Editing Gastrotrich

{{Short description|Phylum of microscopic animals}}
{{Good article}}
{{Automatic taxobox
| image = Gastrotrich.jpg
| image_caption = Darkfield photograph of a gastrotrich
| image2 = Thaumastoderma ramuliferum.jpg
| image2_caption = [[Scanning electron micrograph]] of ''[[Thaumastoderma]] ramuliferum''
| display_parents = 7
| taxon = Gastrotricha
| authority = Metschnikoff, 1865<ref name=WoRMS>{{cite WoRMS |author=Todaro, Antonio |year=2013 |title=Gastrotricha |id=2078 |access-date=2014-01-26 }}</ref>
| subdivision_ranks = Orders
| subdivision = * [[Chaetonotida]]
* [[Macrodasyida]]
}}

The '''gastrotrichs''' ([[phylum]] '''Gastrotricha'''), [[common name|commonly]] referred to as '''hairybellies''' or '''hairybacks''', are a group of [[Microscopic scale|microscopic]] (0.06–3.0&nbsp;mm), cylindrical, [[acoelomate]] [[animal]]s, and are widely distributed and abundant in [[freshwater]] and [[Marine (ocean)|marine]] environments. They are mostly [[Benthos|benthic]] and live within the [[periphyton]], the layer of tiny [[Organism|organisms]] and [[detritus]] that is found on the [[seabed]] and the beds of other [[Body of water|water bodies]]. The majority live on and between particles of [[sediment]] or on other submerged surfaces, but a few species are terrestrial and live on land in the film of water surrounding grains of [[soil]]. Gastrotrichs are divided into two [[Order (biology)|orders]], the [[Macrodasyida]] which are marine (except for two species), and the [[Chaetonotida]], some of which are marine and some freshwater. Nearly 800 species of gastrotrich have been described.

Gastrotrichs have a simple body plan with a head region, with a [[brain]] and [[sensory organs]], and a trunk with a simple gut and the [[reproductive organs]]. They have adhesive glands with which they can anchor themselves to the [[Substrate (biology)|substrate]] and [[cilia]] with which they move around. They feed on detritus, sucking up organic particles with their muscular [[pharynx]]. They are [[hermaphrodite]]s, the marine species producing eggs which develop directly into miniature [[Adult|adults]]. The freshwater species are [[Parthenogenesis|parthenogenetic]], producing unfertilised eggs, and at least one species is [[Viviparity|viviparous]]. Gastrotrichs mature with great rapidity and have lifespans of only a few days.

==Etymology and taxonomy==
The  name ''gastrotrich'' comes from [[Greek language|Greek]] γαστήρ, ''gaster'' 'stomach' and θρίξ, ''thrix'' 'hair'.<ref>{{cite web |url=http://www.thefreedictionary.com/gastrotrich |title=Gastrotrich |work=The Free Dictionary |access-date=2014-01-29}}</ref> The name was coined by the Russian zoologist [[Élie Metchnikoff]] in 1865.<ref name=WoRMS/> The common name ''hairyback'' apparently arises from a mistranslation of ''gastrotrich''.<ref>{{cite book |title=Hairybacks Gastrotricha: Bugs Britannica |last=Marren |first=Peter|author-link=Peter Marren |year=2010 |publisher=Random House |isbn=978-0-7011-8180-2 |page=27 |url=https://books.google.com/books?id=Ah62bUZLDOwC&pg=PA27 }}</ref>

The relationship of gastrotrichs to other phyla is unclear. [[Morphology (biology)|Morphology]] suggests that they are close to the [[Gnathostomulid]]a, the [[Rotifer]]a, or the [[Nematode|Nematoda]]. On the other hand, [[phylogenetics|genetic studies]] place them as close relatives of the [[flatworm|Platyhelminthes]], the [[Ecdysozoa]] or the [[Lophotrochozoa]].<ref name=Todaro/>  As of 2011, around 790 species have been described.<ref name=Zhang2011>{{cite journal| author=Zhang, Z.-Q.| title=Animal biodiversity: An introduction to higher-level classification and taxonomic richness | journal=Zootaxa| volume=3148| year=2011| pages=7–12| doi=10.11646/zootaxa.3148.1.3 | url=http://mapress.com/zootaxa/2011/f/zt03148p012.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://mapress.com/zootaxa/2011/f/zt03148p012.pdf |archive-date=2022-10-09 |url-status=live}}</ref> The phylum contains a single class, divided into two orders: the [[Macrodasyida]] and the [[Chaetonotida]].<ref name=Ruppert>{{cite book |title=Invertebrate Zoology, 7th edition |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard, S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 |pages=753–757 }}</ref> Edward Ruppert ''et al.'' report that the Macrodasyida are wholly marine,<ref name="Ruppert"/> but two rare and poorly known species, ''Marinellina flagellata'' and ''Redudasys fornerise'', are known from fresh water.<ref>{{Cite journal | doi = 10.1371/journal.pone.0031740| pmid = 22348127| title = Gastrotricha: A Marine Sister for a Freshwater Puzzle| journal = PLOS ONE| volume = 7| issue = 2| pages = e31740| date = 14 February 2012| last1 = Todaro | first1 = M. A. | last2 = Dal Zotto | first2 = M. | last3 = Jondelius | first3 = U. | last4 = Hochberg | first4 = R. | last5 = Hummon | first5 = W. D. | last6 = Kånneby | first6 = T. | last7 = Rocha | first7 = C. E. F. | pmc=3279426| bibcode = 2012PLoSO...731740T| doi-access = free}}</ref> The Chaetonotida comprises both marine and freshwater species.<ref name="Ruppert"/>

==Anatomy==
[[File:Lepidodermella squamatum.jpg|275px|left|thumb|''[[Lepidodermella squamata]]'' (Chaetonotida)]]
Gastrotrichs vary in size from about {{convert|0.06|to|3|mm|3|abbr=on}} in body length.<ref name=Todaro/> They are [[symmetry (biology)#Bilateral symmetry|bilaterally symmetrical]], with a transparent strap-shaped or [[bowling pin]]-shaped body, arched dorsally and flattened ventrally. The [[Anatomical terms of location#Anterior and posterior|anterior]] end is not clearly defined as a head but contains the sense organs, brain and pharynx. [[Cilia]] are found around the mouth and on the ventral surface of the head and body. The trunk contains the gut and the reproductive organs. At the [[Anatomical terms of location#Anterior and posterior|posterior]] end of the body are two projections with cement glands that serve in adhesion. This is a double-gland system where one gland secretes the glue and another secretes a de-adhesive agent to sever the connection. In the Macrodasyida, there are additional adhesive glands at the anterior end and on the sides of the body.<ref name=Ruppert/>

[[File:Diplodasys rothei legends.jpg|alt=anatomic diagram|thumb|''[[Diplodasys rothei]]'']]

The body wall consists of a [[cuticle]], an [[Epidermis (zoology)|epidermis]] and longitudinal and circular bands of muscle fibres. In some [[Primitive (phylogenetics)|primitive]] species, each epidermal cell has a single cilium, a feature shared only by the [[gnathostomula]]ns. The whole ventral surface of the animal may be ciliated or the cilia may be arranged in rows, patches or transverse bands. The cuticle is locally thickened in some gastrotrichs and forms scales, hooks and spines. There is no [[coelom]] (body cavity) and the interior of the animal is filled with poorly differentiated [[connective tissue]]. In the macrodasyidans, Y-shaped cells, each containing a [[vacuole]], surround the gut and may function as a [[hydrostatic skeleton]].<ref name=Ruppert/>

The mouth is at the anterior end and opens into an elongated muscular [[pharynx]] with a triangular or Y-shaped [[lumen (anatomy)|lumen]], lined by [[myoepithelium|myoepithelial cells]]. The pharynx opens into a cylindrical intestine, which is lined with glandular and digestive cells. The [[anus]] is located on the ventral surface close to the posterior of the body. In some species, there are pores in the pharynx opening to the ventral surface; these contain valves and may allow [[egestion]] of any excess water swallowed while feeding.<ref name=Ruppert/>

In the chaetonotidans, the excretory system consists of a single pair of [[protonephridia]], which open through separate pores on the lateral underside of the animal, usually in the midsection of the body. In the macrodasyidans, there are several pairs of these opening along the side of the body. [[Nitrogenous waste]] is probably excreted through the body wall, as part of respiration, and the protonephridia are believed to function mainly in [[osmoregulation]].<ref name=Ruppert/> Unusually, the protonephridia do not take the form of [[flame cell]]s, but, instead, the excretory cells consist of a skirt surrounding a series of [[cytoplasm]]ic rods that in turn enclose a central [[flagellum]]. These cells, termed ''cyrtocytes'', connect to a single outlet cell which passes the excreted material into the protonephridial duct.<ref name=IZ>{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |pages= 263–272|isbn= 0-03-056747-5}}</ref>

As is typical for such small animals, there are no respiratory or circulatory organs. The nervous system is relatively simple. The brain consists of two [[ganglion|ganglia]], one on either side of the pharynx, connected by a [[commissure]]. From these lead a pair of nerve cords which run along either side of the body beside the longitudinal muscle bands. The primary sensory organs are the bristles and ciliated tufts of the body surface which function as [[mechanoreceptor]]s. There are also ciliated pits on the head, simple ciliary [[Simple eye in invertebrates|photoreceptors]] and fleshy appendages which act as [[chemoreceptor]]s.<ref name=Ruppert/>

==Distribution and habitat==
[[File:Gatrotricha (South Africa).jpg|thumb|right|A & B = Macrodasyida<br/>C, D, E & F = Chaetonotida]]
Gastrotrichs are [[Cosmopolitan distribution|cosmopolitan]] in distribution. They inhabit the interstitial spaces between particles in marine and freshwater environments, the surfaces of aquatic plants and other submerged objects and the surface film of water surrounding soil particles on land.<ref name=Todaro/> They are also found in stagnant pools and anaerobic mud, where they thrive even in the presence of [[hydrogen sulfide]]. When pools dry up they can survive periods of desiccation as eggs, and some species are capable of forming [[Microbial cyst|cysts]] in harsh conditions.<ref>{{cite book |title=The Ecology of Soil Decomposition: Gastrotrichs |last=Adl |first=Sina M. |year=2003 |publisher=CABI |isbn=978-0-85199-661-5 |page=52 |url=https://books.google.com/books?id=vePuN8aYhyEC&q=gastrotrich+terrestrial+soil&pg=PA52 }}</ref>  In marine sediments they have been known to reach 364 individuals per {{convert|10|cm2|1|abbr=on}} making them the third most common invertebrate in the sediment after [[nematode]]s and [[Harpacticoida|harpacticoid copepods]]. In freshwater they may reach a density of 158 individuals per {{convert|10|cm2|1|abbr=on}} and are the fifth most abundant group of invertebrates in the sediment.<ref name=Todaro/>

==Behaviour and ecology==
In marine and freshwater environments, gastrotrichs form part of the [[benthos|benthic community]]. They are [[detritivore]]s and are microphagous: they feed by sucking small dead or living organic materials, [[diatoms]], [[bacteria]] and small protozoa into their mouths by the muscular action of the pharynx. They are themselves eaten by [[turbellaria]]ns and other small [[macrofauna]].<ref name=Todaro>{{cite web |url=http://www.gastrotricha.unimore.it/overview.htm |title=Gastrotricha |author=Todaro, M. A. |date=2014-01-03 |access-date=2014-01-23}}</ref>

Like many microscopic animals, gastrotrich locomotion is primarily powered by [[hydrostatics]], but movement occurs through different methods in different members of the group. Chaetonotids only have adhesive glands at the back and, in them, locomotion typically proceeds in a smooth gliding manner; the whole body is propelled forward by the rhythmic action of the cilia on the ventral surface. In the [[pelagic]] chaetonotid genus ''[[Stylochaeta]]'', however, movement proceeds in jerks as the long, muscle-activated spines are forced rhythmically towards the side of the body. By contrast, with chaetonotids, macrodasyidans typically have multiple adhesive glands and move forward with a creeping action similar to that of a [[Geometer moth#Caterpillars|"looper" caterpillar]]. In response to a threat, the head and trunk can be rapidly pulled backwards, or the creeping movement can be reversed. Muscular action is important when the animal turns sideways and during copulation, when two individuals twine around each other.<ref name=Ruppert/>

==Reproduction and lifespan==
[[File:Thaumastodermatinae.jpg|thumb|left|''Ptychostomella'' sp., Macrodasyida]]
Gastrotrich reproduction and reproductive behaviour has been little studied. That of [[Macrodasyida|macrodasiyds]] probably most represents that of the ancestral lineage and these more primitive gastrotrichs are simultaneous [[hermaphrodite]]s, possessing both male and female sex organs. There is generally a single pair of [[gonad]]s, the anterior portion of which contains [[sperm]]-producing cells and the posterior portion producing [[ovum|ova]]. The sperm is sometimes packaged in [[spermatophore]]s and is released through male [[gonopore]]s that open, often temporarily, on the underside of the animal, roughly two-thirds of the way along the body. A [[copulatory organ]] on the tail collects the sperm and transfers it to the partner's [[seminal receptacle]] through the female gonopore. Details of the process and the behaviour involved vary with the species, and there is a range of different accessory reproductive organs. During copulation, the "male" individual uses his copulatory organ to transfer sperm to his partner's gonopore and fertilisation is internal. The fertilised eggs are released by rupture of the body wall which afterwards repairs itself. As is the case in most [[protostome]]s, development of the embryo is [[Cleavage (embryo)#Determinate|determinate]], with each cell destined to become a specific part of the animal's body.<ref name=Ruppert/> At least one species of gastrotrich, ''[[Urodasys viviparus]]'', is [[Viviparity|viviparous]].<ref>{{cite journal |author1=Elena, Fregni |author2=Faienza, Maria Grazia |author3=De Zio Grimaldi, Susanna |author4=Tongiorgi, Paolo |author5=Balsamo, Maria |year=1999 |title=Marine gastrotrichs from the Tremiti archipelago in the southern Adriatic Sea, with the description of two new species of ''Urodasys'' |journal=Italian Journal of Zoology |volume= 66|issue=2 |pages=183–194 |doi=10.1080/11250009909356254 |doi-access=free }}</ref>

Many species of chaetotonid gastrotrichs reproduce entirely by [[parthenogenesis]]. In these species, the male portions of the reproductive system are degenerate and non-functional, or, in many cases, entirely absent. Though the eggs have a diameter of less than 50 [[μm]], they are still very large in comparison with the animals' size. Some species are capable of laying eggs that remain [[Dormancy|dormant]] during times of [[desiccation]] or low temperatures; these species, however, are also able to produce regular eggs, which hatch in one to four days, when environmental conditions are more favourable. The eggs of all gastrotrichs undergo [[direct development]] and hatch into miniature versions of the adult. The young typically reach sexual maturity in about three days. In the laboratory, ''[[Lepidodermella squamatum]]'' has lived for up to forty days, producing four or five eggs during the first ten days of life.<ref name=Ruppert/>

Gastrotrichs demonstrate [[eutely]], each species having an invariant genetically fixed number of cells as adults. Cell division ceases at the end of embryonic development and further growth is solely due to cell enlargement.<ref name=Ruppert/>

==Classification==
{{See also|List of bilaterial animal orders}}
Gastrotricha is divided into two orders and a number of families:<ref name=WoRMS/><ref name=Todaro/>

{{col-begin}}
{{col-2}}
'''Order [[Macrodasyida]]'''<small> Remane, 1925 [Rao and Clausen, 1970]</small>
*Family [[Cephalodasyidae]]<small> Hummon & Todaro, 2010</small>
::*Genus ''[[Cephalodasys]]''<small> Remane, 1926</small>
::*Genus ''[[Dolichodasys]]''<small> Gagne, 1977</small>
::*Genus ''[[Megadasys]]''<small> Schmidt, 1974</small>
::*Genus ''[[Mesodasys]]''<small> Remane, 1951</small>
::*Genus ''[[Paradasys]]''<small> Remane, 1934</small>
::*Genus ''[[Pleurodasys]]''<small> Remane, 1927</small>
*Family [[Dactylopodolidae]]<small> Strand, 1929</small>
::*Genus ''[[Dactylopodola]]''<small> Strand, 1929</small>
::*Genus ''[[Dendrodasys]]''<small> Wilke, 1954</small>
::*Genus ''[[Dendropodola]]''<small> Hummon, Todaro & Tongiorgi, 1992</small>
*Family [[Lepidodasyidae]]<small> Remane, 1927</small>
::*Genus ''[[Lepidodasys]]''<small> Remane, 1926</small>
*Family [[Macrodasyidae]]<small> Remane, 1926</small>
::*Genus ''[[Macrodasys]]''<small> Remane, 1924</small>
::*Genus ''[[Urodasys]]''<small> Remane, 1926</small>
*Family [[Planodasyidae]]<small> Rao & Clausen, 1970</small>
::*Genus ''[[Crasiella]]''<small> Clausen, 1968</small>
::*Genus ''[[Planodasys]]''<small> Rao & Clausen, 1970</small>
*Family [[Redudasyidae]]<small> Todaro, Dal Zotto, Jondelius, Hochberg et al., 2012</small>
::*Genus ''[[Anandrodasys]]''<small> Todaro, Dal Zotto, Jondelius, Hochberg et al., 2012</small>
::*Genus ''[[Redudasys]]''<small> Kisielewski, 1987</small>
*Family [[Thaumastodermatidae]]<small> Remane, 1927</small>
**Subfamily [[Diplodasyinae]]<small> Ruppert, 1978</small>
***Genus ''[[Acanthodasys]]''<small> Remane, 1927</small>
***Genus ''[[Diplodasys]]''<small> Remane, 1927</small>
**Subfamily [[Thaumastodermatinae]]<small> Remane, 1927</small>
***Genus ''[[Hemidasys]]''<small> Claparède, 1867</small>
***Genus ''[[Oregodasys]]''<small> Hummon, 2008 =(Platydasys Remane, 1927)</small>
***Genus ''[[Pseudostomella]]''<small> Swedmark, 1956</small>
***Genus ''[[Ptychostomella]]''<small> Remane, 1926</small>
***Genus ''[[Tetranchyroderma]]''<small> Remane, 1926</small>
***Genus ''[[Thaumastoderma]]''<small> Remane, 1926</small>
*Family [[Turbanellidae]]<small> Remane, 1927</small>
::*Genus ''[[Desmodasys]]''<small> Clausen, 1965</small>
::*Genus ''[[Dinodasys]]''<small> Remane, 1927</small>
::*Genus ''[[Paraturbanella]]''<small> Remane, 1927</small>
::*Genus ''[[Prostobuccantia]]''<small> Evans & Hummon, 1991</small>
::*Genus ''[[Pseudoturbanella]]''<small> d'Hondt, 1968</small>
::*Genus ''[[Turbanella]]''<small> Schultze, 1853</small>
*Family [[Xenodasyidae]]<small> Todaro, Guidi, Leasi & Tongiorgi, 2006</small>
::*Genus ''[[Chordodasiopsis]]''<small> Todaro, Guidi, Leasi & Tongiorgi, 2006</small>
::*Genus ''[[Xenodasys]]''<small> Swedmark, 1967</small>

*''[[Incertae sedis]]''
::*Genus ''[[Marinellina]]''<small> Ruttner-Kolisko, 1955</small>

{{col-2}}
'''Order [[Chaetonotida]]'''<small> Remane, 1925 [Rao and Clausen, 1970]</small>

Suborder [[Multitubulatina]]<small> d'Hondt, 1971</small>
*Family [[Neodasyidae]]<small> Remane, 1929</small>
::*Genus ''[[Neodasys]]''<small> Remane, 1927</small>

Suborder [[Paucitubulatina]]<small> d'Hondt, 1971</small>
*Family [[Chaetonotidae]]<small> Gosse, 1864</small>
**Subfamily [[Chaetonotinae]] <small>Kisielewski, 1991</small>
***Genus ''[[Arenotus]]''<small> Kisielewski, 1987</small>
***Genus ''[[Aspidiophorus]]''<small> Voigt, 1903</small>
***Genus ''[[Caudichthydium]]''<small> Schwank, 1990</small>
***Genus ''[[Chaetonotus]]''<small> Ehrenberg, 1830</small>
***Genus ''[[Fluxiderma]]''<small> d'Hondt, 1974</small>
***Genus ''[[Ichthydium]]''<small> Ehrenberg, 1830</small>
***Genus ''[[Halichaetonotus]]''<small> Remane, 1936</small>
***Genus ''[[Heterolepidoderma]]''<small> Remane, 1927</small>
***Genus ''[[Lepidochaetus]]''<small> Kisielewski 1991</small>
***Genus ''[[Lepidodermella]]''<small> Blake, 1933</small>
***Genus ''[[Polymerurus]]''<small> Remane, 1927</small>
***Genus ''[[Rhomballichthys]]''<small> Schwank, 1990</small>
**Subfamily [[Undulinae]]<small> Kisielewski 1991</small>
***Genus ''[[Undula]]''<small> Kisielewski 1991</small>
*Family [[Dasydytidae]]<small> Daday, 1905</small>
::*Genus ''[[Anacanthoderma]]''<small> Marcolongo, 1910</small>
::*Genus ''[[Chitonodytes]]''<small> Remane, 1936</small>
::*Genus ''[[Dasydytes]]''<small> Gosse, 1851</small>
::*Genus ''[[Haltidytes]]''<small> Remane 1936</small>
::*Genus ''[[Ornamentula]]''<small> Kisielewski 1991</small>
::*Genus ''[[Setopus]]''<small> Grünspan, 1908</small>
::*Genus ''[[Stylochaeta]]''<small> Hlava, 1905</small>

*Family [[Dichaeturidae]]<small> Remane, 1927</small>
::*Genus ''[[Dichaetura]]''<small> Lauterborn, 1913</small>

*Family [[Muselliferidae]]<small> Leasi & Todaro, 2008</small>
::*Genus ''[[Diuronotus]]''<small> Todaro, Kristensen & Balsamo, 2005</small>
::*Genus ''[[Musellifer]]''<small> Hummon, 1969</small>
*Family [[Neogosseidae]]<small> Remane, 1927</small>
::*Genus ''[[Neogossea]]''<small> Remane, 1927</small>
::*Genus ''[[Kijanebalola]]''<small> Beauchamp, 1932</small>
*Family [[Proichthydiidae]]<small> Remane, 1927</small>
::*Genus ''[[Proichthydium]]''<small> Cordero, 1918</small>
::*Genus ''[[Proichthydioides]]''<small> Sudzuki, 1971</small>
*Family [[Xenotrichulidae]]<small> Remane, 1927</small>
**Subfamily [[Draculiciterinae]]<small> Ruppert, 1979</small>
***Genus ''[[Draculiciteria]]''<small> Hummon, 1974</small>
**Subfamily [[Xenotrichulinae]]<small> Remane, 1927</small>
***Genus ''[[Heteroxenotrichula]]''<small> Wilke, 1954</small>
***Genus ''[[Xenotrichula]]''<small> Remane, 1927</small>
{{col-end}}

==References==
{{Reflist}}

==External links==
{{Commons category|Gastrotricha}}
{{Wikispecies|Gastrotricha}}
{{EB1911 poster|Gastrotricha}}

*[https://web.archive.org/web/20080916172400/http://www.eol.org/taxa/16100419 Gastrotrichs] in the [[Encyclopedia of Life]]

{{Animalia}}
{{Life on Earth}}
{{Taxonbar|from=Q190115}}
{{Authority control}}

[[Category:Gastrotricha| ]]
<!-- [[Category:Animal phyla]] moved to "Gastrotricha" redirect -->