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Xenoturbella

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Xenoturbella
Xenoturbella japonica. The white arrowhead indicates the ring furrow.
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Xenacoelomorpha
Subphylum: Xenoturbellida
Westblad, 1949
Family: Xenoturbellidae
Westblad, 1949
Genus: Xenoturbella
Westblad, 1949[1][2]

Xenoturbella is a genus of very simple bilaterians up to a few centimeters long. It contains a small number of marine benthic worm-like species.[3]

The first known species (Xenoturbella bocki) was discovered in 1915 by Sixten Bock, but it was only properly described in 1949 by Einar Westblad.[4]

Description

Xenoturbella bockii longitudinal section

Xenoturbella has a very simple body plan. It consists of dorsoventrally flattened acoelomate animals, with an anterior circumferential furrow.[5] It shows two ciliated epithelial layers: an external epidermis and an internal gastrodermis lining the simple sac-like gut. The multiciliated epiderm displays unique interconnected ciliary rootlets and mode of withdrawal and resorption of worn epidermal cells.[5] The mouth is a mid-ventral pore leading to a gastral cavity, and there is no anus:[6][5] waste is dispelled through the same opening as food is taken in.[7]

The nervous system is composed by a net of interconnected neurons beneath the epidermis, without any concentration of neurons forming ganglia or nerve cords.[8][9]

Species of Xenoturbella also lack a respiratory, circulatory and excretory system. In fact, there are no defined organs, except for an anterior statocyst containing flagellated cells and a frontal pore organ.[6][5] 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.[10]

Eggs of Xenoturbella are 0.2 millimetres (0.0079 in) wide, pale orange and opaque.[11] 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.[11] They are similar to the juveniles of acoelomate Neochildia fusca.[11]

Systematics

Etymology

The term Xenoturbella derives from the Ancient Greek word ξένος (xénos), meaning "strange, unusual",[12][13] and from the Latin word turbella meaning "stir, bustle".[14] This refers to the enigmatic, unusual taxonomic status of the animal, initially considered as related to turbellarians, a group of flatworms whose aquatic species stir microscopic particles close to their ciliated epidermis.[15]

Taxonomy

Currently the genus Xenoturbella contains 6 recognized species:[16]

Phylogeny

Among species

To date, the genus Xenoturbella is composed of six species distributed into a shallow-water clade — three species up to 400–650 metres (1,310–2,130 ft) — and a deep-water clade — three species deeper than 1,700 metres (5,600 ft).

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. They form a clade together with a third species, X. japonica, which is slightly over 5 centimetres (2.0 in) long and was found in waters less than 560 metres (1,840 ft) deep.[21] Three larger species, X. monstrosa, X. churro, and X. profunda, which were 10 centimetres (3.9 in) or greater long and lived in deeper waters 1,700–3,700 metres (5,600–12,100 ft), form another clade.[3]

Species-level cladogram of the genus Xenoturbella.
  Xenacoelomorpha  

  Acoelomorpha  

  Xenoturbella  
  'Shallow' clade  
         

  X. japonica

         

  X. bocki

  X. hollandorum

  'Deep' clade  
         

  X. monstrosa

         

  X. churro

  X. profunda

The cladogram has been reconstructed from mitochondrial DNA and protein sequences.[3][21]

Among animals

The systematic and phylogenetic position of Xenoturbella among animals has been considered enigmatic since its discovery. An early DNA analysis suggested a close relationship to molluscs,[23] but it was probably a result from contamination with DNA of molluscs that Xenoturbella consumes.[24]

A subsequent study suggested a placement of the genus in its own phylum, Xenoturbellida, as a deuterostome clade and sister group to the Ambulacraria.[25] The deuterostome affiliations were then recovered by studies that indicate a basal position of this phylum within the deuterostomes[26][27] or in a sister group relationship with the Ambulacraria.[28]

However, morphological characters, such as the structure of epidermal cilia, suggested a close relationship with Acoelomorpha, another problematic group.[29] The study of the embryonic stages of Xenoturbella also showed that it is a direct developer without a feeding larval stage, and this developmental mode is similar to that of acoelomorphs.[11] Molecular studies based on the concatenation of hundreds of proteins revealed indeed a monophyletic group composed by Xenoturbella and Acoelomorpha.[30][28][31] This clade was named Xenacoelomorpha.[28]

The monophyly of Xenacoelomorpha soon became established, but its position as either a basal bilaterian clade or a deuterostome remained unresolved until 2016 when two new studies, with increased gene and taxon sampling, 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.[3][32]

References

  1. ^ Zhang, Zhi-Qiang (2011-12-23). "Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness". Zootaxa. 3148. Magnolia Press: 1–237. doi:10.11646/zootaxa.3148.1.1. Retrieved 2016-02-03.
  2. ^ Tyler, S.; Schilling, S.; Hooge, M.; Bush, L.F. (2006–2016). "Xenoturbella". Turbellarian taxonomic database. Version 1.7. Retrieved 2016-02-03.
  3. ^ a b c d e 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 (7588): 94–97. Bibcode:2016Natur.530...94R. doi:10.1038/nature16545. PMID 26842060. S2CID 3870574.
  4. ^ Westblad, E (1949). "Xenoturbella bocki n. g., n. sp., a peculiar, primitive Turbellarian type". Arkiv för Zoologi. 1: 3–29.
  5. ^ a b c d Giribet, Gonzalo; Edgecombe, Gregory D. (2020-03-03). The Invertebrate Tree of Life. Princeton University Press. p. 88–89. ISBN 978-0-691-17025-1.
  6. ^ a b 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. PMC 1689910.
  7. ^ McCafferty, Georgia (February 4, 2016). "Deep-sea 'purple sock' provides clues to early life". CNN. Archived from the original on February 5, 2016. Retrieved May 6, 2016.
  8. ^ 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. PMID 25696825.
  9. ^ 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 (2): 107–118. doi:10.1007/s004350000028. S2CID 19668017.
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  13. ^ Bailly, Anatole. "Greek-french dictionary online". www.tabularium.be. Retrieved 16 March 2020.
  14. ^ Gaffiot, Félix (1934). Dictionnaire illustré Latin-Français (in French). Paris: Librairie Hachette. p. 1613. Retrieved 16 March 2020.
  15. ^ Ruppert, E.E.; Fox, R.S.; Barnes, R.D. (2004). Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 228. ISBN 0-03-025982-7.
  16. ^ WoRMS: Xenoturbella Westblad, 1949
  17. ^ WoRMS: Xenoturbella bocki (Westblad, 1949)
  18. ^ http://www.marinespecies.org/aphia.php?p=taxdetails&id=879660 AphiaID: 879660 has been DELETED - reason: Database artifact, incorrect authority - WoRMS]
  19. ^ Xenoturbella westbladi Israelsson, 1999 - WoRMS
  20. ^ "Newly discovered deep-sea worms, including one named 'churro,' could shed light on animal evolution". Los Angeles Times. 2016-02-05. Retrieved 2020-02-13.
  21. ^ a b c Nakano, Hiroaki; Miyazawa, Hideyuki; Maeno, Akiteru; Shiroishi, Toshihiko; Kakui, Keiichi; Koyanagi, Ryo; Kanda, Miyuki; Satoh, Noriyuki; Omori, Akihito; Kohtsuka, Hisanori (2017). "A new species of Xenoturbella from the western Pacific Ocean and the evolution of Xenoturbella". BMC Evolutionary Biology. 17 (1): 245. doi:10.1186/s12862-017-1080-2. PMC 5733810. PMID 29249199.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  22. ^ Starr, Michelle. "These Deep Sea Worms Without Butts Likely Haven't Evolved For Millions of Years". ScienceAlert. Retrieved 2020-02-13.
  23. ^ Norén, Michael; Jondelius, Ulf (1997). "Xenoturbella 's molluscan relatives". Nature. 390 (6655): 31–32. Bibcode:1997Natur.390...31N. doi:10.1038/36242. ISSN 0028-0836. S2CID 4426393.
  24. ^ 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. Bibcode:2003Natur.424..925B. doi:10.1038/nature01851. ISSN 0028-0836. PMID 12931184. S2CID 4413357.
  25. ^ 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. Bibcode:2006Natur.444...85B. doi:10.1038/nature05241. ISSN 0028-0836. PMID 17051155. S2CID 4366885.
  26. ^ Perseke, Marleen; Hankeln, Thomas; Weich, Bettina; Fritzsch, Guido; Stadler, Peter F.; Israelsson, Olle; Bernhard, Detlef; Schlegel, Martin (2007-05-10). "The mitochondrial DNA of Xenoturbella bocki: genomic architecture and phylogenetic analysis". Theory in Biosciences. 126 (1): 35–42. doi:10.1007/s12064-007-0007-7. ISSN 1431-7613. PMID 18087755. S2CID 17065867.[permanent dead link‍]
  27. ^ 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. S2CID 5379221.
  28. ^ a b c 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. PMC 4025995. PMID 21307940.
  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 (3): 263–270. doi:10.1111/j.1463-6409.1998.tb00440.x. S2CID 85324766.
  30. ^ 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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  32. ^ Cannon, Johanna T.; Vellutini, Bruno C.; Smith III, Julian.; Ronquist, Frederik; Jondelius, Ulf; Hejnol, Andreas (2016-02-03). "Xenacoelomorpha is the sister group to Nephrozoa". Nature. 530 (7588): 89–93. Bibcode:2016Natur.530...89C. doi:10.1038/nature16520. PMID 26842059. S2CID 205247296.

Further reading

Wikispecies has information related to Xenoturbella.
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