The Spiralia are a morphologically diverse clade of protostome animals, including within their number the molluscs, annelids, platyhelminths and other taxa.[1] The term Spiralia is applied to those phyla that exhibit canonical spiral cleavage, a pattern of early development found in most (but not all) members of the Lophotrochozoa.[2]
Distribution of spiralian development across phylogeny
Members of the molluscs, annelids, platyhelminths and nemerteans have all been shown to exhibit spiral cleavage in its classical form. Other spiralian phyla (rotifers, brachiopods, phoronids, gastrotrichs, and bryozoans) are also said to display a derived form of spiral cleavage in at least a portion of their constituent species, although evidence for this is sparse.[3]
Lophotrochozoa within Spiralia
Previously, spiral cleavage was thought to be unique to the Spiralia in the strictest sense—animals such as molluscs and annelids which exhibit classical spiral cleavage. The presence of spiral cleavage in animals such as platyhelminths could be difficult to correlate with some phylogenies.[4]
Evidence of a close relationship between molluscs, annelids and lophophorates was found in 1995 and Lophotrochozoa was defined as the group containing these taxa and all the descendants of their last common ancestor.[5] More recent research has established the Lophotrochozoa as a superphylum within the Metazoa.[6] With this understanding, the presence of spiral cleavage in polyclad platyhelminths, as well as the more traditional Spiralia, has led to the hypothesis that spiral cleavage was present ancestrally across the Lophotrochozoa as a whole.[3] With the introduction of Platytrochozoa and Rouphozoa, the cladogram is as follows, with an indication approximately how many million years ago (Mya) the clades radiated into newer clades.[7][8][9][10][11][12]
Protostomia Ecdysozoa 
Kimberella †
Spiralia Gnathifera 
Platytrochozoa Mesozoa 
Rouphozoa Gastrotricha 
Platyhelminthes 
Lophotrochozoa Cycliophora 
Annelida 
Mollusca 
Kryptotrochozoa Lophophorata Brachiozoa Brachiopoda 
Phoronida 
Entoprocta 
Ectoprocta (Bryozoa) 
Nemertea 
580 mya 610 mya
An alternative phylogeny was given in 2019, with a basal grouping Mollusca with Entoprocta grouping named Tetraneuralia, and a second grouping of Nemertea with Platyhelminthes named Parenchymia as sister of Annelida. In their proposal and according to the original definition, Lophotrochozoa may become a senior synonym for Platytrochozoa.[13][14][15][16]
Protostomia Ecdysozoa 
Spiralia (s.l.) /
Gnathifera 
Lophotrochozoa /
Tetraneuralia Mollusca 
Entoprocta 
Gastrotricha 
Lophophorata Ectoprocta 
Phoronida 
Brachiopoda
Annelida 
Parenchymia Platyhelminthes 
Nemertea 
Platytrochozoa /
Spiralia (s.s.) Gnathospiralia In 2019 the Rouphozoa was recovered again as a basal Platytrochozoa clade.[17]
A 2022 study supported the Trochozoa and Platyzoa hypotheses, as shown below.[18]
Protostomia Ecdysozoa
Spiralia Trochozoa Mollusca
Nemertea
Annelida
Brachiozoa Brachiopoda
Phoronida
Bryozoa
Bryozoa sensu lato Entoprocta
Cycliophora
Platyzoa Gastrotricha
Platyzoa Gnathifera
Platyhelminthes
Mesozoa Dicyemida 
Orthonectida
References
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^ Giribet, G. (April 2008). "Assembling the lophotrochozoan (=spiralian) tree of life". Philosophical Transactions of the Royal Society B: Biological Sciences. 363 (1496): 1513–22. doi:10.1098/rstb.2007.2241. PMC 2614230. PMID 18192183.
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^ "Explanations.html". Archived from the original on 2013-02-07. Retrieved 2009-06-28.
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^ a b Hejnol, A. (4 August 2010). "A Twist in Time—The Evolution of Spiral Cleavage in the Light of Animal Phylogeny". Integrative and Comparative Biology. 50 (5): 695–706. doi:10.1093/icb/icq103. PMID 21558233.
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^ Boyer, Barbara C.; Henry, Jonathan Q.; Martindale, Mark Q. (1 November 1996). "Dual Origins of Mesoderm in a Basal Spiralian: Cell Lineage Analyses in the Polyclad Turbellarian Hoploplana inquilina". Developmental Biology. 179 (2): 329–338. doi:10.1006/dbio.1996.0264. PMID 8903349.
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^ Halanych, K.; Bacheller, J.; Aguinaldo, A.; Liva, S.; Hillis, D.; Lake, J. (17 March 1995). "Evidence from 18S ribosomal DNA that the lophophorates are protostome animals". Science. 267 (5204): 1641–1643. Bibcode:1995Sci...267.1641H. doi:10.1126/science.7886451. PMID 7886451. S2CID 12196991.
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^ Dunn, C.W.; Hejnol, A.; Matus, D. Q.; Pang, K.; Browne, W. E.; Smith, S.A.; Seaver, E.; Rouse, G.W.; Obst, M.; Sørensen, M. V.; Haddock, S. H. D.; Schmidt-Rhaesa, A.; Okusu, A.; Kristensen, R.M.; Wheeler, W. C.; Martindale, M. Q.; Giribet, G. (10 April 2008). "Broad phylogenomic sampling improves resolution of the animal tree of life". Nature. 452 (7188): 745–749. Bibcode:2008Natur.452..745D. doi:10.1038/nature06614. PMID 18322464. S2CID 4397099.
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^ Struck, Torsten H.; Wey-Fabrizius, Alexandra R.; Golombek, Anja; Hering, Lars; Weigert, Anne; Bleidorn, Christoph; Klebow, Sabrina; Iakovenko, Nataliia; Hausdorf, Bernhard (July 2014). "Platyzoan Paraphyly Based on Phylogenomic Data Supports a Noncoelomate Ancestry of Spiralia". Molecular Biology and Evolution. 31 (7): 1833–1849. doi:10.1093/molbev/msu143. PMID 24748651.
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^ Peterson, Kevin J.; Cotton, James A.; Gehling, James G.; Pisani, Davide (2008-04-27). "The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records". Philosophical Transactions of the Royal Society of London B: Biological Sciences. 363 (1496): 1435–1443. doi:10.1098/rstb.2007.2233. PMC 2614224. PMID 18192191.
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^ Hankeln, Thomas; Wey-Fabrizius, Alexandra; Herlyn, Holger; Witek, Alexander; Weber, Mathias; Nesnidal, Maximilian; Struck, Torsten (2014). "Phylogeny of platyzoan taxa based on molecular data". In Wägele, J. Wolfgang; Bartolomaeus, Thomas (eds.). Deep Metazoan Phylogeny: The Backbone of the Tree of Life. Walter de Gruyter GmbH. pp. 105–125.
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^ Laumer, Christopher E.; Bekkouche, Nicolas; Kerbl, Alexandra; Goetz, Freya; Neves, Ricardo C.; Sørensen, Martin V.; Kristensen, Reinhardt M.; Hejnol, Andreas; Dunn, Casey W. (2015). "Spiralian Phylogeny Informs the Evolution of Microscopic Lineages". Current Biology. 25 (15): 2000–2006. doi:10.1016/j.cub.2015.06.068. PMID 26212884.
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^ Lu, Tsai-Ming; Kanda, Miyuki; Satoh, Noriyuki; Furuya, Hidetaka (2017-05-29). "The phylogenetic position of dicyemid mesozoans offers insights into spiralian evolution". Zoological Letters. 3: 6. doi:10.1186/s40851-017-0068-5. PMC 5447306. PMID 28560048.
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^ Luo, Yi-Jyun; Kanda, Miyuki; Koyanagi, Ryo; Hisata, Kanako; Akiyama, Tadashi; Sakamoto, Hirotaka; Sakamoto, Tatsuya; Satoh, Noriyuki (2017-12-04). "Nemertean and phoronid genomes reveal lophotrochozoan evolution and the origin of bilaterian heads". Nature Ecology and Evolution. 2 (1): 141–151. doi:10.1038/s41559-017-0389-y. PMID 29203924.
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^ Marlétaz, Ferdinand; Peijnenburg, Katja T. C. A.; Goto, Taichiro; Satoh, Noriyuki; Rokhsar, Daniel S. (2019-01-10). "A New Spiralian Phylogeny Places the Enigmatic Arrow Worms among Gnathiferans". Current Biology. 29 (2): 312–318.e3. doi:10.1016/j.cub.2018.11.042. ISSN 0960-9822. PMID 30639106.
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^ Wanninger, Andreas; Wollesen, Tim (2019). "The evolution of molluscs: The evolution of molluscs". Biological Reviews. 94 (1): 102–115. doi:10.1111/brv.12439. PMC 6378612. PMID 29931833.
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^ Telford, Maximilian J. (2019). "Evolution: Arrow Worms Find Their Place on the Tree of Life". Current Biology. 29 (5): R152–R154. doi:10.1016/j.cub.2018.12.029. PMID 30836082.
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^ Laumer, Christopher E.; Fernández, Rosa; Lemer, Sarah; Combosch, David; Kocot, Kevin M.; Riesgo, Ana; Andrade, Sónia C. S.; Sterrer, Wolfgang; Sørensen, Martin V.; Giribet, Gonzalo (2019-07-10). "Revisiting metazoan phylogeny with genomic sampling of all phyla". Proceedings of the Royal Society B: Biological Sciences. 286 (1906): 20190831. doi:10.1098/rspb.2019.0831. ISSN 0962-8452. PMC 6650721. PMID 31288696.
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