Xenoturbella is a genus of small, ciliated marine worms with an unusual and simple morphology (Westblad 1949). Based on both morphological and molecular data, the phylogenetic placement of Xenoturbella in the animal tree has been controversial. Proposals have included a close relationship to Acoelomorpha, to Enteropneusta and Holothuroidea (in Echinodermata), and to Mollusca. It has also been suggested that Xenoturbella represents a basal bilaterian lineage. Recently, phylogenomic analyses by Hejnol et al. (2009) supported a sister group relationship between Xenoturbella and Acoelomorpha, with the clade they form sister to all other Bilateria. Philippe et al. (2011) analyzed a large molecular data set and concluded that Xenoturbella and the Acoelomorpha likely form a clade ("Xenacoelomorpha") that is sister to the Ambulacraria (Hemichordata + Echinodermata) within Deuterostomia. A large analysis of mitochondrial genomes (Perseke et al. 2013) suggested that Xenoturbella is sister to all the deuterostomes excluding tunicates, a finding consistent with most previous analyses based on mitochondrial DNA. Mitochondrial gene order also indicated a close relationship between Xenoturbella and deuterostomes. The phylogenetic position of Xenoturbella in the animal tree has been among the most challenging questions in efforts to resolve the metazoan tree.
(Perseke et al. 2013 and references therein)
Molecular Biology and Genetics
Statistics of barcoding coverage
Specimens with Sequences:4
Specimens with Barcodes:4
Species With Barcodes:1
Xenoturbella is a genus of bilaterian animals; it contains two marine worm-like species. 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.
Taxonomy[edit source | edit]
The genus Xenoturbella contains two species:
Its taxonomic position has been considered enigmatic since its discovery. Earlier it was suspected to be closely related to molluscs (Noren & Jondelius, 1997), but it turned out that the DNA analysis of this study was contaminated with DNA from molluscs which 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.
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  or a sister group relationship with the echinoderms and hemichordates. However, some consider the evidence for a position within deuterostomes weak and favor the placement of Xenoturbella + Acoelomorpha more basally among Metazoa.
Description[edit source | edit]
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 4 centimetres (1.6 in) long, and has been found off the coasts of Sweden, Scotland and Iceland.
Life cycle[edit source | 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.
Eggs of Xenoturbella are 0.2 mm wide, pale orange and opaque. 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. They are similar to the larvae of acoelomate Neochildia fusca
References[edit source | edit]
- Westblad, E (1949) Xenoturbella bocki n. g., n. sp., a peculiar, primitive Turbellarian type. Arkiv för Zoologi 1:3-29
- 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.
- 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.
- 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.
- 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.
- 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 
- 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.
- 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.
- Edgecombe, G. D.; Giribet, G.; Dunn, C. W.; Hejnol, A.; Kristensen, R. M.; Neves, R. C.; Rouse, G. W.; Worsaae, K. et al. (2011). "Higher-level metazoan relationships: Recent progress and remaining questions". Organisms Diversity & Evolution 11 (2): 151. doi:10.1007/s13127-011-0044-4.
- Enigmatic worm identified as mankind's long lost relative – Accessed January 3, 2008
- Xenoturbella – Back to the Basics – Accessed January 3, 2008
Additional material[edit source | 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". Proc. Roy. Soc. 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. doi:10.1038/nature01851. PMID 12931184. 
- 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 
- 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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