name = Ecdysozoa
fossil_range = Early Cambrian - Recent

image_width = 225px
image_caption = Centipede
regnum = Animalia
subregnum = Eumetazoa
unranked_phylum = Bilateria
superphylum = Ecdysozoa
superphylum_authority = Aguinaldo et al., 1997
subdivision_ranks = Phyla
subdivision =
* Kinorhyncha
* Loricifera
* Priapulida
* Nematoda
* Nematomorpha
* Lobopodia
* Onychophora
* Tardigrada
* Arthropoda

The Ecdysozoa (pronEng|ˌɛkdɪsəˈzoʊə) are a grouping of protostome animals, including the Arthropoda (insects, chelicerata, crustaceans, and myriapods), Nematoda, and several smaller phyla. They were first defined by Aguinaldo "et al." in 1997, based mainly on trees constructed using 18S ribosomal RNA genes. [cite journal|last=Aguinaldo|first=A. M. A.|coauthors=J. M. Turbeville, L. S. Linford, M. C. Rivera, J. R. Garey, R. A. Raff, & J. A. Lake|year=1997|title=Evidence for a clade of nematodes, arthropods and other moulting animals|journal=Nature|volume= 387|pages=489–493|doi=10.1038/387489a0] A large study in 2008 by Dunn "et al." strongly supported the ecdysozoa as a natural grouping.cite journal|last=Dunn "et al."|title=Broad phylogenomic sampling improves resolution of the animal tree of life|journal=Nature|volume=452|pages=745–749|url=|doi=10.1038/nature06614|year=2008]

The group is also supported by morphological characters, and can be considered as including all animals that shed their exoskeleton (see ecdysis). Groups corresponding roughly to the Ecdysozoa had been proposed previously by Perrier in 1897 and Seurat in 1920 based on morphology alone.

The group has been contested by a significant minority of biologists. Some have argued for groupings based on more traditional taxonomic techniques,cite book|title=Animal Evolution: Interrelationships of the Living Phyla|first=Claus|last=Nielsen|publisher=Oxford University Press|year=1995|isbn=9780198506829] whilst others have contested the interpretation of the molecular data.cite journal|last=Blair|first=J. E.|coauthors=Kazuho Ikeo, Takashi Gojobori and S. Blair Hedges|url=|title=The evolutionary position of nematodes|journal=BMC Evolutionary Biology|year=2002|volume=2|pages=7|doi=10.1186/1471-2148-2-7] [cite journal|last=Wägele|first=J. W.|coauthors=T. Erikson, P. Lockhart, & B. Misof|year=1999|title=The Ecdysozoa: Artifact or monophylum?|journal=Journal of Zoological Systematics and Evolutionary Research|volume=37|pages=211–223]

Group characters

The most notable characteristic shared by ecdysozoans is a three-layered cuticle composed of organic material, which is periodically molted as the animal grows. This process of molting is called ecdysis and gives the group its name. The Ecdysozoans lack locomotory cilia, produce mostly amoeboid sperm, and their embryos do not undergo spiral cleavage as in most other protostomes. Various other features are found in the group, for instance, both tardigrades, pycnogonids and roundworms have a triradiate pharynx.

The Ecdysozoa include the following phyla: Arthropoda, Onychophora, Tardigrada, Kinorhyncha, Priapulida, Loricifera, Nematoda and Nematomorpha. A few other groups, such as the gastrotrichs, have been considered possible members but lack the main characters of the group, and are now placed elsewhere. The Arthropoda, Onychophora and Tardigrada have been grouped together as the Panarthropoda because they are distinguished by segmented body plans. [ [ Paleos Invertebrates: Panarthropoda] - URL retrieved February 17, 2007] Dunn "et al" in 2008 suggested that the tardigrada could be grouped along with the nematodes, leaving Onychophora as the sister group to the arthropods.

The non-panarthropod members of Ecdysozoa have been grouped as Cycloneuralia but they are more usually considered paraphyletic.

clade| style=font-size:75%;line-height:75%

1=clade |1= clade |1=Tardigrade |2=clade

2= clade

1=Onychophora |2=clade
1=Tetraconata |2=clade
1=Myriapoda |2=Chelicerata


A phylogenetic tree of the Ecdysozoa hypothesis as suggested by Dunn "et al" (2008)


The grouping proposed by Aguinaldo "et al." is not universally accepted. Some zoologists still hold to the original view that Panarthropoda should be classified with Annelida in a group called the Articulata, and that Ecdysozoa are polyphyletic. The highly derived roundworms, with their many highly derived parasitic taxa and a considerable number of autapomorphies continue to pose problems, and are one of the most contested inclusions of grouping.
# Cuticular epithelia are widely spread over diverse phyla of invertebrates (including some groups outside Ecdysozoa, such as annelids and molluscs, where it acts as a skin instead of an exoskeleton) and show a considerable degree of variation. They are believed to have evolved independently, at least in some groups. In Nematoda and Panarthropoda, the cuticle is different in both chemical composition and ultrastructure. While the cuticle in arthropods like insects contains chitin, or can be a combination of both chitin and keratin in crustaceans, chitin has never been found in the complex cuticle of Nematoda which is a fibrous and multilayered structure made of collagen and keratin of types unique to the Nematoda.Fact|date=May 2008
# Molecular evidence for the monophyly of Ecdysozoa is also ambiguous. [cite journal|last=Wägele|first=J. W. |coauthors=B. Misof|year=2001|doi=10.1046/j.1439-0469.2001.00177.x |title=On quality of evidence in phylogeny reconstruction: a reply to Zrzavý's defence of the 'Ecdysozoa' hypothesis|journal=J. Zool. Syst. Evol. Research|volume=39|issue=3|pages=165–176]

One of the proposed solutions is to regard Ecdysozoa as a sister-group of Annelida, [Nielsen, C. (2003) [ Proposing a solution to the Articulata–Ecdysozoa controversy.] "Zoologica Scripta" 32:5, 475-482] however the controversy is still far from closure. [Jenner, Ronald A. [ Unleashing the force of cladistics? Metazoan phylogenetics and hypothesis testing.] "Integrative and Comparative Biology," Feb 2003]


External links

* [ UCMP-Ecdysozoa introduction]

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