|Philippe , Brinkmann , Copley , Moroz , Nakano , Poustka , Wallberg , Peterson & Telford , 2011|
The Xenacoelomorpha are a strain of worm-like animals that was only described in 2009 . It consists of two sub-strains, the Acoelomorpha with about 370 species and the Xenoturbellida, which consists of only six species . Both sub-tribes originally had the rank of tribes themselves.
The species of Xenacoelomorpha are often only a few millimeters in size, the largest known species Xenoturbella monstrosa can, however, reach a length of up to 20 centimeters. They have no continuous intestine, no anus (the mouth also serves as an excretory organ), no gill arches and no coelom (body cavity). They live in the sea, between the particles of the sediment , in planktonic form , on the surface of algae or corals or in the intestines of sea cucumbers , where they mostly feed on organic particles and develop directly without the larval stage.
The Acoelomorpha were originally part of the class of the vortex worms (Turbellaria) and thus to the tribe of the flatworms (Plathelminthes), later placed at the base of the Bilateria . The Xenoturbellida were also assigned to the vortex worms after their discovery and first description. In 2003, after a long period of uncertainty and various hypotheses, they were initially placed in the Neumünder.
Investigations of the microRNA , of amino acids of the completely sequenced mitochondrial DNA of some species, as well as of several hundred genes showed that these animals are related to each other and to the deuterostomia. The RSB66 gene was detected, which so far could only be found in deuterostomies. The microRNA of the Xenacoelomorpha is so far only known from echinoderms and acorn worms , two taxa of the Deuterostomia. According to a hypothesis formulated in 2011, which is based on the comparison of different homologous DNA sequences ( phylogenomics ), the group was positioned within the Deuterostomia. According to this hypothesis, the Xenacoelomorpha have simplified their blueprint in the course of evolution and have lost many of the features characteristic of deuterostomies, so that, with the exception of some features of the fine structure of the epidermis, no morphological features exist for this placement. However, the study was criticized for methodological reasons. Later analyzes showed a different position. Accordingly, the group is more likely the sister group of the rest of the Bilateria put together, that is, the common group of Protostomia and Deuterostomia, known as Taxon Nephrozoa. However, this placement is still fraught with uncertainties and may shift again due to more recent findings.
The systematic position of the Xenacoelomorpha, according to this hypothesis, is shown in the following cladogram :
- Hiroaki Nakano, Kennet Lundin, Sarah J. Bourlat, Maximilian J. Telford, Peter Funch, Jens R. Nyengaard, Matthias Obst, Michael C. Thorndyke: Xenoturbella bocki exhibits direct development with similarities to Acoelomorpha. In: Nature Communications. Article number: 1537, February 2013, doi: 10.1038 / ncomms2556
- Seth Tyler, Stephen Schilling: Phylum Xenacoelomorpha. (PDF; 33 kB). In: Zootaxa . 3148, Magnolia Press, 2011.
- Greg W. Rouse, Nerida G. Wilson, Jose I. Carvajal, Robert C. Vrijenhoek: New deep-sea species of Xenoturbella and the position of Xenacoelomorpha. In: Nature . 530, February 4, 2016, pp. 94–97 doi: 10.1038 / nature16545
- A. Mwinyi, X. Bailly, SJ Bourlat, U. Jondelius, DTJ Littlewood, L. Podsiadlowski: The phylogenetic position of Acoela as revealed by the complete mitochondrial genome of Symsagittifera roscoffensis. In: BMC Evolutionary Biology. 10, 2010, p. 309. doi: 10.1186 / 1471-2148-10-309
- E. Westblad: Xenoturbella Bocki ng, n.sp., a peculiar primitive turbellarian type. In: Arkiv för Zoologi. Vol. 1, 1949, pp. 3-29.
- SJ Bourlat, C. Nielsen, AE Lockyer, D. Timothy, J. Littlewood, MJ Telford: Xenoturbella is a deuterostome that eats molluscs. In: Nature. Vol. 424, 2003, pp. 925–928, (nature.com)
- Hervé Philippe, Henner Brinkmann, Richard R. Copley, Leonid L. Moroz, Hiroaki Nakano, Albert J. Poustka, Andreas Wallberg, Kevin J. Peterson, Maximilian J. Telford: Acoelomorph flatworms are deuterostomes related to Xenoturbella. In: Nature . 470, February 10, 2011, pp. 255-258, doi: 10.1038 / nature09676
- KJ Pedersen, LR Pedersen: Ultrastructural observations on the epidermis of Xenoturbella bocki Westblad, 1949, with a discussion of epidermal cytoplasmic filament systems of invertebrates. In: Acta Zoologica. 69, 1988, pp. 231-246. doi: 10.1111 / j.1463-6395.1988.tb00920.x
- Amy Maxmen: A can of worms. In: Nature. 470, 2011, pp. 161-162. (PDF)
- JT Cannon, B. Cossermelli, J. Smith III, F. Ronquist, U. Jondelius, A. Hejnol: Xenacoelomorpha is the sister group to Nephrozoa. In: Nature. 530, 2016, pp. 89-93. doi: 10.1038 / nature16520
- GW Rouse, NG Wilson, JI Carvajal, RC Vrijenhoek: New deep-sea species of Xenoturbella and the position of Xenacoelomorpha. In: Nature. 530, 2016, pp. 94-97. doi: 10.1038 / nature16545
- Iñaki Ruiz-Trillo, Jordi Paps: Acoelomorpha: earliest branching bilaterians or deuterostomes? In: Organisms Diversity & Evolution. 16 (2), 2016, pp. 391-399. doi: 10.1007 / s13127-015-0239-1
- Max Planck Society: Xenacoelomorpha - a new strain in the animal kingdom
- Phys.org: Xenacoelomorpha - a new phylum in the animal kingdom