Xenoturbella
| Xenoturbella | |
|---|---|
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Xenacoelomorpha |
| Subphylum: | Xenoturbellida |
| Family: | Xenoturbellidae |
| Genus: | Xenoturbella |
| Species | |
| |
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:
- Xenoturbella bocki
- Xenoturbella westbladi
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.[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, Scotland and Iceland.[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
- ↑ 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.
- ↑ 5.0 5.1 5.2 5.3 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.; Sørensen, M. V. (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
- 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". Proceedings of the Royal Society 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. Bibcode:2003Natur.424..925B. 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.
External links
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