Anatomical terms of location#Dorsal and ventral|Dorsal]] sclerites of a body segment, often plate-like, are known as ''[[Tergum|tergites]]''. Similarly the [[Anatomical terms of location#Dorsal and ventral|ventral]] sclerites of a body segment are referred to as ''[[Sternum (arthropod anatomy)|sternites]]''. Separate sclerites on the lateral aspects of body segments, the [[pleuron (insect anatomy)|pleura]], are called ''pleurites''.<ref name="isbn0-412-61390-5">{{cite book |author1=Richards, O. W. |author2=Davies, R.G. |title=Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology |publisher=Springer |location=Berlin |year=1977 |isbn=0-412-61390-5 }}</ref>

==Invertebrates other than arthropods==
[[File:Journal.pone.0079155.g003 Clamps in various genera of Protomicrocotylidae.png|thumb|400 px|Sclerites in [[Clamp (zoology)|clamps]] in [[polyopisthocotylea]]n [[monogenea]]ns; various sclerites are in grey and black]]

Wide ranges of sclerites of various kinds occur in various invertebrate [[phylum|phyla]], including [[Polychaeta]] and [[Mollusca]]. Two [[Taxon|taxa]] that routinely have the term applied however, are the [[Octocorallia|soft corals]] and the [[Sponge|Porifera]]. In both those groups certain of their structures contain mineralised spicules of [[Silicon dioxide|silica]] or [[Aragonite|calcium carbonate]] that are of importance structurally and in defense.

Many other invertebrates grow a few hard parts, largely mineralised, as [[Statocyst|statoliths]] and similar structures, but those are not generally referred to as sclerites. 

[[Clamp (zoology)|Clamps]], the main attachment structure of the parasitic [[polyopisthocotylea]]n [[monogenea]]n flatworms,<ref>[[Boris Evseevitch Bychowsky|Bychowsky, B. E.]] (1961). Monogenetic Trematodes their systematics and phylogeny. English translation edited by W. J. Hargis Jr. Washington: American Institute of Biological Sciences.</ref><ref>Kearn, G. C. (2004). Leeches, Lice and Lampreys. A natural history of skin and gill parasites of fishes. Dordrecht: Springer.</ref> are composed of various sclerites and associated musculature, located on a posterior organ called the [[haptor]]. Clamps are specialized structures attached to the host fish, generally to its gill.

==Prehistory==
[[IMage:Wiwaxia Smithsonian.JPG|The fossilized scleritome of ''[[Wiwaxia corrugata]]''|thumb]]
A '''scleritome''' is a skeleton made of isolated elements, like the scales of organisms such as the [[halwaxiids]], the teeth in a radula,<ref name="Butterfield2008">{{Cite journal| doi = 10.1666/07-066.1| title = An Early Cambrian Radula| year = 2008| last1 = Butterfield | first1 = N. J.| journal = Journal of Paleontology| volume = 82| issue = 3| pages = 543–554| bibcode = 2008JPal...82..543B| s2cid = 86083492}}</ref> [[sponge spicule|spicules]] in a sponge skeleton, or the elements in a [[conodont]] apparatus.<ref name="Bengtson1985">{{Cite journal | last1 = Bengtson | first1 = S. | title = Taxonomy of Disarticulated Fossils | jstor = 1304949 | journal = Journal of Paleontology | volume = 59 | issue = 6 | pages = 1350–1358 | year = 1985 }}</ref> The term was coined by the palaeontologist [[Stefan Bengtson]].<ref name="Bengtson1985" />

Although sclerites are of considerable importance in the study of extant animals, in palaeontology they are of far greater relative importance because they often are the only parts of an animal that fossilize at all, let alone well or clearly. Many extinct groups are known only from sclerites, leaving moot the question of what their gross anatomy might have looked like.

An example of the use of the term in paleontology is to describe hollow calcium carbonate, [[calcium sulfate]] or [[calcium phosphate]] plates grown as body armor by a number of Early [[Cambrian]] animals. Unlike [[Porifera|sponge]] spicules, Early Cambrian sclerites appear to be external armor rather than internal structural elements. Sclerites are found on a curious collection of early animals including a common spongelike animal called ''[[Chancelloria]]''; an armored slug-like form ''[[Wiwaxia]]''; an armored worm with a pair of [[brachiopoda|brachiopod]]-like shells ''[[Halkieria]]''; and another armored worm ''[[Microdictyon]]'' that is generally considered to be a [[lobopodia|lobopod]]/[[onychophora|onychophore]].

It has been suggested that the sclerites of the Cambrian ''Wiwaxia'' are homologous with the bristles of [[annelida|annelid worms]].<ref>{{cite journal | author = Butterfield, N. J. | year = 1990 | title = A Reassessment of the Enigmatic Burgess Shale Fossil ''Wiwaxia corrugata'' (Matthew) and Its Relationship to the Polychaete ''Canadia spinosa'' Walcott |journal = [[Paleobiology (journal)|Paleobiology]] | volume = 16 | issue = 3 | pages = 287–303 | doi = 10.1017/S0094837300010009 | jstor=2400789| bibcode = 1990Pbio...16..287B | s2cid = 88100863 }}</ref>
At least one modern [[gastropoda|gastropod]] mollusc living near deep sea hydrothermal vents has structures made of iron sulfides similar to some Cambrian sclerites.<ref>{{Cite journal |author=Anders Warén, Stefan Bengtson, Shana K. Goffredi & Cindy L. Van Dover |year=2003 |title=A hot-vent gastropod with iron sulfide dermal sclerites |journal=[[Science (journal)|Science]] |volume=302 |issue=5647 |page=1007 |url=http://www.nrm.se/download/18.4e32c81078a8d9249800021562/Waren_etal_2003.pdf |pmid=14605361 |doi=10.1126/science.1087696 |s2cid=38386600 |archive-date=2012-03-09 |access-date=2010-12-05 |archive-url=https://web.archive.org/web/20120309170242/http://www.nrm.se/download/18.4e32c81078a8d9249800021562/Waren_etal_2003.pdf |url-status=dead }}</ref>
[[File:Chrysomallon squamiferum 2.png|thumb|The [[scaly-foot gastropod]] is the only gastropod with sclerites.]]

==References==
{{reflist}}

[[Category:Arthropod anatomy]]
[[Category:Sponge anatomy]]
[[Category:Mollusc anatomy]]