Xenoturbella
| Xenoturbella | |
|---|---|
| Scientific classification | |
| Kingdom: | |
| Superphylum: | |
| (unranked): | |
| Phylum: | Xenoturbellida
|
| Family: | Xenoturbellidae
|
| Genus: | Xenoturbella
|
| Species | |
|
X. bocki Westblad, 1949 | |
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.<ref="Perseke2007"> 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 </ref>
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[1].
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 [2].
The genus Xenoturbella contains two species:
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.
- O. Israelsson (1999). "New light on the enigmatic Xenoturbella (phylum uncertain): ontogeny and phylogeny". Proc. Roy. Soc. B. 266: 835–841.
- 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.
- 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.
{{cite journal}}: CS1 maint: multiple names: authors list (link) [3] - 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.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - Olle Israelsson, Graham E Budd (2005). "Eggs and embryos in Xenoturbella (phylum uncertain) are not ingested prey". Development Genes and Evolution 215: 358-63 [4]