Xenoturbella

Xenoturbella
Scientific classification
Kingdom: Animalia
Phylum: Xenacoelomorpha
Subphylum: Xenoturbellida
Family: Xenoturbellidae
Genus: Xenoturbella
Species

See text.

Xenoturbella is a genus of very simple and primitive bilaterians up to a few centimeters long. It contains a small number of marine benthic worm-like species.[1]

The first known species (Xenoturbella bocki) was discovered in 1915 by Sixten Bock, but it was only properly described in 1949 by Einar Westblad.[2]

Description

Xenoturbella has a very simple body plan. It consists of two ciliated epithelial layers: an external epidermis and an internal gastrodermis lining the simple sack-like gut. The mouth is located ventrally and there is no anus,[3] waste is dispelled through the same opening as food is taken in.[4]

The nervous system is composed by a net of interconnected neurons beneath the epidermis, without any concentration of neurons forming ganglia or nerve cords.[5][6]

Species of Xenoturbella also lack a respiratory, a circulatory and an excretory system. In fact, there are no defined organs, except for a statocyst containing flagellated cells.[3] There are no organized gonads, but gametes are produced. Adults producing sperm are very rarely observed, but eggs and embryos are known to occur in follicles.[7]

Eggs of Xenoturbella are 0.2 millimetres (0.0079 in) wide, pale orange and opaque.[8] 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.[8] They are similar to the larvae of acoelomate Neochildia fusca.[8]

Species

Currently the genus Xenoturbella contains 6 recognized species:[9]

The two smaller species, X. bocki and X. hollandorum, which are up to 4 centimetres (1.6 in) long, are found in shallower waters less than 650 metres (2,130 ft) deep and form a separate clade from the other three larger species, 10 centimetres (3.9 in) or greater long, which live in deeper waters 1,700–3,700 metres (5,600–12,100 ft).[1]

Phylogeny

The systematic position of Xenoturbella has been considered enigmatic since its discovery. An early DNA analysis suggested a close relationship to molluscs,[13] but it was probably a result from contamination with DNA of molluscs that Xenoturbella consumes.[14]

Subsequent studies recovered a result that placed the genus in its own phylum, Xenoturbellida, as a deuterostome clade and sister group to the Ambulacraria.[15] The deuterostome affiliations were recently corroborated by studies that indicate a basal position of this phylum within the deuterostomes[16][17] or in a sister group relationship with the Ambulacraria.[18]

However, morphological characters, such as the structure of epidermal cilia, suggested a close relationship with Acoelomorpha, another problematic group,[19] and recent molecular studies revealed indeed a monophyletic group composed by Xenoturbella and Acoelomorpha, which was named Xenacoelomorpha.[8][20][21]

The monophyly of Xenacoelomorpha soon became established, but its position as either a basal bilaterian clade or a deuterostome remained unresolved until very recently, when a new study, based primarily on transcriptomic data from X. profunda, again placed Xenoturbella as the sister group of Acoelomorpha within Xenacoelomorpha, and placed Xenacoelomorpha as sister to Nephrozoa (Protostomia plus Deuterostomia), and therefore the basalmost bilaterian phylum.[1]

References

  1. 1 2 3 Rouse, Greg W.; Wilson, Nerida G.; Carvajal, Jose I.; Vrijenhoek, Robert C. (2016-02-03). "New deep-sea species of Xenoturbella and the position of Xenacoelomorpha". Nature (530): 94–97. doi:10.1038/nature16545. Retrieved 2016-02-03.
  2. ↑ Westblad, E. (1949) "Xenoturbella bocki n. g., n. sp., a peculiar, primitive Turbellarian type". Arkiv för Zoologi 1:3-29
  3. 1 2 Israelsson, O. (1999). "New light on the enigmatic Xenoturbella (phylum uncertain): ontogeny and phylogeny". Proceedings of the Royal Society B: Biological Sciences. 266 (1421): 835–841. doi:10.1098/rspb.1999.0713. ISSN 0962-8452.
  4. ↑ McCafferty, Georgia (February 4, 2016). "Deep-sea 'purple sock' provides clues to early life". CNN. Archived from the original on May 6, 2016. Retrieved May 6, 2016.
  5. ↑ Perea-Atienza, E.; Gavilan, B.; Chiodin, M.; Abril, J. F.; Hoff, K. J.; Poustka, A. J.; Martinez, P. (2015). "The nervous system of Xenacoelomorpha: a genomic perspective". Journal of Experimental Biology. 218 (4): 618–628. doi:10.1242/jeb.110379. ISSN 0022-0949.
  6. ↑ 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.
  7. ↑ Israelsson, Olle; Budd, Graham E. (2005). "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. ISSN 0949-944X.
  8. 1 2 3 4 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 3586728Freely accessible. PMID 23443565.
  9. ↑ WoRMS: Xenoturbella Westblad, 1949
  10. ↑ WoRMS: Xenoturbella bocki (Westblad, 1949)
  11. ↑ WoRMS: Xenoturbella westbladi Reisinger, 1960
  12. ↑ WoRMS: Xenoturbella westbladi (Israelsson, 1999)
  13. ↑ Norén, Michael; Jondelius, Ulf (1997). "Xenoturbella 's molluscan relatives". Nature. 390 (6655): 31–32. doi:10.1038/36242. ISSN 0028-0836.
  14. ↑ Bourlat, Sarah J.; Nielsen, Claus; Lockyer, Anne E.; Littlewood, D. Timothy J.; Telford, Maximilian J. (2003). "Xenoturbella is a deuterostome that eats molluscs". Nature. 424 (6951): 925–928. doi:10.1038/nature01851. ISSN 0028-0836.
  15. ↑ Bourlat, Sarah J.; Juliusdottir, Thorhildur; Lowe, Christopher J.; Freeman, Robert; Aronowicz, Jochanan; Kirschner, Mark; Lander, Eric S.; Thorndyke, Michael; Nakano, Hiroaki; Kohn, Andrea B.; Heyland, Andreas; Moroz, Leonid L.; Copley, Richard R.; Telford, Maximilian J. (2006). "Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida". Nature. 444 (7115): 85–88. doi:10.1038/nature05241. ISSN 0028-0836.
  16. ↑ 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
  17. ↑ 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.
  18. ↑ 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.
  19. ↑ 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.
  20. ↑ 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.
  21. ↑ 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.

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