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Ecdysozoa

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The Ecdysozoa are a group of protostome animals, including the Arthropoda (insects, arachnids, crustaceans, et cetera), 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. However, the group could also be 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 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 is called ecdysis and gives the group its name. The advocates of the Ecdysosoa hypothesis allege that Ecdysozoans lack locomotory cilia, produce 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 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 Panarthropoda are distinguished by a segmented body plan, and as such were traditionally believed to have evolved from the Annelida (segmented worms), together comprising the Articulata. However, they do not have many other characteristics in common, so it now appears that they developed segmentation separately.

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

Ecdysozoa controversy

It should be noted that the grouping proposed by Aguinaldo et al. is not universally accepted. Many zoologists still hold to the view that Panarthropoda should be classified with Annelida and that Ecdysozoa are polyphyletic. A closer examination of the characters supporting the monophyly of Ecdysozoa shows that some of them are dubious at least if at all factually true. Cuticular epithelia are widely spread over diverse phyla of invertebrates (including some groups outside Ecdysozoa) 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, which means that it is most likely that it has evolved independently. Many members of Panarthropoda do have flagellate (and not amoeboid) spermia (Camatini et al., 2005, Jamieson, 1987), and those with a small amount of yolk in their eggs undergo spiral cleavage. Recent investigations show also that Annelids, as well as some other groups, are moulting animals, too, even though their moulting is less dramatic than in Arthropods (Pilato et al., 2005). Molecular evidence for the monophyly of Ecdysozoa is also not unquestionable, and the whole controversy is still far from closure.

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