Acoelomorpha

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Acoelomorpha
Waminoa on Plerogyra.jpg
The acoelomorph Waminoa sp. (orange structure) on the Plerogyra sp. coral (whitish bubbles).
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Xenacoelomorpha
Subphylum: Acoelomorpha
Ehlers, 1985
Classes

Acoelomorpha is a subphylum of very simple and small soft-bodied animals with planula-like features which live in marine or brackish waters. They usually live between grains of sediment, swimming as plankton, or crawling on other organisms, such as algae and corals. [1] With the exception of two acoel freshwater species, all known acoelomorphs are marine. [2]

Contents

Systematics

Etymology

The term "acoelomorph" derives from the Ancient Greek words ἀ (a), the alpha privative , expressing negation or absence, κοιλία (koilía), meaning "cavity", and μορφή (morphḗ), meaning "form". [3] [4] This refers to the fact that acoelomorphs have a structure lacking a fluid-filled body cavity.

Classification

Various members of the Acoela class. Acoela from Indian Ocean.gif
Various members of the Acoela class.
Various members of the Nemertodermatida class. Nemertodermatida species.png
Various members of the Nemertodermatida class.

The subphylum Acoelomorpha is divided into two classes. There are at least 408 described species, with a majority of these falling within the Crucimusculata infraorder in Acoela.

Phylogeny

The soft bodies of acoelomorphs and the lack of some of the key bilaterian traits make them difficult to classify. [8] [9] Traditionally, based on phenotypic features, acoelomorphs were considered to belong to the phylum Platyhelminthes, which was long seen as the sister group to all other bilaterian phyla. [10] However, a series of molecular phylogenetics studies at the hinge between the 20th and 21st centuries demonstrated that they are fast-evolving organisms not closely related to platyhelminthes, [11] [12] [13] [14] therefore involving the polyphyly of flatworms. [15] [16] [17] [18]

Actually, Acoelomorpha appeared to constitute a separate, deep-branching phylum, kingpin of bilaterian evolution. [19] Yet their evolutionary affinities remain enigmatic as they might be the sister-group either to all other bilateral animals [15] [16] [17] or to all deuterostomes. [18] Resolving this debate would indicate whether acoelomorphs are simple or simplified. If they are the sister group to Bilateria, it would point to a simple body plan for the first bilaterian. Alternatively, if acoelomorphs are related to deuterostomes, this would imply that their organisation is the result of secondary simplification. [20]

Xenoturbella, the sister group to acoelomorphs Xenoturbella japonica.jpg
Xenoturbella , the sister group to acoelomorphs

In addition, comparative analyses of morphological, developmental, and molecular characters raised two points.

Anatomy

Symsagittifera roscoffensis Symsagittifera roscoffensis(Jersey).jpg
Symsagittifera roscoffensis

Acoelomorphs resemble flatworms in many respects, but have a simpler anatomy, not even having a gut. Like flatworms, they have no circulatory or respiratory systems, but they also lack an excretory system. They lack body cavities (acoelomate structure), a hindgut or an anus. [1]

The epidermal cells of acoelomorphs are unable to proliferate, a feature that is only shared with rhabditophoran flatworms and was for some time considered a strong evidence for the position of Acoelomorpha within Platyhelminthes. In both groups, the epidermis is renewed from mesodermal stem cells. [29]

The nervous system of acoelomorphs is formed by a set of longitudinal nerve bundles beneath the ciliated epidermis. Close to the anterior end, these bundles are united by a ring commissure, but do not form a true brain, although it is hypothesized that such organization was the precursor of the cephalization of the nerve system in more derived bilaterians. [30] After decapitation, such a "brain" (rather, a cerebroid ganglion) regenerates in a few weeks. [31]

The sensory organs include a statocyst – which presumably helps them orient to gravity –, and, in some cases, ancestral pigment-spot ocelli capable of detecting light. [32]

Acoelomorphs are simultaneous hermaphrodites, but have no gonads and no ducts associated with the female reproductive system. Instead, gametes are produced from the mesenchymal cells that fill the body between the epidermis and the digestive vacuole. [32]

Related Research Articles

<span class="mw-page-title-main">Flatworm</span> Phylum of soft-bodied invertebrates

The flatworms, flat worms, Platyhelminthes, or platyhelminths are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Being acoelomates, and having no specialised circulatory and respiratory organs, they are restricted to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both ingestion and egestion ; as a result, the food can not be processed continuously.

<span class="mw-page-title-main">Bilateria</span> Animals with embryonic bilateral symmetry

Bilateria is a large clade or infrakingdom of animals called bilaterians, characterized by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front and a rear end, as well as a left–right–symmetrical belly (ventral) and back (dorsal) surface. Nearly all bilaterians maintain a bilaterally symmetrical body as adults; the most notable exception is the echinoderms, which extend to pentaradial symmetry as adults, but are only bilaterally symmetrical as an embryo. Cephalization is also a characteristic feature among most bilaterians, where the special sense organs and central nerve ganglia become concentrated at the front/rostral end.

<span class="mw-page-title-main">Turbellaria</span> Class of flatworms

The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes (flatworms), and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from 1 mm (0.039 in) to large freshwater forms more than 500 mm (20 in) long or terrestrial species like Bipalium kewense which can reach 600 mm (24 in) in length. All the larger forms are flat with ribbon-like or leaf-like shapes, since their lack of respiratory and circulatory systems means that they have to rely on diffusion for internal transport of metabolites. However, many of the smaller forms are round in cross section. Most are predators, and all live in water or in moist terrestrial environments. Most forms reproduce sexually and with few exceptions all are simultaneous hermaphrodites.

<span class="mw-page-title-main">Cephalization</span> Evolutionary trend of a head region developing

Cephalization is an evolutionary trend in animals that, over many generations, the special sense organs and nerve ganglia become concentrated towards the rostral end of the body where the mouth is located, often producing an enlarged head. This is associated with the animal's movement direction and bilateral symmetry, and cephalization of the nervous system led to the formation of a functional centralized brain in three groups of bilaterian animals, namely the arthropods, cephalopod molluscs, and vertebrates (craniates).

<i>Xenoturbella</i> Genus of bilaterians with a simple body plan

Xenoturbella is a genus of very simple bilaterians up to a few centimeters long. It contains a small number of marine benthic worm-like species.

<i>Symsagittifera roscoffensis</i> Species of flatworm-like animal

Symsagittifera roscoffensis, also called the Roscoff worm, the mint-sauce worm, or the shilly-shally worm, is a marine worm belonging to the phylum Xenacoelomorpha. The origin and nature of the green color of this worm stimulated the intrigued zoologists in the 1870's. It was discovered that the coloring resulted from the symbiosis between the animal and a green micro-algae, the species Tetraselmis convolutae, hosted under its epidermis. It is the photosynthetic activity of the micro-algae in hospite that provides the essential nutrients for the worm. This partnership is called photosymbiosis, from "photo", "light", and symbiosis "who lives with". These photosynthetic marine animals live in colonies on the tidal zone.

<span class="mw-page-title-main">Acoela</span> Order of flatworm-like bilaterian animals

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. About 400 species are known, but probably many more not yet described.

The urbilaterian is the hypothetical last common ancestor of the bilaterian clade, i.e., all animals having a bilateral symmetry.

<span class="mw-page-title-main">Deuterostome</span> Superphylum of bilateral animals

Deuterostomes are bilaterian animals of the superphylum Deuterostomia, typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia is further divided into 4 phyla: Chordata, Echinodermata, Hemichordata, and the extinct Vetulicolia known from Cambrian fossils. The extinct clade Cambroernida is also thought to be a member of Deuterostomia.

<span class="mw-page-title-main">Embryological origins of the mouth and anus</span> Important characteristic for separating animals into protostomes and deuterostomes

The embryological origin of the mouth and anus is an important characteristic, and forms the morphological basis for separating bilaterian animals into two natural groupings: the protostomes and deuterostomes.

<span class="mw-page-title-main">Dugesiidae</span> Family of flatworms

Dugesiidae is a family of freshwater planarians distributed worldwide. The type genus is Dugesia Girard, 1850.

<span class="mw-page-title-main">Nephrozoa</span> Clade of animals

Nephrozoa (tube-animals) is a proposed major clade of bilaterian animals. It includes all bilaterians other than Xenacoelomorpha. It contrasts with the Xenambulacraria hypothesis, which instead posits that Xenacoelomorpha is most closely related to Ambulacraria. Which hypothesis is correct is controversial. Authors supporting the Xenambulacraria hypothesis have suggested that the genetic evidence used to support Nephrozoa may be due to systematic error.

<i>Schmidtea</i> Genus of flatworms

Schmidtea is a genus of freshwater triclads. Species of the genus Schmidtea are widely used in regeneration and developmental studies.

Cura is a genus of freshwater flatworm (triclads) belonging to the family Dugesiidae.

<span class="mw-page-title-main">Xenacoelomorpha</span> A deep-branching bilaterian clade of animals with a simple body plan

Xenacoelomorpha is a small phylum of bilaterian invertebrate animals, consisting of two sister groups: xenoturbellids and acoelomorphs. This new phylum was named in February 2011 and suggested based on morphological synapomorphies, which was then confirmed by phylogenomic analyses of molecular data.

<span class="mw-page-title-main">Geoplaninae</span> Subfamily of flatworms

Geoplaninae is a subfamily of land planarians endemic to the Neotropical region. Members of this family are sometimes referred to as the Neotropical land planarians. However, one species, Obama nungara has been introduced in Europe.

Gonzalo Giribet is a Spanish-American invertebrate zoologist and Alexander Agassiz Professor of zoology working on systematics and biogeography at the Museum of Comparative Zoology in Harvard University. He is a past president of the International Society for Invertebrate Morphology, of the Willi Hennig Society, and vice-president of the Sociedad Española de Malacología.

<i>Xenoturbella bocki</i> Species of bilaterians with a simple body plan

Xenoturbella bocki is a marine benthic worm-like species from the genus Xenoturbella. It is found in saltwater sea floor habitats off the coast of Europe, predominantly Sweden. It was the first species in the genus discovered. Initially it was collected by Swedish zoologist Sixten Bock in 1915, and described in 1949 by Swedish zoologist Einar Westblad. The unusual digestive structure of this species, in which a single opening is used to eat food and excrete waste, has led to considerable study and controversy as to its classification. It is a bottom-dwelling, burrowing carnivore that eats mollusks.

<i>Waminoa</i> Genus of acoel

Waminoa is a genus of acoels which are epizoic on living corals, using the coral's mucus as a source of food. Unusually, these acoels harbor two genera of endosymbiotic dinoflagellates: Symbiodinium and Amphidinium; it is not typical for two different genera of dinoflagellates to coexist in a single host. Waminoa's host coral may also contain dinoflagellates of the genus Symbiodinium but not Amphidinium.

<span class="mw-page-title-main">Xenambulacraria</span> Animal clade containing xenoturbellids, acoelomorphs, echinoderms and hemichordates

Xenambulacraria is a proposed clade of animals with bilateral symmetry as an embryo, consisting of the Xenacoelomorpha and the Ambulacraria.

References

  1. 1 2 Cannon, L. R. G. (1986) Turbellaria of the World. A guide to families and genera. Brisbane, Queensland Museum, 136 p.
  2. ↑ Harzsch, Steffen; Purschke, Günter (2016-01-01). Structure and evolution of invertebrate nervous systems. Oxford University Press. pp. 56–61. ISBN   978-0-19-968220-1. OCLC   951605913.
  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 2020-02-27.
  5. ↑ Achatz, Johannes G.; Chiodin, Marta; Salvenmoser, Willi; Tyler, Seth; Martinez, Pedro (2013-06-01). "The Acoela: on their kind and kinships, especially with nemertodermatids and xenoturbellids (Bilateria incertae sedis)". Organisms Diversity & Evolution. 13 (2): 267–286. Bibcode:2013ODivE..13..267A. doi:10.1007/s13127-012-0112-4. ISSN   1618-1077. PMC   3789126 . PMID   24098090.
  6. ↑ Tyler, S.; Artois, T.; Schilling, S.; Hooge, M.; Bush, L.F., eds. (2006–2020). "World List of turbellarian worms: Acoelomorpha, Catenulida, Rhabditophora. Acoela". www.marinespecies.org. WoRMS - World Register of Marine Species. Retrieved 2020-02-12.
  7. ↑ Tyler, S.; Artois, T.; Schilling, S.; Hooge, M.; Bush, L.F., eds. (2006–2020). "World List of turbellarian worms: Acoelomorpha, Catenulida, Rhabditophora. Nemertodermatida". www.marinespecies.org. WoRMS - World Register of Marine Species. Retrieved 2020-02-12.
  8. ↑ Petrov, A.; Hooge, M.; Tyler, S. (2006). "Comparative morphology of the bursal nozzles in acoels (Acoela, Acoelomorpha)". Journal of Morphology. 267 (5): 634–648. doi:10.1002/jmor.10428. PMID   16485278. S2CID   32595353.
  9. ↑ Laden, Greg (2009-10-01). "Why You Should Care About Acoelomorph Flatworms". Smithsonian Magazine. Retrieved 2020-03-02.
  10. ↑ Conway-Morris, S.; George, J. D.; Gibson R.; Platt, H. M. (1985) The Origins and relationships of lower invertebrates. Oxford, Clarendon Press, 397 p.[ page needed ]
  11. ↑ Katayama, Tomoe; Yamamoto, Masamichi; Wada, Hiroshi; Satoh, Noriyuki (1993). "Phylogenetic Position of Acoel Turbellarians Inferred from Partial 18S rDNA Sequences" (PDF). Zoological Science. 10 (3): 529–536. ISSN   0289-0003. PMID   7764139.
  12. ↑ Katayama, T.; Wada, H.; Furuya, H.; Satoh, N.; Yamamoto, M. (1995-10-01). "Phylogenetic Position of the Dicyemid Mesozoa Inferred from 18S rDNA Sequences". The Biological Bulletin. 189 (2): 81–90. doi:10.2307/1542458. ISSN   0006-3185. JSTOR   1542458. PMID   8541419.
  13. ↑ Carranza, S.; Baguñà, J.; Riutort, M. (1997-05-01). "Are the Platyhelminthes a monophyletic primitive group? An assessment using 18S rDNA sequences". Molecular Biology and Evolution. 14 (5): 485–497. doi:10.1093/oxfordjournals.molbev.a025785. ISSN   0737-4038. PMID   9159926.
  14. ↑ Ruiz-Trillo, Iñaki; Riutort, Marta; Littlewood, D. Timothy J.; Herniou, Elisabeth A.; Baguñà, Jaume (March 1999). "Acoel Flatworms: Earliest Extant Bilaterian Metazoans, Not Members of Platyhelminthes". Science. 283 (5409): 1919–1923. Bibcode:1999Sci...283.1919R. doi:10.1126/science.283.5409.1919. ISSN   0036-8075. PMID   10082465.
  15. 1 2 Baguñà, J.; Riutort, M. (2004). "Molecular phylogeny of the Platyhelminthes". Canadian Journal of Zoology. 82 (2): 168–193. doi:10.1139/z03-214.
  16. 1 2 Baguñà, Jaume; Riutort, Marta (2004). "The dawn of bilaterian animals: the case of acoelomorph flatworms". BioEssays. 26 (10): 1046–57. doi:10.1002/bies.20113. PMID   15382134. S2CID   40453683.
  17. 1 2 Ruiz-Trillo, Iñaki; Riutort, Marta; Fourcade, H. Matthew; Baguñà, Jaume; Boore, Jeffrey L. (2004). "Mitochondrial genome data support the basal position of Acoelomorpha and the polyphyly of the Platyhelminthes". Molecular Phylogenetics and Evolution. 33 (2): 321–32. Bibcode:2004MolPE..33..321R. doi:10.1016/j.ympev.2004.06.002. PMID   15336667.
  18. 1 2 Philippe, Hervé; Brinkmann, Henner; Martinez, Pedro; Riutort, Marta; Baguñà, Jaume (2007-08-08). Volff, Jean-Nicolas (ed.). "Acoel Flatworms Are Not Platyhelminthes: Evidence from Phylogenomics". PLOS ONE. 2 (8): e717. Bibcode:2007PLoSO...2..717P. doi: 10.1371/journal.pone.0000717 . ISSN   1932-6203. PMC   1933604 . PMID   17684563.
  19. ↑ Haszprunar, G. (1996). "Plathelminthes and Plathelminthomorpha — paraphyletic taxa". Journal of Zoological Systematics and Evolutionary Research. 34 (1): 41–48. doi: 10.1111/j.1439-0469.1996.tb00808.x . ISSN   1439-0469.
  20. ↑ Ruiz-Trillo, Iñaki; Paps, Jordi (2016-06-01). "Acoelomorpha: earliest branching bilaterians or deuterostomes?". Organisms Diversity & Evolution. 16 (2): 391–399. Bibcode:2016ODivE..16..391R. doi: 10.1007/s13127-015-0239-1 . hdl: 1983/2686c832-dffe-4486-b3b2-17d4966fd64e . ISSN   1618-1077.
  21. 1 2 Philippe, H.; Brinkmann, H.; Copley, R. R.; Moroz, L. L.; Nakano, H.; Poustka, A. J.; Wallberg, A.; Peterson, K. J.; Telford, M. J. (2011). "Acoelomorph flatworms are deuterostomes related to Xenoturbella". Nature. 470 (7333): 255–258. Bibcode:2011Natur.470..255P. doi:10.1038/nature09676. PMC   4025995 . PMID   21307940.
  22. ↑ Lundin, K (1998). "The epidermal ciliary rootlets of Xenoturbella bocki (Xenoturbellida) revisited: new support for a possible kinship with the Acoelomorpha (Platyhelminthes)". Zoologica Scripta. 27 (3): 263–270. doi:10.1111/j.1463-6409.1998.tb00440.x. S2CID   85324766.
  23. ↑ Nakano, H.; Lundin, K.; Bourlat, S. J.; Telford, M. J.; Funch, P.; Nyengaard, J. R.; Obst, M.; Thorndyke, M. C. (2013). "Xenoturbella bocki exhibits direct development with similarities to Acoelomorpha". Nature Communications. 4: 1537–. Bibcode:2013NatCo...4.1537N. doi:10.1038/ncomms2556. PMC   3586728 . PMID   23443565.
  24. ↑ Hejnol, Andreas; Obst, Matthias; Stamatakis, Alexandros; Ott, Michael; Rouse, Greg W.; Edgecombe, Gregory D.; Martinez, Pedro; Baguñà, Jaume; Bailly, Xavier; Jondelius, Ulf; Wiens, Matthias (2009-12-22). "Assessing the root of bilaterian animals with scalable phylogenomic methods". Proceedings of the Royal Society B: Biological Sciences. 276 (1677): 4261–4270. doi:10.1098/rspb.2009.0896. PMC   2817096 . PMID   19759036.
  25. 1 2 Edgecombe, G. D.; Giribet, G.; Dunn, C. W.; Hejnol, A.; Kristensen, R. M.; Neves, R. C.; Rouse, G. W.; Worsaae, K.; Sørensen, M. V. (2011). "Higher-level metazoan relationships: Recent progress and remaining questions" (PDF). Organisms Diversity & Evolution. 11 (2): 151–172. Bibcode:2011ODivE..11..151E. doi:10.1007/s13127-011-0044-4. S2CID   32169826.
  26. 1 2 Philippe, Hervé; Poustka, Albert J.; Chiodin, Marta; Hoff, Katharina J.; Dessimoz, Christophe; Tomiczek, Bartlomiej; Schiffer, Philipp H.; Müller, Steven; Domman, Daryl; Horn, Matthias; Kuhl, Heiner; Timmermann, Bernd; Satoh, Noriyuki; Hikosaka-Katayama, Tomoe; Nakano, Hiroaki; Rowe, Matthew L.; Elphick, Maurice R.; Thomas-Chollier, Morgane; Hankeln, Thomas; Mertes, Florian; Wallberg, Andreas; Rast, Jonathan P.; Copley, Richard R.; Martinez, Pedro; Telford, Maximilian J. (2019). "Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria". Current Biology. 29 (11): 1818–1826.e6. Bibcode:2019CBio...29E1818P. doi:10.1016/j.cub.2019.04.009. hdl: 21.11116/0000-0004-DC4B-1 . ISSN   0960-9822. PMID   31104936. S2CID   155104811.
  27. ↑ Srivastava, M.; Mazza-Curll, K. L.; Van Wolfswinkel, J. C.; Reddien, P. W. (2014). "Whole-Body Acoel Regeneration is Controlled by Wnt and Bmp-Admp Signaling". Current Biology. 24 (10): 1107–13. Bibcode:2014CBio...24.1107S. doi: 10.1016/j.cub.2014.03.042 . PMID   24768051.
  28. ↑ 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. Bibcode:2016Natur.530...89C. doi:10.1038/nature16520. PMID   26842059. S2CID   205247296.
  29. ↑ Michalak, Pawel; Egger, Bernhard; Steinke, Dirk; Tarui, Hiroshi; De Mulder, Katrien; Arendt, Detlev; Borgonie, Gaëtan; Funayama, Noriko; Gschwentner, Robert; Hartenstein, Volker; Hobmayer, Bert; Hooge, Matthew; Hrouda, Martina; Ishida, Sachiko; Kobayashi, Chiyoko; Kuales, Georg; Nishimura, Osamu; Pfister, Daniela; Rieger, Reinhard; Salvenmoser, Willi; Smith, Julian; Technau, Ulrich; Tyler, Seth; Agata, Kiyokazu; Salzburger, Walter; Ladurner, Peter (2009). "To Be or Not to Be a Flatworm: The Acoel Controversy". PLOS ONE. 4 (5): e5502. Bibcode:2009PLoSO...4.5502E. doi: 10.1371/journal.pone.0005502 . PMC   2676513 . PMID   19430533.
  30. ↑ Perea-Atienza, E.; Gavilan, B.; Chiodin, M.; Abril, J. F.; Hoff, K. J.; Poustka, A. J.; Martinez, P. (2015). "The nervous system of Xenacoelomorpha: a genomic perspective". Journal of Experimental Biology. 218 (4): 618–28. doi: 10.1242/jeb.110379 . hdl: 2445/192702 . PMID   25696825.
  31. ↑ Sprecher, Simon G.; & al. (2015). "Functional brain regeneration in the acoel worm Symsagittifera roscoffensis". Biology Open. 4 (12): 1688–1695. doi:10.1242/bio.014266. PMC   4736034 . PMID   26581588.
  32. 1 2 Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. p. 229. ISBN   978-0-03-056747-6.
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