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Ecdysozoa

(Infrakingdom)

Overview

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The Ecdysozoa are a grouping of animals,1] 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.[2] A large study in 2008 by Dunn et al. strongly supported the ecdysozoa as a natural grouping.[3]

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,[4] whilst others have contested the interpretation of the molecular data.[5][6]

Group Characters

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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, prod uce 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, 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.[7] 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.[3]

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

Criticism

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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.

  1. Cuticular epithelia are widely spread over diverse phyla of invertebrates (including some groups outside Ecdysozoa, such as annelids and molluscs, whe re 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.[citation needed]
  2. Molecular evidence for the monophyly of Ecdysozoa is also ambiguous.[5][8]

One of the proposed solutions is to regard Ecdysozoa as a sister-group of Annelida,[9] however the controversy is still far from closure.[10]

Photos

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Taxonomy

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The Infrakingdom Ecdysozoa is a member of the Branch Protostomia. Here is the complete "parentage" of Ecdysozoa:

The Infrakingdom Ecdysozoa is further organized into finer groupings including:

Phyla

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Arthropoda

An arthropod is an that has an exoskeleton (external skeleton), a segmented body, and jointed attachments called appendages. Arthropods are animals belonging to the Phylum Arthropoda (from Greek ?????? arthron, "joint", and p?d?? podos "foot", which together mean "jointed feet"), and include the insects, arachnids, crustaceans, and others. Arthropods are characterized by their jointed limbs and cuticles, which are mainly made of a-chitin; the cuticles of crustaceans are also biomineralized with calcium carbonate. The rigid cuticle inhibits growth, so arthropods replace it periodically by molting. The arthropod body plan consists of repeated segments, each with a pair of appendages. It is so versatile that they have been compared to Swiss Army knives, and it has enabled them to become the most species-rich members of all ecological guilds in most environments. They have over a million described species, making up more than 80% of all described living species, and are one of only two animal groups that are really successful in dry environments – the other being the amniotes. They range in size from microscopic plankton up to forms a few meters long. [more]

Chaetognatha

Chaetognatha, meaning hair-jaws, is a of predatory marine worms that are a major component of plankton worldwide. About 20% of the known species are benthic and can attach to algae or rocks. They are found in all marine waters from surface tropical waters and shallow tide pools to the deep sea and polar regions. Most chaetognaths are transparent and are torpedo shaped. Some deep-sea species are orange. They range in size from 2 mm to 12 cm. The common term for the phylum is Arrow Worms. There are more than 120 modern species assigned to over 20 genera. Despite the limited diversity of species, the number of individuals is staggering. [more]

Kinorhyncha

Kinorhyncha (Gr. ?????, kineo 'move' + ??????, rhynchos 'snout') is a of small (1 mm or less) marine pseudocoelomate invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos. They are also called mud dragons. [more]

Loricifera

Nematoda

The "roundworms" or "nematodes" ( Nematoda) are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 80,000 have been described, of which over 15,000 are parasitic. It has been estimated that the total number of described and undescribed roundworms might be more than 500,000. Unlike cnidarians or flatworms, roundworms have a digestive system that is like a tube at both ends. [more]

Nematomorpha

Nematomorpha (sometimes called Gordiacea, and commonly known as Horsehair worms or Gordian worms) are a of parasitic animals that are morphologically and ecologically similar to nematode worms, hence the name. They range in size from 1cm to 1 meter long, and 1 to 3 millimetres in diameter. Horsehair worms can be discovered in damp areas such as watering troughs, streams, puddles, and cisterns. The adult worms are free living, but the larvae are parasitic on beetles, cockroaches, Orthoptera and crustaceans. About 326 species are known and a conservative estimate suggests that there may be about 2000 species worldwide. The name "Gordian" stems from the legendary Gordian knot. This relates to the fact that nematomorpha often tie themselves in knots. [more]

Onychophora

The velvet worms (Onychophora — literally "claw bearers") form a within the Ecdysozoa and can be simply described as "worms with legs". Most common in the Southern Hemisphere, they prey on smaller animals such as insects, which they catch by squirting a sticky slime. In modern zoology they are particularly renowned for their curious mating behaviour and for bearing live young. They are becoming increasingly popular in the 'exotic pets' trade, due to their bizarre appearance and eating habits. The Lobopodia, possible ancestors of velvet worms from the Cambrian period, are of great interest in paleontology. [more]

Priapulida

Priapulida (priapulid worms or penis worms, from Gr. p???p??, priapos '' + Lat. -ul-, diminutive) are a phylum of marine worms with an extensible spiny proboscis. Priapulid fossils are known at least as far back as the Middle Cambrian. Their nearest relatives are probably Kinorhyncha and Loricifera with which they constitute the taxon Scalidophora. Besides arthropods and velvet worms, it is only among Priapulida that we can find members of the Ecdysozoa which are relatively large in size. They were likely major predators of the Cambrian period. There are 16 known species of Priapulid worms. [more]

Tardigrada

Tardigrades (commonly known as water bears) form the Tardigrada, part of the superphylum Ecdysozoa. They are microscopic, water-dwelling, segmented animals with eight legs. Tardigrades were first described by Johann August Ephraim Goeze in 1773 (kleiner Wasserbär = little water bear). The name Tardigrada means "slow walker" and was given by Lazzaro Spallanzani in 1777. The name water bear comes from the way they walk, reminiscent of a bear's gait. The biggest adults may reach a body length of 1.5 mm, the smallest below 0.1 mm. Freshly hatched larvae may be smaller than 0.05 mm. [more]

At least 1,104 species and subspecies belong to the Phylum Tardigrada.

More info about the Phylum Tardigrada may be found here.

References

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  1. ^ Telford MJ, Bourlat SJ, Economou A, Papillon D, Rota-Stabelli O (April 2008). "The evolution of the Ecdysozoa". Philos. Trans. R. Soc. Lond., B, Biol. Sci. 363 (1496): 1529–37. doi:10.1098/rstb.2007.2243. PMID 18192181. PMC: 2614232. http://rstb.royalsocietypublishing.org/cgi/pmidlookup?view=long&pmid=18192181. 
  2. ^ Aguinaldo, A. M. A.; J. M. Turbeville, L. S. Linford, M. C. Rivera, J. R. Garey, R. A. Raff, & J. A. Lake (1997). "Evidence for a clade of nematodes, arthropods and other moulting animals". Nature 387: 489–493. doi:10.1038/387489a0. 
  3. ^ a b Dunn et al. (2008). "Broad phylogenomic sampling improves resolution of the animal tree of life". Nature 452: 745–749. doi:10.1038/nature06614. http://www.nature.com/nature/journal/v452/n7188/abs/nature06614.html. 
  4. ^ Nielsen, Claus (1995). Animal Evolution: Interrelationships of the Living Phyla. Oxford University Press. ISBN 9780198506829. 
  5. ^ a b Blair, J. E.; Kazuho Ikeo, Takashi Gojobori and S. Blair Hedges (2002). "The evolutionary position of nematodes". BMC Evolutionary Biology 2: 7. doi:10.1186/1471-2148-2-7. http://www.biomedcentral.com/1471-2148/2/7. 
  6. ^ Wägele, J. W.; T. Erikson, P. Lockhart, & B. Misof (1999). "The Ecdysozoa: Artifact or monophylum?". Journal of Zoological Systematics and Evolutionary Research 37: 211–223. 
  7. ^ Paleos Invertebrates: Panarthropoda - URL retrieved February 17, 2007
  8. ^ Wägele, J. W.; B. Misof (2001). "On quality of evidence in phylogeny reconstruction: a reply to Zrzavý's defence of the 'Ecdysozoa' hypothesis". J. Zool. Syst. Evol. Research 39 (3): 165–176. doi:10.1046/j.1439-0469.2001.00177.x. 
  9. ^ Nielsen, C. (2003) Proposing a solution to the Articulata–Ecdysozoa controversy. Zoologica Scripta 32:5, 475-482
  10. ^ Jenner, Ronald A. Unleashing the force of cladistics? Metazoan phylogenetics and hypothesis testing. Integrative and Comparative Biology, Feb 2003

Sources

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Last Revised: September 22, 2009
2009/09/22 05:53:08