Acoela

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.[1][2]

Acoela
Many flatworm-like, orange individuals of the Waminoa acoel on a Plerogyra coral (whitish bubbles)
Scientific classification
Kingdom: Animalia
Phylum: Xenacoelomorpha
Subphylum: Acoelomorpha
Order: Acoela
Uljanin, 1870
Neochildia fusca, a member of Convolutidae (= Anaperidae)

The etymology of "acoel" is from the Ancient Greek words ἀ (a), the alpha privative, expressing negation or absence, and κοιλία (koilía), meaning "cavity".[3][4] 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 2 millimetres (0.079 in) in length, but some larger species, such as Symsagittifera roscoffensis, may reach up to 15 millimetres (0.59 in).[5]

They are found worldwide in marine and brackish waters, usually having a benthic lifestyle, although some species are epibionts.[6]

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.[7][8] 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 there is sometimes a short pharynx leading from the mouth to the vacuole. All other bilateral animals (apart from tapeworms) have a gut lined with epithelial cells. As a result, the acoels appear to be solid-bodied.

As the basal lineage of bilateral animals, the Acoela provide interesting insights into early animal evolution and development.[9][10] The most thoroughly studied animal in this group is the species Isodiametra pulchra. Acoela used to be classified in the phylum Platyhelminthes. However, Acoela was separated from this phylum after molecular analyses showed that it had diverged before the three main bilaterian clades had formed.

Taxonomy

The following sub-taxa are recognised in the order Acoela:[11]

References
  1. Cannon, Johanna Taylor; Vellutini, Bruno Cossermelli; Smith, Julian; Ronquist, Fredrik; Jondelius, Ulf; Hejnol, Andreas (2016). "Xenacoelomorpha is the sister group to Nephrozoa". Nature. 530 (7588): 89–93. doi:10.1038/nature16520. ISSN 0028-0836. PMID 26842059.
  2. Jimenez-Guri E, Paps J, Garcia-Fernandez J, Salo E (2006), "Hox and ParaHox genes in Nemertodermatida, a basal bilaterian clade", Int. J. Dev. Biol., 50 (8): 675–9, doi:10.1387/ijdb.062167ej, PMID 17051477.
  3. Bailly, Anatole (1981-01-01). Abrégé du dictionnaire grec français. Paris: Hachette. ISBN 2010035283. OCLC 461974285.
  4. Bailly, Anatole. "Greek-french dictionary online". www.tabularium.be. Retrieved 2020-03-02.
  5. Hooge, M. D.; Tyler, S. (2005). "New tools for resolving phylogenies: a systematic revision of the Convolutidae (Acoelomorpha, Acoela)". Journal of Zoological Systematics and Evolutionary Research. 43 (2): 100–113. doi:10.1111/j.1439-0469.2005.00301.x. ISSN 0947-5745.
  6. Ogunlana, M. V.; Hooge, M. D.; Tekle, Y. I.; Benayahu, Y.; Barneah, O.; Tyler, Seth (2005). "Waminoa brickneri n. sp. (Acoela: Acoelomorpha) associated with corals in the Red Sea". Zootaxa. 1008: 1–11. doi:10.11646/zootaxa.1008.1.1.
  7. Boll, Piter Kehoma; Rossi, Ilana; Amaral, Silvana Vargas do; Oliveira, Simone Machado de; Müller, Eliara Solange; Lemos, Virginia Silva; Leal-Zanchet, Ana Maria (2013). "Platyhelminthes ou apenas semelhantes a Platyhelminthes? Relações filogenéticas dos principais grupos de turbelários". Neotropical Biology and Conservation (in Portuguese). 8 (1). doi:10.4013/nbc.2013.81.06. ISSN 2236-3777.
  8. "The Platyhelminthes and the Acoela". Archived from the original on 2008-05-09. Retrieved 2009-03-21.
  9. Hejnol A, Martindale MQ (2008), "Acoel development supports a simple planula-like urbilaterian", Phil. Trans. R. Soc. B, 363 (1496): 1493–1501, doi:10.1098/rstb.2007.2239, PMC 2614228, PMID 18192185
  10. Hejnol A, Martindale MQ (2008), "Acoel development indicates the independent evolution of the bilaterian mouth and anus", Nature, 456 (7220): 382–386, doi:10.1038/nature07309, PMID 18806777
  11. "WoRMS - World Register of Marine Species - Acoela". www.marinespecies.org. Retrieved 2020-02-08.
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