Acoela

{{Short description|Order of flatworm-like bilaterian animals}} {{Automatic taxobox | fossil_range = {{Fossil range|Middle Triassic|Recent}}<ref name="Knaust 2021">{{Cite journal |last=Knaust |first=Dirk |date=2021-10-07 |title=A microbialite with its entombed benthic community from the Middle Triassic (Anisian-Ladinian) Muschelkalk Group of Germany |url=http://www.schweizerbart.de/papers/pala/detail/320/99661/A_microbialite_with_its_entombed_benthic_community?af=crossref |journal=Palaeontographica Abteilung A |language=en |volume=320 |issue=1-3 |pages=1–63 |doi=10.1127/pala/2021/0114 |issn=0375-0442|url-access=subscription }}</ref> | image = Waminoa on Plerogyra.jpg | image_caption = Many flatworm-like, orange individuals of the ''Waminoa'' acoel on a ''Plerogyra'' coral (whitish bubbles) | taxon = Acoela | authority = Uljanin, 1870 }}

[[Image:Neochildia fusca.jpg|thumb|right|''Neochildia fusca'', a member of Convolutidae (= Anaperidae)]]

'''Acoela''', or the '''acoels''', is an order of small and simple invertebrates in the subphylum Acoelomorpha of phylum Xenacoelomorpha, a deep-branching bilaterian group of animals, which resemble flatworms. Historically, they were treated as an order of turbellarian flatworms.<ref name="CannonVellutini2016">{{cite journal|last1=Cannon|first1=Johanna Taylor|last2=Vellutini|first2=Bruno Cossermelli|last3=Smith|first3=Julian|last4=Ronquist|first4=Fredrik|last5=Jondelius|first5=Ulf|last6=Hejnol|first6=Andreas|title=Xenacoelomorpha is the sister group to Nephrozoa|journal=Nature|volume=530|issue=7588|year=2016|pages=89–93|issn=0028-0836|doi=10.1038/nature16520|pmid=26842059|bibcode=2016Natur.530...89C|s2cid=205247296|url=http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-1844}}</ref><ref name="pmid17051477">{{Citation |vauthors=Jimenez-Guri E, Paps J, Garcia-Fernandez J, Salo E |title=Hox and ParaHox genes in Nemertodermatida, a basal bilaterian clade |journal=Int. J. Dev. Biol. |volume=50 |issue=8 |pages=675–9 |year=2006 |pmid=17051477 |doi=10.1387/ijdb.062167ej |postscript=.|doi-access=free }}</ref> About 400 species are known, but probably many more are not yet described.<ref>[https://bioone.org/journals/zoological-science/volume-39/issue-1/zs210058/A-New-Species-of-Acoela-Possessing-a-Middorsal-Appendage-with/10.2108/zs210058.full A New Species of Acoela Possessing a Middorsal Appendage with a Possible Sensory Function]</ref>

The etymology of "acoel" is from the Ancient Greek words {{wikt-lang|grc|ἀ}} ({{grc-transl|ἀ}}), the ''alpha privative'', expressing negation or absence, and {{wikt-lang|grc|κοιλία}} ({{grc-transl|κοιλία}}), meaning "cavity".<ref>{{Cite book | title=Abrégé du dictionnaire grec français | last=Bailly | first=Anatole | date=1981-01-01 | publisher=Hachette | isbn=2-01-003528-3 | location=Paris | oclc=461974285}}</ref><ref>{{Cite web | url=http://www.tabularium.be/bailly/ | title=Greek-french dictionary online | last=Bailly | first=Anatole | website=www.tabularium.be | access-date=2020-03-02 | archive-date=2022-03-18 | archive-url=https://web.archive.org/web/20220318000653/http://www.tabularium.be/bailly/ }}</ref> This refers to the fact that acoels have a structure lacking a fluid-filled body cavity.

== Description == Acoels are very small, flattened worms, usually under {{convert|2|mm|in}} in length, but some larger species, such as ''Symsagittifera roscoffensis'', may reach up to {{convert|15|mm|in}}.<ref name="HoogeTyler2005">{{cite journal|last1=Hooge|first1=M. D.|last2=Tyler|first2=S.|title=New tools for resolving phylogenies: a systematic revision of the Convolutidae (Acoelomorpha, Acoela)|journal=Journal of Zoological Systematics and Evolutionary Research|volume=43|issue=2|year=2005|pages=100–113|issn=0947-5745|doi=10.1111/j.1439-0469.2005.00301.x}}</ref> They are bilaterally symmetric and microscopic.<ref>{{Cite journal|last1=Achatz|first1=Johannes G.|last2=Chiodin|first2=Marta|last3=Salvenmoser|first3=Willi|last4=Tyler|first4=Seth|last5=Martinez|first5=Pedro|date=2013|title=The Acoela: on their kind and kinships, especially with nemertodermatids and xenoturbellids (Bilateria incertae sedis)|journal=Organisms, Diversity & Evolution|volume=13|issue=2|pages=267–286|doi=10.1007/s13127-012-0112-4|issn=1439-6092|pmc=3789126|pmid=24098090}}</ref>

They are found worldwide in marine and brackish waters, usually having a benthic lifestyle, although some species are epibionts.<ref name="Ogunlana2005">{{cite journal|last1=Ogunlana|first1=M. V.|last2=Hooge|first2=M. D.|last3=Tekle|first3=Y. I.|last4=Benayahu|first4=Y.|last5=Barneah|first5=O.|last6=Tyler|first6=Seth|title=''Waminoa brickneri'' n. sp. (Acoela: Acoelomorpha) associated with corals in the Red Sea|journal=Zootaxa|volume=1008|year=2005|pages=1–11|doi=10.11646/zootaxa.1008.1.1|doi-access=free}}</ref> Two species, ''Limnoposthia polonica'' and ''Oligochoerus limnophilus'', are known to live in freshwater.<ref>{{cite book | url=https://books.google.com/books?id=YHetDwAAQBAJ&dq=limonoposthia+polonica+oligochoerus+limnophilus&pg=PA92 | title=The Invertebrate Tree of Life | isbn=978-0-691-17025-1 | last1=Giribet | first1=Gonzalo | last2=Edgecombe | first2=Gregory D. | date=3 March 2020 | publisher=Princeton University Press }}</ref>

Species in the family Convolutidae often form endosymbiotic relationship with microalgae. In one of the genera, ''Waminoa'', the algae are transmitted vertically from parents to offspring. In addition to Convolutidae, there appears to be a potential new and yet unnamed family of acoels that also live in relationships with microalgal endosymbionts.<ref>{{Cite journal |last1=Riewluang |first1=Siratee |last2=Wakeman |first2=Kevin C. |date=2023 |title=Biodiversity of symbiotic microalgae associated with meiofaunal marine acoels in Southern Japan |journal=PeerJ |volume=11 |article-number=e16078 |doi=10.7717/peerj.16078 |doi-access=free |pmid=37814628|pmc=10560497 }}</ref>

Members of the class Acoela lack a conventional gut, so that the mouth opens directly into the mesenchyme, i.e., the layer of tissue that fills the body.<ref name="BollRossi2013">{{cite journal|last1=Boll|first1=Piter Kehoma|last2=Rossi|first2=Ilana|last3=Amaral|first3=Silvana Vargas do|last4=Oliveira|first4=Simone Machado de|last5=Müller|first5=Eliara Solange|last6=Lemos|first6=Virginia Silva|last7=Leal-Zanchet|first7=Ana Maria|title=Platyhelminthes ou apenas semelhantes a Platyhelminthes? Relações filogenéticas dos principais grupos de turbelários|journal=Neotropical Biology and Conservation|volume=8|issue=1|year=2013|issn=2236-3777|doi=10.4013/nbc.2013.81.06|language=pt|doi-access=free}}</ref><ref name="The Platyhelminthes and the Acoela">{{cite web|url=http://devbio.umesci.maine.edu/styler/globalworming/platyhelm2003.html |title=The Platyhelminthes and the Acoela |access-date=2009-03-21 |archive-url=https://web.archive.org/web/20080509144930/http://devbio.umesci.maine.edu/styler/globalworming/platyhelm2003.html |archive-date=2008-05-09 }}</ref> Digestion is accomplished by means of a syncytium that forms a vacuole around ingested food. There are no epithelial cells lining the digestive vacuole, but in the families Diopisthoporidae, Hallangiidae, Hofsteniidae, and Solenofilomorphidae, and the genera Oligochoerus (Convolutidae) and Proporus (Proporidae), there is a short pharynx leading from the mouth to the vacuole.<ref>[The Acoela: on their kind and kinships, especially with nemertodermatids and xenoturbellids (Bilateria incertae sedis) The Acoela: on their kind and kinships, especially with nemertodermatids and xenoturbellids (Bilateria incertae sedis)]</ref> All other bilateral animals (apart from tapeworms) have a gut lined with epithelial cells. As a result, the acoels appear to be solid-bodied.

The Acoela provide interesting insights into early animal evolution and development.<ref>{{citation |title=Acoel development supports a simple planula-like urbilaterian |vauthors=Hejnol A, Martindale MQ |journal=Phil. Trans. R. Soc. B |volume=363 |issue=1496 |pages=1493–1501 |year=2008 |doi=10.1098/rstb.2007.2239 |pmc=2614228 |pmid=18192185}}</ref><ref>{{citation |title=Acoel development indicates the independent evolution of the bilaterian mouth and anus |vauthors=Hejnol A, Martindale MQ |journal=Nature |volume=456 |issue=7220 |pages=382–386 |year=2008 |bibcode=2008Natur.456..382H |doi=10.1038/nature07309 |pmid=18806777 |s2cid=4403355}}</ref> The most thoroughly studied animal in this group is the species ''Isodiametra pulchra''.

==Taxonomy== Acoela used to be classified in the phylum Platyhelminthes.{{Citation needed|date=January 2026}} However, Acoela was separated from this phylum after molecular analyses showed that it had diverged before the three main bilaterian clades had formed, making flatworms, as traditionally understood, an evolutionary grade rather than a monophyletic clade.{{Citation needed|date=January 2026}}

Acoela is considered a member of the phylum Xenacoelomorpha, within which it is sister to the Nemertodermatida.<ref name=":0">{{Cite journal |last=Redmond |first=Anthony K. |date=September 2024 |title=Acoelomorph flatworm monophyly is a long-branch attraction artefact obscuring a clade of Acoela and Xenoturbellida |url=https://royalsocietypublishing.org/doi/10.1098/rspb.2024.0329 |journal=Proceedings of the Royal Society B: Biological Sciences |language=en |volume=291 |issue=2031 |doi=10.1098/rspb.2024.0329 |issn=0962-8452 |pmc=11407873 |pmid=39288803}}</ref> However, this apparent relationship may result erroneously from long branch attraction, with Acoela instead forming a clade with ''Xenoturbella''.<ref name=":0" />

The following sub-taxa are recognised in the order Acoela:<ref>{{Cite web|url=http://www.marinespecies.org/aphia.php?p=taxdetails&id=2847|title=WoRMS - World Register of Marine Species - Acoela|website=www.marinespecies.org|access-date=2020-02-08}}</ref>

* Family Actinoposthiidae Hooge, 2001 * Family Antigonariidae Dörjes, 1968 * Family Antroposthiidae Faubel, 1976 * Family Diopisthoporidae Westblad, 1940 * Family Nadinidae Dörjes, 1968 * Family Paratomellidae Dörjes, 1966 * Family Taurididae Kostenko, 1989 * Suborder Bursalia Jondelius ''et al''., 2011 ** Infraorder Crucimusculata Jondelius ''et al''., 2011 *** Family Dakuidae Hooge, 2003 *** Family Isodiametridae Hooge & Tyler, 2005 *** Family Otocelididae Westblad, 1948 *** Family Proporidae Graff, 1882 *** Superfamily Aberrantospermata Jondelius ''et al''., 2011 **** Family Convolutidae Graff, 1905 **** Family Mecynostomidae Dörjes, 1968 ** Infraorder Prosopharyngida Jondelius ''et al''., 2011 *** Family Hallangiidae Westblad, 1946 *** Family Hofsteniidae Bock, 1923 *** Family Solenofilomorphidae Dörjes, 1968

==References== {{Commons category}} {{Wikispecies|Acoela}} {{Reflist}}

Category:Acoela