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Xenoturbella japonica.jpg
Xenoturbella japonica
Scientific classification

Westblad 1949

Westblad 1949

Westblad 1949[1][2]

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.[3]

The first known species (Xenoturbella bocki) was discovered in 1915 by Sixten Bock, but it was only properly described in 1949 by Einar Westblad.[4]


Xenoturbella has a very simple body plan. It consists of two ciliated epithelial layers: an external epidermis and an internal gastrodermis lining the simple sack-like gut. The mouth is located ventrally and there is no anus,[5] waste is dispelled through the same opening as food is taken in.[6]

The nervous system is composed by a net of interconnected neurons beneath the epidermis, without any concentration of neurons forming ganglia or nerve cords.[7][8]

Species of Xenoturbella also lack a respiratory, a circulatory and an excretory system. In fact, there are no defined organs, except for a statocyst containing flagellated cells.[5] There are no organized gonads, but gametes are produced. Adults producing sperm are very rarely observed, but eggs and embryos are known to occur in follicles.[9]

Eggs of Xenoturbella are 0.2 millimetres (0.0079 in) wide, pale orange and opaque.[10] 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.[10] They are similar to the juveniles of acoelomate Neochildia fusca.[10]


Currently the genus Xenoturbella contains 6 recognized species:[11]

The two smaller species, X. bocki and X. hollandorum, which are up to 4 centimetres (1.6 in) long, are found in shallower waters less than 650 metres (2,130 ft) deep and form a separate clade from the other three larger species, 10 centimetres (3.9 in) or greater long, which live in deeper waters 1,700–3,700 metres (5,600–12,100 ft).[3]

In 2017, a further species, X. japonica has been proposed.[15]


The systematic position of Xenoturbella has been considered enigmatic since its discovery. An early DNA analysis suggested a close relationship to molluscs,[16] but it was probably a result from contamination with DNA of molluscs that Xenoturbella consumes.[17]

A subsequent study suggested a placement of the genus in its own phylum, Xenoturbellida, as a deuterostome clade and sister group to the Ambulacraria.[18] The deuterostome affiliations were then recovered by studies that indicate a basal position of this phylum within the deuterostomes[19][20] or in a sister group relationship with the Ambulacraria.[21]

However, morphological characters, such as the structure of epidermal cilia, suggested a close relationship with Acoelomorpha, another problematic group.[22] The study of the embryonic stages of Xenoturbella also showed that it is a direct developer without a feeding larval stage, and this developmental mode is similar to that of acoelomorphs.[10] Molecular studies based on the concatenation of hundreds of proteins revealed indeed a monophyletic group composed by Xenoturbella and Acoelomorpha.[23][21][24] This clade was named Xenacoelomorpha.[21]

The monophyly of Xenacoelomorpha soon became established, but its position as either a basal bilaterian clade or a deuterostome remained unresolved until 2016 when two new studies, with increased gene and taxon sampling, again placed Xenoturbella as the sister group of Acoelomorpha within Xenacoelomorpha, and placed Xenacoelomorpha as sister to Nephrozoa (Protostomia plus Deuterostomia), and therefore the basalmost bilaterian phylum.[3][25]


  1. ^ Zhang, Zhi-Qiang (2011-12-23). "Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness". Zootaxa. Magnolia Press. 3148: 1–237. Retrieved 2016-02-03.
  2. ^ Tyler, S.; Schilling, S.; Hooge, M.; Bush, L.F. (2006–2016). "Xenoturbella". Turbellarian taxonomic database. Version 1.7. Retrieved 2016-02-03.
  3. ^ a b c 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 (7588): 94–97. Bibcode:2016Natur.530...94R. doi:10.1038/nature16545. PMID 26842060. Retrieved 2016-02-03.
  4. ^ Westblad, E (1949). "Xenoturbella bocki n. g., n. sp., a peculiar, primitive Turbellarian type". Arkiv för Zoologi. 1: 3–29.
  5. ^ a b Israelsson, O. (1999). "New light on the enigmatic Xenoturbella (phylum uncertain): ontogeny and phylogeny". Proceedings of the Royal Society B: Biological Sciences. 266 (1421): 835–841. doi:10.1098/rspb.1999.0713. ISSN 0962-8452. PMC 1689910.
  6. ^ McCafferty, Georgia (February 4, 2016). "Deep-sea 'purple sock' provides clues to early life". CNN. Archived from the original on May 6, 2016. Retrieved May 6, 2016.
  7. ^ 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): 618–628. doi:10.1242/jeb.110379. ISSN 0022-0949. PMID 25696825.
  8. ^ 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 (2): 107–118. doi:10.1007/s004350000028.
  9. ^ Israelsson, Olle; Budd, Graham E. (2005). "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. ISSN 0949-944X. PMID 15818482.
  10. ^ 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–. Bibcode:2013NatCo...4E1537N. doi:10.1038/ncomms2556. PMC 3586728. PMID 23443565.
  11. ^ WoRMS: Xenoturbella Westblad, 1949
  12. ^ WoRMS: Xenoturbella bocki (Westblad, 1949)
  13. ^ WoRMS: Xenoturbella westbladi Reisinger, 1960
  14. ^ WoRMS: Xenoturbella westbladi (Israelsson, 1999)
  15. ^ Nakano, Hiroaki; Miyazawa, Hideyuki; Maeno, Akiteru; Shiroishi, Toshihiko; Kakui, Keiichi; Koyanagi, Ryo; Kanda, Miyuki; Satoh, Noriyuki; Omori, Akihito; Kohtsuka, Hisanori (2017). "A new species of Xenoturbella from the western Pacific Ocean and the evolution of Xenoturbella". BMC Evolutionary Biology. 17. doi:10.1186/s12862-017-1080-2.
  16. ^ Norén, Michael; Jondelius, Ulf (1997). "Xenoturbella 's molluscan relatives". Nature. 390 (6655): 31–32. Bibcode:1997Natur.390...31N. doi:10.1038/36242. ISSN 0028-0836.
  17. ^ Bourlat, Sarah J.; Nielsen, Claus; Lockyer, Anne E.; Littlewood, D. Timothy J.; Telford, Maximilian J. (2003). "Xenoturbella is a deuterostome that eats molluscs". Nature. 424 (6951): 925–928. Bibcode:2003Natur.424..925B. doi:10.1038/nature01851. ISSN 0028-0836. PMID 12931184.
  18. ^ Bourlat, Sarah J.; Juliusdottir, Thorhildur; Lowe, Christopher J.; Freeman, Robert; Aronowicz, Jochanan; Kirschner, Mark; Lander, Eric S.; Thorndyke, Michael; Nakano, Hiroaki; Kohn, Andrea B.; Heyland, Andreas; Moroz, Leonid L.; Copley, Richard R.; Telford, Maximilian J. (2006). "Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida". Nature. 444 (7115): 85–88. Bibcode:2006Natur.444...85B. doi:10.1038/nature05241. ISSN 0028-0836. PMID 17051155.
  19. ^ 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]
  20. ^ 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.
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  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.
  23. ^ 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.
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