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

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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 [2]
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External links

  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.
  5. ↑ 5.0 5.1 5.2 5.3 Lua error in package.lua at line 80: module 'strict' not found.
  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 [3]
  7. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  8. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  9. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  10. ↑ Enigmatic worm identified as mankind's long lost relative – Accessed January 3, 2008
  11. ↑ Xenoturbella – Back to the Basics – Accessed January 3, 2008
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