Xenoturbella Explained

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.[1]

The first known species (Xenoturbella bocki) was collected in 1878 and 1879 in the Gullmar fiord on the Swedish west coast by August Malm and is stored in the collection of the Gothenburg Natural History Museum.[2] A specimen is on display in the exhibition. It was collected again in the Gullmar fiord in 1915 by Sixten Bock, but it was only properly described in 1949 by Einar Westblad.[3] The type specimens are kept at the Swedish national museum of natural history in Stockholm.

Description

Xenoturbella has a very simple body plan. It consists of a dorsoventrally flattened acoelomate body, with a ventral furrow on each side running down from the anterior tip till they are stopped by an anterior circumferential furrow.[4] [5] It shows two ciliated epithelial layers: an external epidermis and an internal gastrodermis lining the simple sac-like gut. The epidermis and gastrodermis is separated by a thick and multilayered basement membrane called the "subepidermal membrane complex", a major part of the extracellular matrix.[6] [7] The multiciliated epidermis displays unique interconnected ciliary rootlets and mode of withdrawal and resorption of worn epidermal cells.[4] The mouth is a mid-ventral pore leading to a gastral cavity, and there is no anus:[8] [4] waste is dispelled through the same opening as food is taken in.[9]

The nervous system is composed by a net of interconnected neurons beneath the epidermis, without any concentration of neurons forming ganglia or nerve cords.[10] [11]

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.[8] [4] 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.[12]

Research on the species Xenoturbella bocki has shown it to have external fertilization, with eggs and sperm being released from new openings in the body wall. Gametes released into the water through ruptures also occurs in Xenoturbella's closest relatives the acoels and nemertodermatids. No examples of hermaphroditism was reported.[13] [14]

Eggs of Xenoturbella are wide, pale orange and opaque.[15] 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.[15] They are similar to the juveniles of acoelomate Neochildia fusca.[15]

Systematics

Etymology

The term Xenoturbella derives from the Ancient Greek word, meaning "strange, unusual",[16] [17] and from the Latin word meaning "stir, bustle".[18] 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.[19]

Taxonomy

Currently the genus Xenoturbella contains six recognized species:[20]

Phylogeny

Among species

To date, the genus Xenoturbella is composed of six species distributed into a shallow-water clade—three species up to 400-—and a deep-water clade—three species deeper than .

The two smaller species, X. bocki and X. hollandorum, which are up to long, are found in shallower waters less than deep. They form a clade together with a third species, X. japonica, which is slightly over long and was found in waters less than deep.[24] Three larger species, X. monstrosa, X. churro, and X. profunda, which were or greater long and lived in deeper waters 1700-, form another clade.[1]

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,[26] but it was probably a result from contamination with DNA of molluscs that Xenoturbella consumes.[27]

A subsequent study suggested a placement of the genus in its own phylum, Xenoturbellida, as a deuterostome clade and sister group to the Ambulacraria.[28] The deuterostome affiliations were then recovered by studies that indicate a basal position of this phylum within the deuterostomes[29] [30] or in a sister group relationship with the Ambulacraria.[31]

However, morphological characters, such as the structure of epidermal cilia, suggested a close relationship with Acoelomorpha, another problematic group.[32] 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. Molecular studies based on the concatenation of hundreds of proteins revealed indeed a monophyletic group composed by Xenoturbella and Acoelomorpha.[33] [34] This clade was named Xenacoelomorpha.

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.[1] [35]

Further reading

External links

Notes and References

  1. 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 . 10.1038/nature16545 . 26842060. 2016Natur.530...94R . 3870574 .
  2. News: Paulsen . Hege . Vår urmor i havet . 17 August 2023 . abc/nyheter . March 17, 2013 . paulsen2013.
  3. Westblad . E . 1949 . Xenoturbella bocki n. g., n. sp., a peculiar, primitive Turbellarian type . Arkiv för Zoologi . 1 . 3–29 .
  4. Book: Giribet . Gonzalo . Edgecombe . Gregory D. . The Invertebrate Tree of Life . 2020-03-03 . Princeton University Press . 88–89 . 978-0-691-17025-1 . en.
  5. https://zoologicalletters.biomedcentral.com/articles/10.1186/s40851-015-0018-z What is Xenoturbella? | Zoological Letters - BioMed Central
  6. https://books.google.com/books?id=YHetDwAAQBAJ&dq=basement+membrane+termed+%22subepidermal+membrane+complex%22&pg=PA86 The Invertebrate Tree of Life
  7. https://discovery.ucl.ac.uk/id/eprint/10038990/1/Helen_Robertson_Final_thesis.pdf Molecular approaches for studying the evolution of the Xenacoelomorpha
  8. Israelsson. O.. New light on the enigmatic Xenoturbella (phylum uncertain): ontogeny and phylogeny. Proceedings of the Royal Society B: Biological Sciences. 266. 1421. 1999. 835–841. 0962-8452. 10.1098/rspb.1999.0713. 1689910.
  9. Web site: Deep-sea 'purple sock' provides clues to early life . CNN . Georgia . McCafferty. February 4, 2016 . https://web.archive.org/web/20160205094344/http://www.cnn.com/2016/02/04/world/deep-sea-purple-sock-early-life/ . February 5, 2016 . live . May 6, 2016.
  10. Perea-Atienza. E.. Gavilan. B.. Chiodin. M.. Abril. J. F.. Hoff. K. J.. Poustka. A. J.. Martinez. P.. The nervous system of Xenacoelomorpha: a genomic perspective. Journal of Experimental Biology. 218. 4. 2015. 618–628. 0022-0949. 10.1242/jeb.110379. 25696825. free. 2445/192702. free.
  11. 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 . 10.1007/s004350000028. 19668017 .
  12. Israelsson. Olle. Budd. Graham E.. Eggs and embryos in Xenoturbella (phylum uncertain) are not ingested prey. Development Genes and Evolution. 215. 7. 2005. 358–363. 0949-944X. 10.1007/s00427-005-0485-x. 15818482. 9078623.
  13. https://www.techexplorist.com/unique-reproductive-strategy-discovered-deep-sea-worm-creatures/57243/ A unique reproductive strategy discovered in deep-sea worm-like creatures
  14. 10.1038/s42003-023-04549-z . Induced spawning with gamete release from body ruptures during reproduction of Xenoturbella bocki . 2023 . Nakano . Hiroaki . Nakano . Ako . Maeno . Akiteru . Thorndyke . Michael C. . Communications Biology . 6 . 1 . 172 . 36805023 . 9938242 .
  15. 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–. 10.1038/ncomms2556. 3586728. 23443565. Nakano. H.. 2013NatCo...4.1537N.
  16. Book: Bailly, Anatole . Abrégé du dictionnaire grec français . 1981-01-01 . Hachette . 2010035283 . Paris . 461974285.
  17. Web site: Greek-french dictionary online . Bailly . Anatole . www.tabularium.be . 16 March 2020.
  18. Book: Gaffiot, Félix . fr . 1934 . Dictionnaire illustré Latin-Français . Paris . Librairie Hachette . 1613 . 16 March 2020.
  19. Book: Ruppert . E.E. . Fox . R.S. . Barnes . R.D. . Invertebrate Zoology . Brooks / Cole . 7 . 0-03-025982-7 . 2004 . 228 . registration .
  20. http://www.marinespecies.org/aphia.php?p=taxdetails&id=138665 WoRMS: Xenoturbella Westblad, 1949
  21. http://www.marinespecies.org/aphia.php?p=taxdetails&id=141971 WoRMS: Xenoturbella bocki (Westblad, 1949)
  22. http://www.marinespecies.org/aphia.php?p=taxdetails&id=141972 Xenoturbella westbladi Israelsson, 1999 - WoRMS
  23. Web site: Newly discovered deep-sea worms, including one named 'churro,' could shed light on animal evolution . 2016-02-05 . Los Angeles Times . en-US . 2020-02-13.
  24. 10.1186/s12862-017-1080-2 . 29249199 . A new species of Xenoturbella from the western Pacific Ocean and the evolution of Xenoturbella . BMC Evolutionary Biology . 17 . 1 . 245 . 2017 . Nakano . Hiroaki . Miyazawa . Hideyuki . Maeno . Akiteru . Shiroishi . Toshihiko . Kakui . Keiichi . Koyanagi . Ryo . Kanda . Miyuki . Satoh . Noriyuki . Omori . Akihito . Kohtsuka . Hisanori . 5733810 . free .
  25. Web site: These Deep Sea Worms Without Butts Likely Haven't Evolved For Millions of Years . Starr . Michelle . ScienceAlert . 20 December 2017 . en-gb . 2020-02-13.
  26. Norén. Michael. Jondelius. Ulf. Xenoturbella 's molluscan relatives. Nature. 390. 6655. 1997. 31–32. 0028-0836. 10.1038/36242. 1997Natur.390...31N. 4426393.
  27. 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. 10.1038/nature01851. 0028-0836. Bourlat. Sarah J.. 12931184. 2003Natur.424..925B. 4413357.
  28. 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.. Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida. Nature. 444. 7115. 2006. 85–88. 0028-0836. 10.1038/nature05241. 17051155. 2006Natur.444...85B. 4366885.
  29. 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. 10.1007/s12064-007-0007-7. 18087755. 17065867. 1431-7613. https://web.archive.org/web/20220324010641/https://ul.qucosa.de/api/qucosa%3A31999/attachment/ATT-0/. dead. March 24, 2022.
  30. Telford . M. J. . Xenoturbellida: the fourth deuterostome phylum and the diet of worms . 10.1002/dvg.20414 . Genesis . 46 . 11 . 580–586 . 2008 . 18821586 . 5379221 . free .
  31. Philippe . H. . Brinkmann . H. . Copley . R. R. . Moroz . L. L. . Nakano . H. . Poustka . A. J. . Wallberg . A. . Peterson . K. J. . Telford . M. J. . Acoelomorph flatworms are deuterostomes related to Xenoturbella . Nature . 470 . 255–258 . 2011 . 21307940 . 10.1038/nature09676 . 2011Natur.470..255P . 7333 . 4025995.
  32. 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 . 10.1111/j.1463-6409.1998.tb00440.x. 85324766 .
  33. 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.
  34. 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. 151–172. 10.1007/s13127-011-0044-4. Edgecombe. G. D.. 32169826.
  35. 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 . 10.1038/nature16520 . 26842059. 2016Natur.530...89C . 205247296 .

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