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Xenoturbella

Xenoturbella is a genus of bilaterian animals; it contains two marine worm-like species. Its taxonomic position has been considered enigmatic since its discovery in 1949, but a 2003 DNA study has positioned it as a primitive deuterostome outside the established phyla (Bourlat et al., 2003). Earlier it was suspected to be closely related to molluscs (Noren & Jondelius, 1997), but it turned out that the DNA test was contaminated with DNA from molluscs which it may have eaten (Bourlat et al., 2003; Israelsson & Budd, 2006). The earlier results were recently corroborated; the genus is now the sole member of its own phylum Xenoturbellida (Haszprunar et al., 1991; Bourlat et al., 2006). It appears that this phylum is basal within the deuterostomes.^[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 4 cm long, and has been found off the coasts of Sweden, Scotland and Iceland.^[2]

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 that the Xenoturbella larval stage develops as an internal parasite of certain molluscus.^[3]

The genus Xenoturbella contains two species:

• Xenoturbella bocki
• Xenoturbella westbladi

Notes

References

• 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: 31-32. doi:10.1038/36242. 
• O. Israelsson (1999). "New light on the enigmatic Xenoturbella (phylum uncertain): ontogeny and phylogeny". Proc. Roy. Soc. B 266: 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: 358-363. doi:10.1007/s00427-005-0485-x. 
• 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: 925-928. doi:10.1038/nature01851.  [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: 85-88. doi:10.1038/nature05241. 
• 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]

External links

• A PCR Survey of Xenoturbella bocki Hox Genes