Xenoturbella

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

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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.[1]

Taxonomy

The genus Xenoturbella contains two species:

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.[2][3][4][5]

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[6][7] or a sister group relationship with the echinoderms and hemichordates.[8] However, some consider the evidence for a position within deuterostomes weak and favor the placement of Xenoturbella + Acoelomorpha more basally among Metazoa.[9]

Description

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 and Scotland.[10]

Life cycle

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.[11]

Eggs of Xenoturbella are 0.2 mm wide, pale orange and opaque.[5] 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.[5] They are similar to the larvae of acoelomate Neochildia fusca.[5]

References

  1. Westblad, E (1949) Xenoturbella bocki n. g., n. sp., a peculiar, primitive Turbellarian type. Arkiv för Zoologi 1:3-29
  2. 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.
  3. 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.
  4. 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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  6. 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]
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  10. Enigmatic worm identified as mankind's long lost relative – Accessed January 3, 2008
  11. Xenoturbella – Back to the Basics – Accessed January 3, 2008

Additional material

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  • 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
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  • 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]
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External links