Difference between revisions of "Team:Uppsala"

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{{about|the organism|the infection|Helminthiasis}}
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{{automatic taxobox
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| taxon = Nematoda
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| name = Roundworms
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| fossil_range = {{fossil range|635|0|[[Precambrian]]–Recent <ref>{{cite web |url=http://www.fossilmuseum.net/fossils/Nematda-fossils.htm |title=Nematode Fossils — Nematoda |website=The Virtual Fossil Museum }}{{dead link|date=March 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>}}
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| image = CelegansGoldsteinLabUNC.jpg
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| image_caption = ''[[Caenorhabditis elegans]]'',<br/> a model species of roundworm
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| authority = [[Karl Moriz Diesing|Diesing]], 1861
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| subdivision_ranks = [[Class (biology)|Class]]es
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| subdivision =
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* [[Chromadorea]]
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* [[Enoplea]]
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* [[Secernentea]]
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* [[Dorylaimea]]
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(see [[#Nematode systematics|text]])
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| synonyms ={{Plainlist|
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* Nematodes <small>[[Hermann Burmeister|Burmeister]], 1837</small>
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* Nematoidea <small>''sensu stricto'' Cobb, 1919</small>
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* Nemates <small>Cobb, 1919</small>
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* Nemata <small>Cobb, 1919 ''emend.''</small>
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}}
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}}
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The '''nematodes''' ({{IPAc-en|UK|ˈ|n|ɛ|m|ə|t|oʊ|d|z}}, {{IPAc-en|US|ˈ|n|iː|m|ə|ˌ|t|oʊ|d|z}}) or '''roundworms'''  constitute the [[phylum]] '''Nematoda''' (also called ''Nemathelminthes'').<ref>[http://plpnemweb.ucdavis.edu/nemaplex/General/animpara.htm Classification of Animal Parasites<!-- Bot generated title -->]</ref><ref name="Garcia">{{cite web|last1=Garcia|first1=Lynne|title=Classification of Human Parasites, Vectors, and Similar Organisms|url=https://cid.oxfordjournals.org/content/29/4/734.full.pdf|publisher=Department of Pathology and Laboratory Medicine, UCLA Medical Center|accessdate=July 21, 2017|location=Los Angeles, California|date=October 29, 1999}}</ref> They are a diverse animal phylum inhabiting a broad range of environments. Nematode [[species]] can be difficult to distinguish,  and although over 25,000 have been described,<ref>{{cite journal | last1 = Hodda | first1 = M | year = 2011 | title = Phylum Nematoda Cobb, 1932. In: Zhang, Z.-Q. (Ed.) Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness | url = | journal = Zootaxa | volume = 3148 | issue = | pages = 63–95 }}</ref><ref>{{cite journal | last1 = Zhang | first1 = Z | year = 2013 | title = Animal biodiversity: An update of classification and diversity in 2013. In: Zhang, Z.-Q. (Ed.) Animal Biodiversity: An Outline of Higher-level Classification and Survey of Taxonomic Richness (Addenda 2013) | url = | journal = Zootaxa | volume = 3703 | issue = 1| pages = 5–11 | doi = 10.11646/zootaxa.3703.1.3 }}</ref> of which more than half are [[parasitism|parasitic]], it is estimated that over 40,000 species exist; estimates of 500,000 to 1 million species<ref name="Lambshead 1993">
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{{cite journal
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| author = Lambshead PJD
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| title = Recent developments in marine benthic biodiversity research
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| journal = Oceanis
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| year = 1993
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| volume = 19
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| issue = 6
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| pages = 5–24
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}}
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</ref> are not supported by evidence.<ref name=n>{{cite book|author=Roy C. Anderson|title=Nematode Parasites of Vertebrates: Their Development and Transmission|url=https://books.google.com/books?id=lEERbfsvP1EC|date=8 February 2000|publisher=CABI|isbn=978-0-85199-786-5|page=1|quote=Estimates of 500,000 to a million species have no basis in fact.}}</ref> Nematodes are classified along with [[insecta|insect]]s and other [[moulting]] [[animalia|animal]]s in the [[clade]] [[Ecdysozoa]], and, unlike [[platyhelminthe|flatworm]]s, have tubular [[digestion|digestive system]]s  with openings at both ends.
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Nematodes have successfully adapted to nearly every [[ecosystem]] from marine (salt water) to fresh water, to soils, and from the polar regions to the tropics, as well as the highest to the lowest of elevations. They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and [[species]] counts, and are found in locations as diverse as mountains, deserts and [[oceanic trench]]es. They are found in every part of the earth's [[lithosphere]],<ref name="Borgonie 2011" /> even at great depths, {{convert|0.9|–|3.6|km|ft |abbr=on |sigfig=2}}, below the surface of the Earth in gold mines in South Africa.<ref name="Lemonick 2011" /><ref name="Bhanoo 2011" /><ref name="Nature 2011-06-02" /><ref name="Drake 2011-06-01" /><ref name="Borgonie 2011-06-02" /> They represent  90% of all animals on the [[Seabed|ocean floor]].<ref name="pmid18164201">{{cite journal |vauthors=Danovaro R, Gambi C, Dell'Anno A, Corinaldesi C, Fraschetti S, Vanreusel A, Vincx M, Gooday AJ | title = Exponential decline of deep-sea ecosystem functioning linked to benthic biodiversity loss | journal = Curr. Biol. | volume = 18 | issue = 1 | pages = 1–8 |date=January 2008 | pmid = 18164201 | doi = 10.1016/j.cub.2007.11.056 | laysummary = http://www.eurekalert.org/pub_releases/2007-12/cp-dsl122007.php | laysource =  EurekAlert! }}</ref> Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity of life cycles, and their presence at various trophic levels point at an important role in many ecosystems.<ref name="isbn0-903874-22-9">{{cite book | author = Platt HM | chapter = foreword |veditors=Lorenzen S, Lorenzen SA | title = The phylogenetic systematics of freeliving nematodes | publisher = The Ray Society | location = London | year = 1994 | pages = | isbn = 0-903874-22-9 }}</ref> There are about 2271 [[genera]] in 256 [[Family (biology)|families]].<ref name=n/> The many parasitic forms include [[pathogen]]s in most plants and animals. A third of the genera occur as [[parasite]]s of [[vertebrate]]s; about 35 nematode species occur in humans.<ref name=n/>
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[[Nathan Cobb]], a nematologist, described the ubiquity of nematodes on Earth thus:<blockquote>In short, if all the matter in the universe  except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its  mountains, hills, vales, rivers, lakes, and  oceans represented by a film of nematodes. The location of towns would  be decipherable, since for every massing of human beings there would be a corresponding massing of certain nematodes. Trees would still stand  in ghostly rows representing  our streets and highways. The location of the various plants and  animals would still be decipherable, and, had  we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites.<ref name = "Cobb_1914">{{cite book|last=Cobb|first=Nathan|authorlink=Nathan Cobb| chapter=Nematodes and their relationships|title=Yearbook United States Department of Agriculture|publisher=[[United States Department of Agriculture]]|year=1914|pages=457–90|url= http://naldc.nal.usda.gov/catalog/IND43748196}} Quote on p. 472.</ref></blockquote>
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== Taxonomy and systematics ==
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{{see also|List of nematode families}}
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[[File:Eophasma jurasicum.JPG|thumb|''Eophasma jurasicum'', a fossilized nematode]]
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[[File:Celegans wt nhr80rnai.png|thumb|''Caenorhabditis elegans'']]
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[[File:Hookworms.JPG|thumb|Rhabditia]]
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[[File:Gravid adult female Nippostrongylus brasiliensis - image.pntd.v07.i08.g001.png|thumb|right|''[[Nippostrongylus brasiliensis]]'']]
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[[File:Anisakids.jpg|thumb|right|Unidentified [[Anisakidae]] ([[Ascaridina]]: [[Ascaridoidea]])]]
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[[File:Threadworm.jpg|thumb|right|''Oxyuridae'' Threadworm]]
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[[File:Microfilaria.jpg|thumb|right|Spiruridae ''Dirofilaria immitis'']]
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=== History ===
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In 1758, Linnaeus described some nematode genera (e.g., ''[[Ascaris]]''), then included in [[Vermes]].
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The name of the group Nematoda, informally called "nematodes", came from [[Nematoidea]], originally defined by [[Karl Rudolphi]] (1808),<ref name="phylumname">{{cite journal | author = Chitwood BG | title = The English word "Nema" Revised | journal = Systematic Zoology in Nematology Newsletter | volume = 4 | issue = 45 | pages = 1619 |date=1957 | pmid = | doi = 10.2307/sysbio/6.4.184 | url = http://plpnemweb.ucdavis.edu/Nemaplex/General/Phylumname.htm }}</ref> from [[Ancient Greek]] νῆμα (''nêma, nêmatos'', 'thread') and -eiδἠς (''-eidēs'', 'species'). It was treated as [[family (biology)|family]] Nematodes by [[Hermann Burmeister|Burmeister]] (1837).<ref name="phylumname" />
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At its origin, the "Nematoidea" erroneously included Nematodes and [[Nematomorpha]], attributed by von Siebold (1843). Along with [[Acanthocephala]], [[Trematoda]] and [[Cestoidea]], it formed the obsolete group [[Entozoa]],<ref name="isbn0-85199-202-1">{{cite book | author = Siddiqi MR | title = Tylenchida: parasites of plants and insects | publisher = CABI Pub | location = Wallingford, Oxon, UK | year = 2000 | pages = | isbn = 0-85199-202-1 }}</ref> created by Rudolphi (1808).<ref>Schmidt-Rhaesa, A. (2014). Gastrotricha, Cycloneuralia and Gnathifera: General History and Phylogeny. In: Schmidt-Rhaesa, A. (ed.). ''Handbook of Zoology'' (founded by W. Kükenthal); Gastrotricha, Cycloneuralia and Gnathifera; Vol 1, Nematomorpha, Priapulida, Kinorhyncha, Loricifera. de Gruyter: Berlin-Boston.</ref> They were also classed along with Acanthocephala in the obsolete [[phylum]] [[Aschelminth|Nemathelminthes]] by Gegenbaur (1859).
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In 1861, [[Karl Moriz Diesing|K. M. Diesing]] treated the group as order Nematoda.<ref name="phylumname" /> In 1877, the [[taxon]] Nematoidea, including the family Gordiidae (horsehair worms), was promoted to the rank of phylum by [[Ray Lankester]]. In 1919, [[Nathan Cobb]] proposed that nematodes should be recognized alone as a phylum.<ref>Cobb, N. A. (1919). The orders and classes of nemas. ''Contrib. Sci. Nematol.'' 8: 213–216, [https://www.biodiversitylibrary.org/part/58037].</ref> He argued they should be called "nema" in English rather than "nematodes"{{efn|Note that words as nematologist, nematosis, nematocide, etc. are based on ''nema, nematos'' and not on "nematode".}} and defined the taxon Nemates (later emended as Nemata, Latin plural of ''nema''), listing Nematoidea ''sensu restricto'' as a synonym. Since Cobb was the first to exclude all but nematodes from the group, some sources consider the valid taxon name to be Nemates or Nemata, rather than Nematoda.<ref name="ITIS Nematoda" />
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=== Phylogeny ===
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The [[Phylogeny|phylogenetic]] relationships of the nematodes and their close relatives among the [[protostome|protostomia]]n [[Animal|Metazoa]] are unresolved. Traditionally, they were held to be a lineage of their own, but in the 1990s, they were proposed to form the group [[Ecdysozoa]] together with [[moulting]] animals, such as [[arthropod]]s. The identity of the closest living relatives of the Nematoda has always been considered to be well resolved. Morphological characters and molecular phylogenies agree with placement of the roundworms as a [[sister taxon]] to the parasitic [[Nematomorpha]]; together they make up the [[Nematoida]]. Along with the [[Scalidophora]] (formerly Cephalorhyncha), the Nematoida form the clade [[Cycloneuralia]], but much disagreement occurs both between and among the available morphological and molecular data. The Cycloneuralia or the Introverta—depending on the validity of the former—are often ranked as a [[superphylum]].<ref name = "ToL_2002_Bilateria">{{cite web |publisher=[[Tree of Life Web Project]] |title=Bilateria |date=2002 |url=http://tolweb.org/Bilateria/2459/2002.01.01 |work=Tree of Life Web Project |accessdate=2008-11-02}}</ref>
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=== Nematode systematics ===
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Due to the lack of knowledge regarding many nematodes, their systematics is contentious. An earliest and influential classification was proposed by Chitwood and Chitwood<ref name="Chitwood1933">{{cite journal |vauthors=Chitwood BG, Chitwood MB | title = The characters of a protonematode | journal = J Parasitol | volume = 20 | issue = | pages = 130 | year = 1933 }}</ref>—later revised by Chitwood<ref name="Chitwood1937">{{cite book | author = Chitwood BG | chapter = A revised classification of the ''Nematoda'' | title = Papers on Helminthology published in commemoration of the 30 year Jubileum of ... K.J. Skrjabin... | pages = 67–79 | year = 1937 | location = Moscow | publisher =  All-Union Lenin Academy of Agricultural Sciences}}</ref>—who divided the phylum into two—the [[Aphasmidia]] and the [[Secernentea|Phasmidia]]. These were later renamed [[Adenophorea]] (gland bearers) and [[Secernentea]] (secretors), respectively.<ref name="Chitwood1958">{{cite journal | author = Chitwood BG | url = https://www.biodiversitylibrary.org/part/19410#/summary | title = The designation of official names for higher taxa of invertebrates | journal = Bull Zool Nomencl | volume = 15 | issue = | pages = 860–95 | year = 1958 | doi = 10.5962/bhl.part.19410}}</ref> The Secernentea share several characteristics, including the presence of [[phasmid (nematode)|phasmid]]s, a pair of sensory organs located in the lateral posterior region, and this was used as the basis for this division. This scheme was adhered to in many later classifications, though the Adenophorea were not in a uniform group.
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Initial studies of incomplete [[Nucleic acid sequence|DNA sequence]]s<ref>{{cite journal |last1=Coghlan |first1=A. |title=Nematode genome evolution |date=7 Sep 2005 |publisher=The ''C. elegans'' Research Community |journal=WormBook |doi=10.1895/wormbook.1.15.1 |url=http://www.wormbook.org/chapters/www_genomevol/genomevol.pdf |accessdate=13 January 2016}}</ref> suggested the existence of five [[clade]]s:<ref name = "Blaxter_1998">{{cite journal |vauthors=Blaxter ML, De Ley P, Garey JR, Liu LX, Scheldeman P, Vierstraete A, Vanfleteren JR, Mackey LY, Dorris M, Frisse LM, Vida JT, Thomas WK | title = A molecular evolutionary framework for the phylum Nematoda | journal = Nature | volume = 392 | issue = 6671 | pages = 71–5 |date=March 1998 | pmid = 9510248 | doi = 10.1038/32160 }}</ref>
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* [[Dorylaimida]]
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* [[Enoplia]]
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* [[Spirurina]]
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* [[Tylenchina]]
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* [[Rhabditina]]
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As it seems, the [[Secernentea]] are indeed a natural group of closest relatives. But the "Adenophorea" appear to be a [[paraphyletic]] assemblage of roundworms simply retaining a good number of [[plesiomorph|ancestral traits]]. The old [[Enoplia]] do not seem to be monophyletic either, but to contain two distinct lineages. The old group "[[Chromadoria]]" seem to be another paraphyletic assemblage, with the [[Monhysterida]] representing a very ancient minor group of nematodes. Among the Secernentea, the [[Diplogasteria]] may need to be united with the [[Rhabditia]], while the Tylenchia might be paraphyletic with the Rhabditia.<ref name = "ToL:2002_Nematoda">{{cite web |publisher=[[Tree of Life Web Project]] |title=Nematoda |date=2002 |url=http://tolweb.org/Nematoda/2472/2002.01.01 |work=Tree of Life Web Project |accessdate=2008-11-02}}</ref>
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The understanding of roundworm systematics and [[phylogeny]] as of 2002 is summarised below:
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'''Phylum Nematoda'''
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* [[Basal (evolution)|Basal]] order [[Monhysterida]]
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* Class [[Dorylaimida]]
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* Class [[Enoplea]]
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* Class [[Secernentea]]
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** Subclass [[Diplogasteria]] (disputed)
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** Subclass [[Rhabditia]] (paraphyletic?)
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** Subclass [[Spiruria]]
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** Subclass [[Tylenchia]] (disputed)
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* "[[Chromadorea]]" assemblage
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Later work has suggested the presence of 12 clades.<ref name="Holterman2006" /> The Secernentea—a group that includes virtually all major animal and plant 'nematode' parasites—apparently arose from within the Adenophorea.
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A major effort to improve the systematics of this phylum is in progress and being organised by the 959 Nematode Genomes.<ref name="Nematodes.org Genomes" />
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A complete checklist of the World's nematode species can be found in the World Species Index: Nematoda.<ref>{{cite book | title = World Species Index: Nematoda | year = 2012}}</ref>
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An analysis of the mitochondrial DNA suggests that the following groupings are valid<ref name=Liu2013>{{cite journal | last1 = Liu | first1 = GH | last2 = Shao | first2 = R | last3 = Li | first3 = JY | last4 = Zhou | first4 = DH | last5 = Li | first5 = H | last6 = Zhu | first6 = XQ | year = 2013 | title = The complete mitochondrial genomes of three parasitic nematodes of birds: a unique gene order and insights into nematode phylogeny | journal = BMC Genomics | volume = 14 | issue = 1| page = 414 | doi=10.1186/1471-2164-14-414 | pmid=23800363 | pmc=3693896}}</ref>
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*subclass [[Dorylaimia]]
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*orders [[Rhabditida]], [[Trichinellida]] and [[Mermithida]]
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*suborder [[Rhabditina]]
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*infraorders [[Spiruromorpha]] and [[Oxyuridomorpha]]
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The [[Ascaridomorpha]], [[Rhabditomorpha]] and [[Diplogasteromorpha]] appear to be related.
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The suborders [[Spirurina]] and [[Tylenchina]] and the infraorders [[Rhabditomorpha]], [[Panagrolaimomorpha]] and [[Tylenchomorpha]] are paraphytic.
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The monophyly of the [[Ascaridomorph]] is uncertain.
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== Anatomy ==
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[[File:C elegans male.svg|thumb|Internal anatomy of a male ''C. elegans'' nematode]]
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Nematodes are small, slender worms: typically approximately 5 to 100&nbsp;µm thick, and 0.1 to 2.5&nbsp;mm long.<ref name="brady">{{cite book |author1=Nyle C. Brady |author2=Ray R. Weil |lastauthoramp=yes |title=Elements of the Nature and Properties of Soils |edition=3rd |publisher=Prentice Hall |year=2009 |isbn=9780135014332}}</ref> The smallest nematodes are microscopic, while free-living species can reach as much as {{convert|5|cm|in |abbr=on |sigfig=1}}, and some parasitic species are larger still, reaching over {{convert|1|m|ft |abbr=on |sigfig=1<!--approx value only-->}} in length.<ref name="Ruppert_2004">{{cite book |vauthors=Ruppert EE, Fox RS, Barnes RD |year=2004 |title=Invertebrate Zoology: A Functional Evolutionary Approach |edition=7th |publisher=Brooks/Cole |location=Belmont, California |isbn=978-0-03-025982-1}}</ref>{{rp|271}} The body is often ornamented with ridges, rings, bristles, or other distinctive structures.<ref name="Weischer 2000"/>
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The head of a nematode is relatively distinct. Whereas the rest of the body is bilaterally symmetrical, the head is radially symmetrical, with sensory bristles and, in many cases, solid 'head-shields' radiating outwards around the mouth. The mouth has either three or six lips, which often bear a series of teeth on their inner edges. An adhesive 'caudal gland' is often found at the tip of the tail.<ref name="Barnes_1980"/>
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The [[Epidermis (zoology)|epidermis]] is either a [[syncytium]] or a single layer of cells, and is covered by a thick [[collagen]]ous [[cuticle]]. The cuticle is often of complex structure, and may have two or three distinct layers. Underneath the epidermis lies a layer of longitudinal [[muscle]] cells. The relatively rigid cuticle works with the muscles to create a hydroskeleton as nematodes lack circumferential muscles. Projections run from the inner surface of muscle cells towards the [[Ventral nerve cord|nerve cord]]s; this is a unique arrangement in the animal kingdom, in which nerve cells normally extend fibres into the muscles rather than ''vice versa''.<ref name="Barnes_1980"/>
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=== Digestive system ===
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The oral cavity is lined with cuticle, which is often strengthened with ridges or other structures, and, especially in carnivorous species, may bear a number of teeth. The mouth often includes a sharp [[stylet (anatomy)|stylet]], which the animal can thrust into its prey. In some species, the stylet is hollow, and can be used to suck liquids from plants or animals.<ref name="Barnes_1980"/>
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The oral cavity opens into a muscular, sucking [[pharynx]], also lined with cuticle. Digestive glands are found in this region of the gut, producing [[digestive enzyme|enzyme]]s that start to break down the food. In stylet-bearing species, these may even be injected into the prey.<ref name="Barnes_1980"/>
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There is no [[stomach]], with the pharynx connecting directly to a muscleless [[intestine]] that forms the main length of the gut. This produces further [[enzyme]]s, and also absorbs nutrients through its single cell thick lining. The last portion of the intestine is lined by cuticle, forming a [[rectum]], which expels waste through the [[anus]] just below and in front of the tip of the tail. Movement of food through the digestive system is the result of body movements of the worm. The intestine has valves or [[sphincter]]s at either end to help control the movement of food through the body.<ref name="Barnes_1980"/>
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=== Excretory system ===
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[[Nitrogenous waste]] is excreted in the form of [[ammonia]] through the body wall, and is not associated with any specific organs. However, the structures for excreting salt to maintain [[osmoregulation]] are typically more complex.<ref name="Barnes_1980"/>
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In many marine nematodes, one or two unicellular '[[renette gland]]s' excrete salt through a pore on the underside of the animal, close to the pharynx. In most other nematodes, these specialised cells have been replaced by an organ consisting of two parallel ducts connected by a single transverse duct. This transverse duct opens into a common canal that runs to the excretory pore.<ref name="Barnes_1980"/>
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=== Nervous system ===
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{{see also|Muscle arms}}
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Four peripheral [[nerve]]s run the length of the body on the dorsal, ventral, and lateral surfaces. Each nerve lies within a cord of connective tissue lying beneath the [[cuticle]] and between the [[muscle]] cells. The ventral nerve is the largest, and has a double structure forward of the [[excretion|excretory]] [[wiktionary:pore|pore]]. The dorsal nerve is responsible for motor control, while the lateral nerves are sensory, and the ventral combines both functions.<ref name="Barnes_1980"/>
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The nervous system is also the only place in the nematode body that contains [[cilia]], which are all non-motile and with a sensory function.<ref>[http://www.wormbook.org/chapters/www_ciliumbiogenesis.2/ciliumbiogenesis.html The sensory cilia of Caenorhabditis elegans]</ref><ref>{{cite journal | pmc = 2870953 | pmid=20215474 | doi=10.1534/genetics.110.114009 | volume=185 | title=Hearing in Drosophila requires TilB, a conserved protein associated with ciliary motility | journal=Genetics | date=May 2010 | pages=177–88 | last1 = Kavlie | first1 = RG | last2 = Kernan | first2 = MJ | last3 = Eberl | first3 = DF}}</ref>
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At the anterior end of the animal, the nerves branch from a dense, circular nerve ([[nerve ring]]) round surrounding the [[pharynx]], and serving as the [[brain]]. Smaller nerves run forward from the ring to supply the sensory organs of the head.<ref name="Barnes_1980"/>
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The bodies of nematodes are covered in numerous sensory [[bristle]]s and [[:wikt:papilla|papillae]] that together provide a sense of touch. Behind the sensory bristles on the head lie two small pits, or '[[amphid]]s'. These are well supplied with nerve cells, and are probably [[chemoreceptor|chemoreception]] organs. A few aquatic nematodes possess what appear to be [[pigment]]ed eye-spots, but is unclear whether or not these are actually sensory in nature.<ref name="Barnes_1980"/>
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== Reproduction ==
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[[File:Eucoleus aerophilus male spicule.jpg|thumb|Extremity of a male nematode showing the [[Spicule (nematode)|spicule]], used for copulation. Bar = 100 µm <ref>{{Cite journal  | last1 = Lalošević | first1 = V. | last2 = Lalošević | first2 = D. | last3 = Capo | first3 = I. | last4 = Simin | first4 = V. | last5 = Galfi | first5 = A. | last6 = Traversa | first6 = D. | title = High infection rate of zoonotic ''Eucoleus aerophilus'' infection in foxes from Serbia. | journal = Parasite | volume = 20 | issue =  | pages = 3 | month =  | year = 2013 | doi = 10.1051/parasite/2012003 | PMID = 23340229 | PMC = 3718516 }}</ref>]]
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Most nematode species are [[dioecious]], with separate male and female individuals, though some, such as ''[[Caenorhabditis elegans]]'', are [[androdioecious]], consisting of [[hermaphrodite]]s and rare males. Both sexes possess one or two tubular [[gonad]]s. In males, the sperm are produced at the end of the gonad and migrate along its length as they mature. The testis opens into a relatively wide [[seminal vesicle]] and then during intercourse into a glandular and muscular ejaculatory duct associated with the [[vas deferens]] and [[cloaca]]. In females, the ovaries each open into an [[oviduct]] (in hermaphrodites, the eggs enter a [[spermatheca]] first) and then a glandular [[uterus]]. The uteri both open into a common vulva/ vagina, usually located in the middle of the morphologically ventral surface.<ref name="Barnes_1980"/>
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Reproduction is usually sexual, though hermaphrodites are capable of self-fertilization. Males are usually smaller than females/ hermaphrodites (often much smaller) and often have a characteristically bent or fan-shaped tail. During [[copulation]], one or more [[chitin]]ized [[Spicule (nematode)|spicule]]s move out of the cloaca and are inserted into the genital pore of the female. [[Amoeboid]] [[sperm]] crawl along the spicule into the female worm. Nematode sperm is thought to be the only [[eukaryotic cell]] without the globular protein [[G-actin]].
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Eggs may be [[embryonated]] or unembryonated when passed by the female, meaning their fertilized eggs may not yet be developed. A few species are known to be [[ovoviviparous]]. The eggs are protected by an outer shell, secreted by the uterus. In free-living roundworms, the eggs hatch into [[larva]]e, which appear essentially identical to the adults, except for an underdeveloped reproductive system; in parasitic roundworms, the life cycle is often much more complicated.<ref name="Barnes_1980"/>
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Nematodes as a whole possess a wide range of modes of reproduction.<ref name = "Bell_1982">{{cite book | author = Bell G | title = The masterpiece of nature: the evolution and genetics of sexuality | publisher = University of California Press | location = Berkeley | year = 1982 | pages = | isbn = 0-520-04583-1  }}</ref> Some nematodes, such as ''[[Heterorhabditis]]'' spp., undergo a process called ''[[endotokia matricida]]'': intrauterine birth causing maternal death.<ref name = "Johnigk_Ehlers_1999">{{cite journal |vauthors=Johnigk SA, Ehlers RU | year = 1999 | title = ''Endotokia matricida'' in hermaphrodites of ''Heterorhabditis'' spp. and the effect of the food supply | journal = Nematology | issn = 1388-5545 | volume = 1 | issue = 7–8 | pages = 717–726 | doi = 10.1163/156854199508748 }}</ref> Some nematodes are [[hermaphrodite|hermaphroditic]], and keep their self-fertilized eggs inside the [[uterus]] until they hatch. The juvenile nematodes will then ingest the parent nematode. This process is significantly promoted in environments with a low food supply.<ref name = "Johnigk_Ehlers_1999"/>
 +
 +
The nematode model species ''[[Caenorhabditis elegans]]'' and ''[[Caenorhabditis briggsae|C. briggsae]]'' exhibit [[androdioecy]], which is very rare among animals. The single [[genus]] ''[[Meloidogyne]]'' (root-knot nematodes) exhibit a range of reproductive modes, including [[sexual reproduction]], [[facultative sexuality]] (in which most, but not all, generations reproduce asexually), and both [[meiosis|meiotic]] and [[mitosis|mitotic]] [[parthenogenesis]].
 +
 +
The genus ''[[Mesorhabditis]]'' exhibits an unusual form of parthenogenesis, in which sperm-producing males copulate with females, but the sperm do not fuse with the ovum. Contact with the sperm is essential for the ovum to begin dividing, but because there is no fusion of the cells, the male contributes no genetic material to the offspring, which are essentially [[cloning|clones]] of the female.<ref name="Barnes_1980">{{cite book | author = Barnes RG | title = Invertebrate zoology | publisher = Sanders College | location = Philadelphia | year = 1980 | pages = | isbn = 0-03-056747-5 }}</ref>
 +
 +
== Free-living species ==
 +
Different free-living species feed on materials as varied as [[algae]], [[fungi]], small animals, fecal matter, dead organisms, and living tissues. Free-living marine nematodes are important and abundant members of the [[meiobenthos]]. They play an important role in the decomposition process, aid in recycling of nutrients in marine environments, and are sensitive to changes in the environment caused by pollution. One roundworm of note, ''[[Caenorhabditis elegans]]'', lives in the soil and has found much use as a [[model organism]]. ''C. elegans'' has had its entire genome sequenced, as well as the developmental fate of every cell determined, and every neuron mapped.
 +
 +
== Parasitic species ==
 +
[[File:Parasite140080-fig3 Gastrointestinal parasites in seven primates of the Taï National Park - Helminths.png|thumb|Eggs (mostly nematodes) from stools of wild [[primates]]]]
 +
 +
Nematodes commonly parasitic on humans include [[ascarid]]s (''Ascaris''), [[filaria]]s, [[hookworm]]s, [[pinworm]]s (''Enterobius''), and [[whipworm]]s (''Trichuris trichiura''). The species ''[[Trichinella spiralis]]'', commonly known as the 'trichina worm', occurs in rats, pigs, and humans, and is responsible for the disease [[trichinosis]]. ''[[Baylisascaris]]'' usually infests wild animals, but can be deadly to humans, as well. ''[[Dirofilaria immitis]]'' are known for causing heartworm disease by inhabiting the hearts, arteries, and lungs of dogs and some cats. ''[[Haemonchus contortus]]'' is one of the most abundant infectious agents in sheep around the world, causing great economic damage to sheep. In contrast, [[entomopathogenic nematode]]s parasitize insects and are mostly considered beneficial by humans, but some attack beneficial insects.
 +
 +
One form of nematode is entirely dependent upon [[fig wasp]]s, which are the sole source of [[ficus|fig]] fertilization. They prey upon the wasps, riding them from the ripe fig of the wasp's birth to the fig flower of its death, where they kill the wasp, and their offspring await the birth of the next generation of wasps as the fig ripens.
 +
 +
A newly discovered parasitic tetradonematid nematode, ''[[Myrmeconema neotropicum]]'',  apparently induces fruit mimicry in the tropical ant ''[[Cephalotes atratus]]''. Infected ants develop bright red [[gaster (insect anatomy)|gaster]]s (abdomens), tend to be more sluggish, and walk with their gasters in a conspicuous elevated position. It is likely that these changes cause [[frugivorous]] birds to confuse the infected ants for berries, and eat them. Parasite eggs passed in the bird's [[feces]] are subsequently collected by foraging ''Cephalotes atratus'' and are fed to their [[larva]]e, thus completing the life cycle of ''M. neotropicum''.<ref name="pmid18279076">{{cite journal |vauthors=Yanoviak SP, Kaspari M, Dudley R, Poinar G | title = Parasite-induced fruit mimicry in a tropical canopy ant | journal = Am. Nat. | volume = 171 | issue = 4 | pages = 536–44 |date=April 2008 | pmid = 18279076 | doi = 10.1086/528968 }}</ref>
 +
 +
Similarly, multiple varieties of nematodes have been found in the abdominal cavities of the primitively social sweat bee, ''[[Lasioglossum zephyrum]]''. Inside the female body, the nematode hinders ovarian development and renders the bee less active and thus less effective in pollen collection.<ref>{{Cite journal|title = Organisms Associated with Lasioglossum zephyrum (Hymenoptera: Halictidae)|jstor = 25083474|journal = Journal of the Kansas Entomological Society|date = 1965-10-01|pages = 367–389|volume = 38|issue = 4|first = Suzanne W. T.|last = Batra}}</ref>
 +
 +
[[File:Soybean cyst nematode and egg SEM.jpg|thumb|Colorized [[electron micrograph]] of [[soybean cyst nematode]] (''Heterodera'' sp.) and egg]]
 +
Plant-parasitic nematodes include several groups causing severe crop losses. The most common genera are ''[[Aphelenchoides]]'' ([[foliar nematode]]s), ''[[Ditylenchus]]'', ''[[Globodera]]'' (potato cyst nematodes), ''[[Heterodera]]'' (soybean cyst nematodes), ''[[Longidorus]]'', ''[[Meloidogyne]]'' ([[root-knot nematodes]]), ''[[Nacobbus]]'', ''[[Pratylenchus]]'' (lesion nematodes), ''[[Trichodorus]]'' and ''[[Xiphinema]]'' (dagger nematodes). Several phytoparasitic nematode species cause histological damages to roots, including the formation of visible galls (e.g. by root-knot nematodes), which are useful characters for their diagnostic in the field. Some nematode species transmit plant viruses through their feeding activity on roots. One of them is ''[[Xiphinema index]]'', vector of [[grapevine fanleaf virus]], an important disease of grapes, another one is ''[[Xiphinema diversicaudatum]]'', vector of [[arabis mosaic virus]]''.''
 +
 +
Other nematodes attack bark and forest trees. The most important representative of this group is ''[[Bursaphelenchus xylophilus]]'', the pine wood nematode, present in Asia and America and recently discovered in Europe.
 +
 +
=== Agriculture and horticulture ===
 +
 +
Depending on the species, a nematode may be beneficial or detrimental to plant health. From agricultural and [[horticulture]] perspectives, the two categories of nematodes are the predatory ones, which will kill garden pests like [[cutworm]]s and [[Helicoverpa zea|corn earworm moths]], and the pest nematodes, like the [[root-knot nematode]], which attack plants, and those that act as [[vector (epidemiology)|vectors]] spreading [[plant virus]]es between crop plants.<ref>{{cite journal|vauthors=Purcell M, Johnson MW, Lebeck LM, Hara AH|title=Biological Control of Helicoverpa zea (Lepidoptera: Noctuidae) with Steinernema carpocapsae (Rhabditida: Steinernematidae) in Corn Used as a Trap Crop|journal=Environmental Entomology|date=1992|volume=21|issue=6|pages=1441–1447|doi=10.1093/ee/21.6.1441}}</ref> Predatory nematodes can be bred by soaking a specific recipe of leaves and other [[detritus]] in water, in a dark, cool place, and can even be purchased as an [[Organic movement|organic]] form of [[pest control]].
 +
 +
Rotations of plants with nematode-resistant species or varieties is one means of managing parasitic nematode infestations. For example, [[Marigold (common)|marigolds]], grown over one or more seasons (the effect is cumulative), can be used to control nematodes.<ref name="Riotte" /> Another is treatment with natural antagonists such as the fungus ''[[Gliocladium roseum]]''. [[Chitosan]], a natural biocontrol, elicits plant defense responses to destroy parasitic [[cyst]] nematodes on roots of [[soybean]], [[corn]], [[sugar beet]], [[potato]], and [[tomato]] crops without harming beneficial nematodes in the soil.<ref name="US-2008/072494">{{ cite patent | country = US | number = 2008072494 | status = application | title = Micronutrient elicitor for treating nematodes in field crops | pubdate = 2008-03-27 | fdate = 2006-09-07 | inventor = Stoner RJ, Linden JC | assign1 = }}</ref> [[Soil steam sterilization|Soil steaming]] is an efficient method to kill nematodes before planting a crop, but indiscriminately eliminates both harmful and beneficial soil fauna.
 +
 +
The golden nematode ''[[Globodera rostochiensis]]'' is a particularly harmful variety of nematode pest that has resulted in quarantines and crop failures worldwide. [[CSIRO]] has found a 13- to 14-fold reduction of nematode population densities in plots having [[Indian mustard]] ''[[Brassica juncea]]'' green manure or seed meal in the soil.<ref name="CSIRO" />
 +
 +
== Epidemiology ==
 +
[[File:Intestinal nematode infections world map - DALY - WHO2002.svg|thumb|right|upright=1.23|[[Disability-adjusted life year]] for intestinal nematode infections per 100,000 in 2002.{{div col|colwidth=6em}} {{legend|#ffff65|&nbsp;&nbsp;<&nbsp; 25}} {{legend|#fff200|25–50}} {{legend|#ffdc00|50–75}} {{legend|#ffc600|75–100}} {{legend|#ffb000|100–120}} {{legend|#ff9a00|120–140}} {{legend|#ff8400|140–160}} {{legend|#ff6e00|160–180}} {{legend|#ff5800|180–200}} {{legend|#ff4200|200–220}} {{legend|#ff2c00|220–240}} {{legend|#cb0000|&nbsp;&nbsp;>&nbsp; 240}} {{legend|#b3b3b3|no data}} {{div col end}}]]
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[[File:Anthelmintic effect of papain on Heligmosomoides bakeri.ogv|thumb|[[Anthelmintic]] effect of [[papain]] on ''[[Heligmosomoides bakeri]]'']]
 +
 +
A number of intestinal nematodes cause diseases affecting human beings, including [[ascariasis]], [[trichuriasis]] and [[hookworm disease]]. Filarial nematodes cause [[filariasis]].
 +
 +
==Soil ecosystems==
 +
Ninety percent of nematodes reside in the top 15&nbsp;cm of soil. Nematodes do not decompose organic matter, but, instead, are parasitic and free-living organisms that feed on living material. Nematodes can effectively regulate bacterial population and community composition — they may eat up to 5,000 bacteria per minute. Also, nematodes can play an important role in the [[nitrogen cycle]] by way of nitrogen mineralization.<ref name="brady"/>
 +
 +
One group of [[carnivorous fungus|carnivorous fungi]], the [[Nematophagous fungus|nematophagous fungi]], are predators of soil nematodes. They set enticements for the nematodes in the form of lassos or adhesive structures.<ref name="Pramer 1964" /><ref name="Hauser_1985">{{cite journal|date=December 1985|title=Nematode-trapping fungi|url=http://www.carnivorousplants.org/cpn/articles/CPNv14n1p8_11.pdf|journal=Carnivorous Plant Newsletter|volume=14|issue=1|pages=8–11|doi=|author=Hauser JT}}</ref><ref name="Ahrén 1998" />
 +
 +
==Society and culture==
 +
 +
Nematode worms (''C. elegans''), part of an ongoing research project conducted on the [[Space Shuttle Columbia]] mission [[STS-107]], survived the [[Space Shuttle Columbia disaster#Recovery of debris|re-entry breakup]]. It is believed to be the first known life-form to survive a virtually unprotected atmospheric descent to Earth's surface.<ref>{{cite magazine |magazine=Astrobiology Magazine |url=http://www.astrobio.net/topic/origins/extreme-life/columbia-survivors/ |title=Columbia Survivors |date=Jan 1, 2006}}</ref><ref>{{cite journal |last1=Szewczyk |first1=Nathaniel J. |last2=Mancinelli |first2=Rocco L. |last3=McLamb |first3=William |last4=Reed |first4=David |last5=Blumberg |first5=Baruch S. |last6=Conley |first6=Catharine A. |title=''Caenorhabditis elegans'' Survives Atmospheric Breakup of STS–107, Space Shuttle Columbia |journal=Astrobiology |date=December 2005 |volume=5 |issue=6 |pages=690–705 |doi=10.1089/ast.2005.5.690 |url=https://dx.doi.org/10.1089/ast.2005.5.690 |accessdate=12 January 2016 |bibcode=2005AsBio...5..690S |pmid=16379525}}</ref>
 +
 +
== See also ==
 +
* [[Biological pest control]]
 +
* [[Capillaria (nematode)|Capillaria]]
 +
* ''[[Caenorhabditis elegans]]'': An important [[model organism]] often used to study [[cellular differentiation]], sometimes simply referred to as "worm" by scientists
 +
* [[List of organic gardening and farming topics]]
 +
* [[List of parasites of humans]]
 +
* [[Toxocariasis]]: A [[helminth]] infection of humans caused by the [[dog]] or [[cat]] roundworm, ''[[Toxocara canis]]'' or ''[[Toxocara cati]]''
 +
 +
== Notes ==
 +
{{notes}}
 +
 +
== References ==
 +
{{Reflist |colwidth=30em |refs=
 +
 +
<ref name="Borgonie 2011">
 +
{{cite journal
 +
|vauthors=Borgonie G, García-Moyano A, Litthauer D, Bert W, Bester A, van Heerden E, Möller C, Erasmus M, Onstott TC | title = Nematoda from the terrestrial deep subsurface of South Africa
 +
| journal = Nature
 +
|date=June 2011
 +
| volume = 474
 +
| issue = 7349
 +
| pages = 79–82
 +
| doi = 10.1038/nature09974
 +
| pmid = 21637257
 +
}}
 +
</ref>
 +
 +
<ref name="Pramer 1964">
 +
{{cite journal
 +
| author = Pramer C
 +
| year = 1964
 +
| title = Nematode-trapping fungi
 +
| url =
 +
| journal = Science
 +
| volume = 144
 +
| issue = 3617
 +
| pages = 382–388
 +
| doi = 10.1126/science.144.3617.382
 +
| pmid = 14169325
 +
}}
 +
</ref>
 +
 +
<ref name="Ahrén 1998">
 +
{{cite journal
 +
|vauthors=Ahrén D, Ursing BM, Tunlid A | year = 1998
 +
| title = Phylogeny of nematode-trapping fungi based on 18S rDNA sequences
 +
| url =
 +
| journal = FEMS Microbiology Letters
 +
| volume = 158
 +
| issue = 2
 +
| pages = 179–184
 +
| pmid = 9465391
 +
| doi=10.1016/s0378-1097(97)00519-3
 +
}}
 +
</ref>
 +
 +
<ref name="Lemonick 2011">
 +
{{cite news
 +
| author = Lemonick MD
 +
| title = Could 'worms from Hell' mean there's life in space?
 +
| work = Time
 +
| issn=0040-781X
 +
| url = http://www.time.com/time/health/article/0,8599,2076281,00.html
 +
| accessdate = 2011-06-08
 +
| date = 2011-06-08
 +
}}
 +
</ref>
 +
 +
<ref name="Bhanoo 2011">
 +
{{cite news
 +
| author = Bhanoo SN
 +
| date = 2011-06-01
 +
| title = Nematode found in mine is first subsurface multicellular organism
 +
| work = The New York Times
 +
| issn = 0362-4331
 +
| url = https://www.nytimes.com/2011/06/07/science/07obworm.html?_r=1&ref=southafrica
 +
| accessdate = 2011-06-13
 +
}}
 +
</ref>
 +
 +
<ref name="Nature 2011-06-02">{{cite journal | title = Gold mine | journal = Nature |date=June 2011 | volume = 474 | issue = 7349 | pages = 6 | doi = 10.1038/474006b}}</ref>
 +
 +
<ref name="Drake 2011-06-01">
 +
{{cite web
 +
| author = Drake N
 +
| date = 2011-06-01
 +
| title = Subterranean worms from hell: Nature News
 +
| work = Nature News
 +
| url = http://www.nature.com/news/2011/110601/full/news.2011.342.html
 +
| doi = 10.1038/news.2011.342
 +
| accessdate = 2011-06-13
 +
}}
 +
</ref>
 +
 +
<ref name="Borgonie 2011-06-02">
 +
{{cite journal
 +
|vauthors=Borgonie G, García-Moyano A, Litthauer D, Bert W, Bester A, van Heerden E, Möller C, Erasmus M, Onstott TC | date = 2011-06-02
 +
| title = Nematoda from the terrestrial deep subsurface of South Africa
 +
| journal = Nature
 +
| volume = 474
 +
| issue = 7349
 +
| pages = 79–82
 +
| doi = 10.1038/nature09974
 +
| issn = 0028-0836
 +
| pmid = 21637257
 +
}}
 +
</ref>
 +
 +
<ref name="ITIS Nematoda">
 +
{{cite web
 +
| url = https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=59490
 +
| title = ITIS report: Nematoda
 +
| publisher = Itis.gov
 +
| date =
 +
| accessdate = 2012-06-12
 +
}}
 +
</ref>
 +
 +
<ref name="Holterman2006">
 +
{{cite journal
 +
|vauthors=Holterman M, van der Wurff A, van den Elsen S, van Megen H, Bongers T, Holovachov O, Bakker J, Helder J | year = 2006
 +
| title = Phylum-wide analysis of SSU rDNA reveals deep phylogenetic relationships among nematodes and accelerated evolution toward crown Clades
 +
| url =
 +
| journal = Mol Biol Evol
 +
| volume = 23
 +
| issue = 9
 +
| pages = 1792–1800
 +
| doi = 10.1093/molbev/msl044
 +
| pmid = 16790472
 +
}}
 +
</ref>
 +
 +
<ref name="Nematodes.org Genomes">
 +
{{cite web
 +
| url = http://www.nematodes.org/nematodegenomes/index.php/959_Nematode_Genomes
 +
| title = 959 Nematode Genomes&nbsp;– NematodeGenomes
 +
| publisher = Nematodes.org
 +
| date = 2011-11-11
 +
| accessdate = 2012-06-12
 +
}}
 +
</ref>
 +
 +
<ref name="Weischer 2000">
 +
{{cite book
 +
|vauthors=Weischer B, Brown DJ | title = An Introduction to Nematodes: General Nematology
 +
| year = 2000
 +
| publisher = Pensoft
 +
| location = Sofia, Bulgaria
 +
| isbn = 978-954-642-087-9
 +
| pages = 75–76
 +
}}
 +
</ref>
 +
 +
<ref name="Riotte">
 +
{{cite book
 +
| author = Riotte L
 +
| title = Secrets of companion planting for successful gardening
 +
| year = 1975
 +
| page=7
 +
}}
 +
</ref>
 +
 +
<ref name="CSIRO">
 +
{{cite journal
 +
| url = http://www.publish.csiro.au/paper/AP04081
 +
| vauthors = Loothfar R, Tony S
 +
| title = Suppression of root knot nematode (Meloidogyne javanica) after incorporation of Indian mustard cv. Nemfix as green manure and seed meal in vineyards
 +
| journal = [[Australasian Plant Pathology]]
 +
| volume = 34
 +
| issue = 1
 +
| date = 2005-03-22
 +
| pages = 77–83
 +
| doi = 10.1071/AP04081
 +
| publisher = CSIRO Publishing
 +
| accessdate = 2010-06-14
 +
}}
 +
</ref>
 +
 +
}}
 +
 +
== Further reading ==
 +
{{refbegin|30em}}
 +
* {{cite journal | last1 = Atkinson | first1 = H.J. | year = 1973 | title = The respiratory physiology of the marine nematodes ''Enoplus brevis'' (Bastian) and ''E. communis'' (Bastian): I. The influence of oxygen tension and body size | url = http://jeb.biologists.org/cgi/reprint/59/1/255.pdf | format = PDF | journal = [[Journal of Experimental Biology|J. Exp. Biol.]] | volume = 59 | issue = 1| pages = 255–266 }}
 +
* {{cite web |website=[[BBC News]] |url=http://news.bbc.co.uk/1/hi/sci/tech/2992123.stm |title=Worms survived Columbia disaster |date=1 May 2003 |accessdate=4 Nov 2008}}
 +
* {{cite journal | last1 = Gubanov | first1 = N.M. | year = 1951 | title = Giant nematoda from the placenta of Cetacea; ''Placentonema gigantissima'' nov. gen., nov. sp. | url = | journal = [[Proceedings of the USSR Academy of Sciences|Proc. USSR Acad. Sci.]] | volume = 77 | issue = 6| pages = 1123–1125 }} [in Russian].
 +
* {{cite book|authors=Kaya, Harry K.|chapter=An Overview of Insect-Parasitic and Entomopathogenic Nematodes|editors=Bedding, R.A.|title=Nematodes and the Biological Control of Insect Pests|publisher=Csiro Publishing|year=1993|isbn=9780643105911|url=https://books.google.com/books?id=drhdg7UmNnAC&pg=PT8|display-authors=etal}}
 +
* {{cite web |website=[[Merck Veterinary Manual]] (MVM) |date=2006 |url=http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/130506.htm |title=Giant kidney worm infection in mink and dogs |accessdate=10 Feb 2007}}
 +
* {{cite journal |vauthors=White JG, Southgate E, Thomson JN, Brenner S | title = The structure of the ventral nerve cord of Caenorhabditis elegans | journal = Philos. Trans. R. Soc. Lond. B Biol. Sci. | volume = 275 | issue = 938 | pages = 327–48 |date=August 1976 | pmid = 8806 | doi =  10.1098/rstb.1976.0086}}
 +
* {{cite book|editor1-last=Lee|editor1-first=Donald L|title=The biology of nematodes|date=2010|publisher=Taylor & Francis|location=London|isbn=0415272114|url=https://books.google.ca/books?id=omQLBAAAQBAJ|accessdate=16 December 2014}}
 +
* {{cite book |authors=De Ley, P & Blaxter, M |date=2004 |chapter=A new system for Nematoda: combining morphological characters with molecular trees, and translating clades into ranks and taxa |editors=R Cook & DJ Hunt |title=Nematology Monographs and Perspectives |volume=vol. 2 |publisher=E.J. Brill, Leiden |pages=633–653}}
 +
{{refend}}
 +
 +
== External links ==
 +
{{Commons category|Nematoda}}
 +
{{refbegin|30em}}
 +
* [https://web.archive.org/web/20101230000456/http://www.harper-adams.ac.uk/groups/crops/nematology/ Harper Adams University College Nematology Research]
 +
* [https://wayback.archive-it.org/all/20110721030344/http://knol.google.com/k/nematodes-roundworms-of-man#view Nematodes/roundworms of man]
 +
* http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html
 +
* [http://www.esn-online.org/ European Society of Nematologists]
 +
* [http://www.nematode.net/ Nematode.net: Repository of parasitic nematode sequences.]
 +
* http://www.nematodes.org/
 +
* [https://web.archive.org/web/20061027152335/http://intramar.ugent.be/nemys/start.asp?group=2 NeMys World free-living Marine Nematodes database]
 +
* [http://faculty.ucr.edu/%7Epdeley/lab/taxonomy.html Nematode Virtual Library]
 +
* [http://www.ifns.org/ International Federation of Nematology Societies]
 +
* [http://nematologists.org/ Society of Nematologists]
 +
* [http://nematologists.org.au/ Australasian Association of Nematologists]
 +
* [http://news.bbc.co.uk/2/hi/science/nature/6612411.stm Research on nematodes and longevity]
 +
* [http://www.bbc.co.uk/dna/h2g2/alabaster/A2467299 Nematode on BBC]
 +
* [http://www.theaquariumwiki.com/Nematode Nematode worms in an aquarium]
 +
* [http://entnemdept.ufl.edu/creatures/main/search_higher_nematodes.htm Phylum Nematoda – nematodes] on the [[University of Florida|UF]] / *[[Institute of Food and Agricultural Sciences|IFAS]] Featured Creatures Web site
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{{refend}}
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{{Animalia}}
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{{Helminthiases}}
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{{Taxonbar|from=Q5185}}
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{{Authority control}}
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[[Category:Nematodes| ]]
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<!-- [[Category:Animal phyla]] moved to "Nematoda" redirect -->
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[[Category:Articles containing video clips]]
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[[Category:Nematoida]]
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[[Category:Composting]]
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[[Category:Garden pests]]
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[[Category:Organic gardening]]
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[[Category:Pest control]]
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[[Category:Ediacaran first appearances]]
 
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Revision as of 16:36, 11 April 2018

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The nematodes (Template:IPAc-en, Template:IPAc-en) or roundworms constitute the phylum Nematoda (also called Nemathelminthes).[1][2] They are a diverse animal phylum inhabiting a broad range of environments. Nematode species can be difficult to distinguish, and although over 25,000 have been described,[3][4] of which more than half are parasitic, it is estimated that over 40,000 species exist; estimates of 500,000 to 1 million species[5] are not supported by evidence.[6] Nematodes are classified along with insects and other moulting animals in the clade Ecdysozoa, and, unlike flatworms, have tubular digestive systems with openings at both ends.

Nematodes have successfully adapted to nearly every ecosystem from marine (salt water) to fresh water, to soils, and from the polar regions to the tropics, as well as the highest to the lowest of elevations. They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as mountains, deserts and oceanic trenches. They are found in every part of the earth's lithosphere,[7] even at great depths, Template:Convert, below the surface of the Earth in gold mines in South Africa.[8][9][10][11][12] They represent 90% of all animals on the ocean floor.[13] Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity of life cycles, and their presence at various trophic levels point at an important role in many ecosystems.[14] There are about 2271 genera in 256 families.[6] The many parasitic forms include pathogens in most plants and animals. A third of the genera occur as parasites of vertebrates; about 35 nematode species occur in humans.[6]

Nathan Cobb, a nematologist, described the ubiquity of nematodes on Earth thus:
In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable, since for every massing of human beings there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites.[15]

Taxonomy and systematics

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File:Eophasma jurasicum.JPG
Eophasma jurasicum, a fossilized nematode
File:Celegans wt nhr80rnai.png
Caenorhabditis elegans
File:Threadworm.jpg
Oxyuridae Threadworm
File:Microfilaria.jpg
Spiruridae Dirofilaria immitis

History

In 1758, Linnaeus described some nematode genera (e.g., Ascaris), then included in Vermes.

The name of the group Nematoda, informally called "nematodes", came from Nematoidea, originally defined by Karl Rudolphi (1808),[16] from Ancient Greek νῆμα (nêma, nêmatos, 'thread') and -eiδἠς (-eidēs, 'species'). It was treated as family Nematodes by Burmeister (1837).[16]

At its origin, the "Nematoidea" erroneously included Nematodes and Nematomorpha, attributed by von Siebold (1843). Along with Acanthocephala, Trematoda and Cestoidea, it formed the obsolete group Entozoa,[17] created by Rudolphi (1808).[18] They were also classed along with Acanthocephala in the obsolete phylum Nemathelminthes by Gegenbaur (1859).

In 1861, K. M. Diesing treated the group as order Nematoda.[16] In 1877, the taxon Nematoidea, including the family Gordiidae (horsehair worms), was promoted to the rank of phylum by Ray Lankester. In 1919, Nathan Cobb proposed that nematodes should be recognized alone as a phylum.[19] He argued they should be called "nema" in English rather than "nematodes"Template:Efn and defined the taxon Nemates (later emended as Nemata, Latin plural of nema), listing Nematoidea sensu restricto as a synonym. Since Cobb was the first to exclude all but nematodes from the group, some sources consider the valid taxon name to be Nemates or Nemata, rather than Nematoda.[20]

Phylogeny

The phylogenetic relationships of the nematodes and their close relatives among the protostomian Metazoa are unresolved. Traditionally, they were held to be a lineage of their own, but in the 1990s, they were proposed to form the group Ecdysozoa together with moulting animals, such as arthropods. The identity of the closest living relatives of the Nematoda has always been considered to be well resolved. Morphological characters and molecular phylogenies agree with placement of the roundworms as a sister taxon to the parasitic Nematomorpha; together they make up the Nematoida. Along with the Scalidophora (formerly Cephalorhyncha), the Nematoida form the clade Cycloneuralia, but much disagreement occurs both between and among the available morphological and molecular data. The Cycloneuralia or the Introverta—depending on the validity of the former—are often ranked as a superphylum.[21]

Nematode systematics

Due to the lack of knowledge regarding many nematodes, their systematics is contentious. An earliest and influential classification was proposed by Chitwood and Chitwood[22]—later revised by Chitwood[23]—who divided the phylum into two—the Aphasmidia and the Phasmidia. These were later renamed Adenophorea (gland bearers) and Secernentea (secretors), respectively.[24] The Secernentea share several characteristics, including the presence of phasmids, a pair of sensory organs located in the lateral posterior region, and this was used as the basis for this division. This scheme was adhered to in many later classifications, though the Adenophorea were not in a uniform group.

Initial studies of incomplete DNA sequences[25] suggested the existence of five clades:[26]

As it seems, the Secernentea are indeed a natural group of closest relatives. But the "Adenophorea" appear to be a paraphyletic assemblage of roundworms simply retaining a good number of ancestral traits. The old Enoplia do not seem to be monophyletic either, but to contain two distinct lineages. The old group "Chromadoria" seem to be another paraphyletic assemblage, with the Monhysterida representing a very ancient minor group of nematodes. Among the Secernentea, the Diplogasteria may need to be united with the Rhabditia, while the Tylenchia might be paraphyletic with the Rhabditia.[27]

The understanding of roundworm systematics and phylogeny as of 2002 is summarised below:

Phylum Nematoda

Later work has suggested the presence of 12 clades.[28] The Secernentea—a group that includes virtually all major animal and plant 'nematode' parasites—apparently arose from within the Adenophorea.

A major effort to improve the systematics of this phylum is in progress and being organised by the 959 Nematode Genomes.[29]

A complete checklist of the World's nematode species can be found in the World Species Index: Nematoda.[30]

An analysis of the mitochondrial DNA suggests that the following groupings are valid[31]

The Ascaridomorpha, Rhabditomorpha and Diplogasteromorpha appear to be related.

The suborders Spirurina and Tylenchina and the infraorders Rhabditomorpha, Panagrolaimomorpha and Tylenchomorpha are paraphytic.

The monophyly of the Ascaridomorph is uncertain.

Anatomy

File:C elegans male.svg
Internal anatomy of a male C. elegans nematode

Nematodes are small, slender worms: typically approximately 5 to 100 µm thick, and 0.1 to 2.5 mm long.[32] The smallest nematodes are microscopic, while free-living species can reach as much as Template:Convert, and some parasitic species are larger still, reaching over Template:Convert in length.[33]Template:Rp The body is often ornamented with ridges, rings, bristles, or other distinctive structures.[34]

The head of a nematode is relatively distinct. Whereas the rest of the body is bilaterally symmetrical, the head is radially symmetrical, with sensory bristles and, in many cases, solid 'head-shields' radiating outwards around the mouth. The mouth has either three or six lips, which often bear a series of teeth on their inner edges. An adhesive 'caudal gland' is often found at the tip of the tail.[35]

The epidermis is either a syncytium or a single layer of cells, and is covered by a thick collagenous cuticle. The cuticle is often of complex structure, and may have two or three distinct layers. Underneath the epidermis lies a layer of longitudinal muscle cells. The relatively rigid cuticle works with the muscles to create a hydroskeleton as nematodes lack circumferential muscles. Projections run from the inner surface of muscle cells towards the nerve cords; this is a unique arrangement in the animal kingdom, in which nerve cells normally extend fibres into the muscles rather than vice versa.[35]

Digestive system

The oral cavity is lined with cuticle, which is often strengthened with ridges or other structures, and, especially in carnivorous species, may bear a number of teeth. The mouth often includes a sharp stylet, which the animal can thrust into its prey. In some species, the stylet is hollow, and can be used to suck liquids from plants or animals.[35]

The oral cavity opens into a muscular, sucking pharynx, also lined with cuticle. Digestive glands are found in this region of the gut, producing enzymes that start to break down the food. In stylet-bearing species, these may even be injected into the prey.[35]

There is no stomach, with the pharynx connecting directly to a muscleless intestine that forms the main length of the gut. This produces further enzymes, and also absorbs nutrients through its single cell thick lining. The last portion of the intestine is lined by cuticle, forming a rectum, which expels waste through the anus just below and in front of the tip of the tail. Movement of food through the digestive system is the result of body movements of the worm. The intestine has valves or sphincters at either end to help control the movement of food through the body.[35]

Excretory system

Nitrogenous waste is excreted in the form of ammonia through the body wall, and is not associated with any specific organs. However, the structures for excreting salt to maintain osmoregulation are typically more complex.[35]

In many marine nematodes, one or two unicellular 'renette glands' excrete salt through a pore on the underside of the animal, close to the pharynx. In most other nematodes, these specialised cells have been replaced by an organ consisting of two parallel ducts connected by a single transverse duct. This transverse duct opens into a common canal that runs to the excretory pore.[35]

Nervous system

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Four peripheral nerves run the length of the body on the dorsal, ventral, and lateral surfaces. Each nerve lies within a cord of connective tissue lying beneath the cuticle and between the muscle cells. The ventral nerve is the largest, and has a double structure forward of the excretory pore. The dorsal nerve is responsible for motor control, while the lateral nerves are sensory, and the ventral combines both functions.[35]

The nervous system is also the only place in the nematode body that contains cilia, which are all non-motile and with a sensory function.[36][37]

At the anterior end of the animal, the nerves branch from a dense, circular nerve (nerve ring) round surrounding the pharynx, and serving as the brain. Smaller nerves run forward from the ring to supply the sensory organs of the head.[35]

The bodies of nematodes are covered in numerous sensory bristles and papillae that together provide a sense of touch. Behind the sensory bristles on the head lie two small pits, or 'amphids'. These are well supplied with nerve cells, and are probably chemoreception organs. A few aquatic nematodes possess what appear to be pigmented eye-spots, but is unclear whether or not these are actually sensory in nature.[35]

Reproduction

File:Eucoleus aerophilus male spicule.jpg
Extremity of a male nematode showing the spicule, used for copulation. Bar = 100 µm [38]

Most nematode species are dioecious, with separate male and female individuals, though some, such as Caenorhabditis elegans, are androdioecious, consisting of hermaphrodites and rare males. Both sexes possess one or two tubular gonads. In males, the sperm are produced at the end of the gonad and migrate along its length as they mature. The testis opens into a relatively wide seminal vesicle and then during intercourse into a glandular and muscular ejaculatory duct associated with the vas deferens and cloaca. In females, the ovaries each open into an oviduct (in hermaphrodites, the eggs enter a spermatheca first) and then a glandular uterus. The uteri both open into a common vulva/ vagina, usually located in the middle of the morphologically ventral surface.[35]

Reproduction is usually sexual, though hermaphrodites are capable of self-fertilization. Males are usually smaller than females/ hermaphrodites (often much smaller) and often have a characteristically bent or fan-shaped tail. During copulation, one or more chitinized spicules move out of the cloaca and are inserted into the genital pore of the female. Amoeboid sperm crawl along the spicule into the female worm. Nematode sperm is thought to be the only eukaryotic cell without the globular protein G-actin.

Eggs may be embryonated or unembryonated when passed by the female, meaning their fertilized eggs may not yet be developed. A few species are known to be ovoviviparous. The eggs are protected by an outer shell, secreted by the uterus. In free-living roundworms, the eggs hatch into larvae, which appear essentially identical to the adults, except for an underdeveloped reproductive system; in parasitic roundworms, the life cycle is often much more complicated.[35]

Nematodes as a whole possess a wide range of modes of reproduction.[39] Some nematodes, such as Heterorhabditis spp., undergo a process called endotokia matricida: intrauterine birth causing maternal death.[40] Some nematodes are hermaphroditic, and keep their self-fertilized eggs inside the uterus until they hatch. The juvenile nematodes will then ingest the parent nematode. This process is significantly promoted in environments with a low food supply.[40]

The nematode model species Caenorhabditis elegans and C. briggsae exhibit androdioecy, which is very rare among animals. The single genus Meloidogyne (root-knot nematodes) exhibit a range of reproductive modes, including sexual reproduction, facultative sexuality (in which most, but not all, generations reproduce asexually), and both meiotic and mitotic parthenogenesis.

The genus Mesorhabditis exhibits an unusual form of parthenogenesis, in which sperm-producing males copulate with females, but the sperm do not fuse with the ovum. Contact with the sperm is essential for the ovum to begin dividing, but because there is no fusion of the cells, the male contributes no genetic material to the offspring, which are essentially clones of the female.[35]

Free-living species

Different free-living species feed on materials as varied as algae, fungi, small animals, fecal matter, dead organisms, and living tissues. Free-living marine nematodes are important and abundant members of the meiobenthos. They play an important role in the decomposition process, aid in recycling of nutrients in marine environments, and are sensitive to changes in the environment caused by pollution. One roundworm of note, Caenorhabditis elegans, lives in the soil and has found much use as a model organism. C. elegans has had its entire genome sequenced, as well as the developmental fate of every cell determined, and every neuron mapped.

Parasitic species

Nematodes commonly parasitic on humans include ascarids (Ascaris), filarias, hookworms, pinworms (Enterobius), and whipworms (Trichuris trichiura). The species Trichinella spiralis, commonly known as the 'trichina worm', occurs in rats, pigs, and humans, and is responsible for the disease trichinosis. Baylisascaris usually infests wild animals, but can be deadly to humans, as well. Dirofilaria immitis are known for causing heartworm disease by inhabiting the hearts, arteries, and lungs of dogs and some cats. Haemonchus contortus is one of the most abundant infectious agents in sheep around the world, causing great economic damage to sheep. In contrast, entomopathogenic nematodes parasitize insects and are mostly considered beneficial by humans, but some attack beneficial insects.

One form of nematode is entirely dependent upon fig wasps, which are the sole source of fig fertilization. They prey upon the wasps, riding them from the ripe fig of the wasp's birth to the fig flower of its death, where they kill the wasp, and their offspring await the birth of the next generation of wasps as the fig ripens.

A newly discovered parasitic tetradonematid nematode, Myrmeconema neotropicum, apparently induces fruit mimicry in the tropical ant Cephalotes atratus. Infected ants develop bright red gasters (abdomens), tend to be more sluggish, and walk with their gasters in a conspicuous elevated position. It is likely that these changes cause frugivorous birds to confuse the infected ants for berries, and eat them. Parasite eggs passed in the bird's feces are subsequently collected by foraging Cephalotes atratus and are fed to their larvae, thus completing the life cycle of M. neotropicum.[41]

Similarly, multiple varieties of nematodes have been found in the abdominal cavities of the primitively social sweat bee, Lasioglossum zephyrum. Inside the female body, the nematode hinders ovarian development and renders the bee less active and thus less effective in pollen collection.[42]

Plant-parasitic nematodes include several groups causing severe crop losses. The most common genera are Aphelenchoides (foliar nematodes), Ditylenchus, Globodera (potato cyst nematodes), Heterodera (soybean cyst nematodes), Longidorus, Meloidogyne (root-knot nematodes), Nacobbus, Pratylenchus (lesion nematodes), Trichodorus and Xiphinema (dagger nematodes). Several phytoparasitic nematode species cause histological damages to roots, including the formation of visible galls (e.g. by root-knot nematodes), which are useful characters for their diagnostic in the field. Some nematode species transmit plant viruses through their feeding activity on roots. One of them is Xiphinema index, vector of grapevine fanleaf virus, an important disease of grapes, another one is Xiphinema diversicaudatum, vector of arabis mosaic virus.

Other nematodes attack bark and forest trees. The most important representative of this group is Bursaphelenchus xylophilus, the pine wood nematode, present in Asia and America and recently discovered in Europe.

Agriculture and horticulture

Depending on the species, a nematode may be beneficial or detrimental to plant health. From agricultural and horticulture perspectives, the two categories of nematodes are the predatory ones, which will kill garden pests like cutworms and corn earworm moths, and the pest nematodes, like the root-knot nematode, which attack plants, and those that act as vectors spreading plant viruses between crop plants.[43] Predatory nematodes can be bred by soaking a specific recipe of leaves and other detritus in water, in a dark, cool place, and can even be purchased as an organic form of pest control.

Rotations of plants with nematode-resistant species or varieties is one means of managing parasitic nematode infestations. For example, marigolds, grown over one or more seasons (the effect is cumulative), can be used to control nematodes.[44] Another is treatment with natural antagonists such as the fungus Gliocladium roseum. Chitosan, a natural biocontrol, elicits plant defense responses to destroy parasitic cyst nematodes on roots of soybean, corn, sugar beet, potato, and tomato crops without harming beneficial nematodes in the soil.[45] Soil steaming is an efficient method to kill nematodes before planting a crop, but indiscriminately eliminates both harmful and beneficial soil fauna.

The golden nematode Globodera rostochiensis is a particularly harmful variety of nematode pest that has resulted in quarantines and crop failures worldwide. CSIRO has found a 13- to 14-fold reduction of nematode population densities in plots having Indian mustard Brassica juncea green manure or seed meal in the soil.[46]

Epidemiology

A number of intestinal nematodes cause diseases affecting human beings, including ascariasis, trichuriasis and hookworm disease. Filarial nematodes cause filariasis.

Soil ecosystems

Ninety percent of nematodes reside in the top 15 cm of soil. Nematodes do not decompose organic matter, but, instead, are parasitic and free-living organisms that feed on living material. Nematodes can effectively regulate bacterial population and community composition — they may eat up to 5,000 bacteria per minute. Also, nematodes can play an important role in the nitrogen cycle by way of nitrogen mineralization.[32]

One group of carnivorous fungi, the nematophagous fungi, are predators of soil nematodes. They set enticements for the nematodes in the form of lassos or adhesive structures.[47][48][49]

Society and culture

Nematode worms (C. elegans), part of an ongoing research project conducted on the Space Shuttle Columbia mission STS-107, survived the re-entry breakup. It is believed to be the first known life-form to survive a virtually unprotected atmospheric descent to Earth's surface.[50][51]

See also

Notes

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References

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Further reading

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External links

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  19. Cobb, N. A. (1919). The orders and classes of nemas. Contrib. Sci. Nematol. 8: 213–216, [1].
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