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Scientific classification e
Kingdom: Animalia
Phylum: Xenacoelomorpha
Subphylum: Xenoturbellida
Family: Xenoturbellidae
Genus: Xenoturbella
  • Xenoturbella bocki Westblad 1949
  • Xenoturbella westbladi Israelsson 1999

Xenoturbella is a genus of bilaterian animals; it contains a small number of marine worm-like species.[1] The first known species (Xenoturbella bocki) was discovered in 1915 by Sixten Bock but the first published description was only in 1949 by Einar Westblad.[2]


The genus Xenoturbella contains the species

  • Xenoturbella bocki
  • Xenoturbella westbladi.

In 2016, four new species were described:[1][3][4]

  • Xenoturbella monstrosa, nicknamed "purple sock"[4]
  • Xenoturbella hollandorum
  • Xenoturbella profunda
  • Xenoturbella churro.

The smaller (up to 4 centimetres (1.6 in) long), shallower water (less than 650 metres (2,130 ft) deep) species, X. bocki and X. hollandorum (with X. westbladi considered a junior synonym of X. bocki), are in a separate clade from the other three larger (10 centimetres (3.9 in) or greater long), deeper dwelling (1,700–3,700 metres (5,600–12,100 ft)) species, with X. churro and X. profunda being sister species within the latter.[1] Rouse and colleagues also argue that X. bocki and X. westbladi are the same species.[1]

Its taxonomic position has been considered enigmatic since its discovery. An early DNA analysis suggested a close relationship to molluscs (Noren & Jondelius, 1997), but turned out to have been contaminated with DNA from molluscs that Xenoturbella may have eaten (Bourlat et al., 2003; Israelsson & Budd, 2006). The genus is now the sole member of its own phylum Xenoturbellida (Haszprunar et al., 1991; Bourlat et al., 2006), and there is strong support from both morphological and molecular studies for a close relationship with Acoelomorpha.[5][6][7][8]

A 2003 DNA study positioned Xenoturbella as a primitive deuterostome outside the established phyla (Bourlat et al., 2003). The deuterostome affiliations were recently corroborated by studies that indicate a basal position of this phylum within the deuterostomes[9][10] or a sister group relationship with the echinoderms and hemichordates.[11] However, some consider the evidence for a position within deuterostomes weak and favor the placement of Xenoturbella + Acoelomorpha more basally among Metazoa.[12]

A 2016 study, based primarily on transcriptomic data from X. profunda, again placed Xenoturbella as the sister group of Acoelomorpha within Xenacoelomorpha, and placed Xenacoelomorpha as either sister to Nephrozoa (Protostomia plus Deuterostomia) or as sister to Protostomia.[1]


Xenoturbella has a very simple body plan: it has no brain, no through gut, no excretory system, no organized gonads (but does have gametes; eggs and embryos occur in follicles [Israelsson and Budd]), or any other defined organs except for a statocyst containing flagellated cells; it has cilia and a diffuse nervous system. The animal is up to 20 centimetres (7.9 in) long (X. monstrosa), and has been found off the coasts of Sweden and Scotland,[13] in Monterey Canyon off California, and in the Gulf of California.[1]

Life cycle[edit]

The association of specimens of Xenoturbella with mollusc larva has led many to suggest that they are molluscivores. However, a more radical interpretation, of this and other data, is that the Xenoturbella larval stage develops as an internal parasite of certain molluscs.[14]

Eggs of Xenoturbella are 0.2 millimetres (0.0079 in) wide, pale orange and opaque.[8] Newly hatched embryos are free-swimming (tending to stay close to water surface) and ciliated. They feature no mouth and they do not apparently feed.[8] They are similar to the larvae of acoelomate Neochildia fusca.[8]


  1. ^ a b c d e f Rouse, Greg W.; Wilson, Nerida G.; Carvajal, Jose I.; Vrijenhoek, Robert C. (2016-02-03). "New deep-sea species of Xenoturbella and the position of Xenacoelomorpha". Nature (530): 94–97. doi:10.1038/nature16545. Retrieved 2016-02-03. 
  2. ^ Westblad, E (1949) Xenoturbella bocki n. g., n. sp., a peculiar, primitive Turbellarian type. Arkiv för Zoologi 1:3-29
  3. ^ "Scripps-Led Team Discovers Four New Deep-Sea Worm Species". Scripps Institution of Oceanography, UC San Diego. 2016-02-03. Retrieved 2016-02-03. 
  4. ^ a b Morelle, Rebecca (2016-02-03). "Mystery of deep-sea 'purple sock' solved". BBC. Retrieved 2016-02-03. 
  5. ^ 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, 263–270.
  6. ^ Raikova, O. I., Reuter, M., Jondelius, U., & Gustafsson, M. K. S. (2000). An immunocytochemical and ultrastructural study of the nervous and muscular systems of Xenoturbella westbladi (Bilateria inc. sed.). Zoomorphology, 120, 107–118.
  7. ^ Hejnol, A., Obst, M., Stamatakis, A., Ott, M., Rouse, G. W., Edgecombe, G. D., et al. (2009). Assessing the root of bilaterian animals with scalable phylogenomic methods. Proceedings of the Royal Society, Series B, 276, 4261–4270.
  8. ^ a b c d 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–. doi:10.1038/ncomms2556. PMC 3586728. PMID 23443565. 
  9. ^ Perseke M, Hankeln T, Weich B, Fritzsch G, Stadler PF, Israelsson O, Bernhard D, Schlegel M. (2007) "The mitochondrial DNA of Xenoturbella bocki: genomic architecture and phylogenetic analysis". Theory Biosci. 126(1):35-42. Available on-line at [1]
  10. ^ Telford, M. J. (2008). "Xenoturbellida: the fourth deuterostome phylum and the diet of worms". Genesis 46 (11): 580–586. doi:10.1002/dvg.20414. PMID 18821586. 
  11. ^ 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. PMID 21307940. 
  12. ^ 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". Organisms Diversity & Evolution 11 (2): 151. doi:10.1007/s13127-011-0044-4. 
  13. ^ Enigmatic worm identified as mankind's long lost relative – Accessed January 3, 2008
  14. ^ Xenoturbella – Back to the Basics – Accessed January 3, 2008

Additional material[edit]

  • E. Westblad (1949). "Xenoturbella bocki n.g., n.sp., a peculiar, primitive turbellarian type". Arkiv för Zoologi 1: 3–29. 
  • G. Haszprunar, R.M. Rieger, P. Schuchert (1991). "Extant 'Problematica' within or near the Metazoa." In: Simonetta, A.M. & Conway Morris, S. (eds.): The Early Evolution of Metazoa and the Significance of Problematic Taxa. Oxford Univ. Press, Cambridge. pp. 99–105
  • M. Noren, U. Jondelius (1997). "Xenoturbella's molluscan relatives..". Nature 390 (6655): 31–32. Bibcode:1997Natur.390...31N. doi:10.1038/36242. 
  • O. Israelsson (1999). "New light on the enigmatic Xenoturbella (phylum uncertain): ontogeny and phylogeny". Proceedings of the Royal Society B 266 (1421): 835–841. doi:10.1098/rspb.1999.0713. 
  • O. Israelsson O, G. E. Budd G E (2006). "Eggs and embryos in Xenoturbella (phylum uncertain) are not ingested prey". Development Genes and Evolution 215 (7): 358–363. doi:10.1007/s00427-005-0485-x. PMID 15818482. 
  • S. J. Bourlat, C. Nielsen, A. E. Lockyer, D. Timothy, J. Littlewood, M. J. Telford (2003). "Xenoturbella is a deuterostome that eats molluscs". Nature 424 (6951): 925–928. Bibcode:2003Natur.424..925B. doi:10.1038/nature01851. PMID 12931184.  [2]
  • S. J. Bourlat, T. Juliusdottir, C. J. Lowe, R. Freeman, J. Aronowicz, M. Kirschner, E. S. Lander, M. Thorndyke, H. Nakano, A. B. Kohn, A. Heyland, L. L. Moroz, R. R. Copley, M. J. Telford (2006). "Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida". Nature 444 (7115): 85–88. Bibcode:2006Natur.444...85B. doi:10.1038/nature05241. PMID 17051155. 
  • Olle Israelsson, Graham E Budd (2005). "Eggs and embryos in Xenoturbella (phylum uncertain) are not ingested prey". Development Genes and Evolution 215: 358-63 [3]
  • K. U. Kjeldsen, M. Obst, H. Nakano, P. Funch, A. Schramm (2010). "Two Types of Endosymbiotic Bacteria in the Enigmatic Marine Worm Xenoturbella bocki". Applied and Environmental Microbiology 76 (8): 2657–2662. doi:10.1128/aem.01092-09. PMC 2849209. PMID 20139320. 

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