Ecdysozoa
Encyclopedia
Ecdysozoa is a group of protostome
animals, including Arthropod
a (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. A large study in 2008 by Dunn et al. strongly supported the Ecdysozoa as a clade
, that is, a group consisting of a common ancestor and all its descendants.
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, while others have contested the interpretation of the molecular data.
A phylogenetic tree of the Ecdysozoa hypothesis as suggested by Dunn et al. (2008)
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 embryo
s 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 gastrotrich
s, 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. 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
.
should be classified with Annelida in a group called the Articulata
, and that Ecdysozoa are polyphyletic. Nielsen has suggested that a possible solution is to regard Ecdysozoa as a sister-group of Annelida.
Inclusion of the roundworms within the Ecdysozoa was initially contested but since 2003, a broad consensus has formed supporting the Ecdysozoa and in 2011 the Darwin-Wallace Medal
was awarded for the discovery of the New Animal Phylogeny consisting of the Ecdysozoa, the Lophotrochozoa, and the Deuterostomia.
Protostome
Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
animals, including Arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
a (insects, chelicerata
Chelicerata
The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda, and includes horseshoe crabs, scorpions, spiders and mites...
, 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
Ribosomal RNA
Ribosomal ribonucleic acid is the RNA component of the ribosome, the enzyme that is the site of protein synthesis in all living cells. Ribosomal RNA provides a mechanism for decoding mRNA into amino acids and interacts with tRNAs during translation by providing peptidyl transferase activity...
genes. A large study in 2008 by Dunn et al. strongly supported the Ecdysozoa as a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
, that is, a group consisting of a common ancestor and all its descendants.
The group is also supported by morphological
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
characters, and can be considered as including all animals that shed their exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
(see ecdysis
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
).
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, while others have contested the interpretation of the molecular data.
Group characters
The most notable characteristic shared by ecdysozoans is a three-layered cuticle
Cuticle
A cuticle , or cuticula, is a term used for any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticles" are non-homologous; differing in their origin, structure, function, and chemical composition...
composed of organic material, which is periodically molted as the animal grows. This process of molting is called ecdysis
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
and gives the group its name. The Ecdysozoans lack locomotory cilia, produce mostly amoeboid
Amoeboid
Amoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amœba" are often used interchangeably even by biologists, and especially refer to a creature moving by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...
sperm, and their embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s 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
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
.
The Ecdysozoa include the following phyla: Arthropoda, Onychophora, Tardigrada, Kinorhyncha
Kinorhyncha
Kinorhyncha is a phylum of small marine pseudocoelomate invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos...
, Priapulida
Priapulida
Priapulida is a phylum of marine worms. They are named for their extensible spiny proboscis, which, in some species, may have a shape like that of a human penis...
, Loricifera
Loricifera
Loricifera is a phylum of very small to microscopic marine sediment-dwelling animals with twenty-two described species, in eight genera. Aside from these described species, there are approximately 100 more that have been collected and not yet described. Their size ranges from 100 µm to ca....
, Nematoda and Nematomorpha
Nematomorpha
Nematomorpha is a phylum of parasitic animals that are superficially morphologically similar to nematode worms, hence the name. They range in size in most species from long and can reach in extreme cases up to 2 metres, and in diameter...
. A few other groups, such as the gastrotrich
Gastrotrich
The gastrotrichs are a phylum of microscopic animals abundant in fresh water and marine environments. Most fresh water species are part of the periphyton and benthos...
s, 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
Panarthropoda
Panarthropoda is a taxon combining the phyla Arthropoda, Tardigrada, and Onychophora. Dunn et al. in 2008 suggested that the tardigrades could be grouped along with the nematodes, leaving Onychophora as the sister group to the arthropods...
because they are distinguished by segmented body plans. 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
Cycloneuralia
Cycloneuralia is a clade of ecdysozoan animals including the Scalidophora and the Nematoida . Its sister group is the panarthropoda....
but they are more usually considered paraphyletic
Paraphyly
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...
.
Criticism
The grouping proposed by Aguinaldo et al. is widely accepted, although some zoologists still hold to the original view that PanarthropodaPanarthropoda
Panarthropoda is a taxon combining the phyla Arthropoda, Tardigrada, and Onychophora. Dunn et al. in 2008 suggested that the tardigrades could be grouped along with the nematodes, leaving Onychophora as the sister group to the arthropods...
should be classified with Annelida in a group called the Articulata
Articulata Hypothesis
The Articulata Hypothesis is the grouping in a higher taxon of animals with segmented bodies, consisting of Annelida and Panarthropoda. This theory states that these groups are descended from a common segmented ancestor...
, and that Ecdysozoa are polyphyletic. Nielsen has suggested that a possible solution is to regard Ecdysozoa as a sister-group of Annelida.
Inclusion of the roundworms within the Ecdysozoa was initially contested but since 2003, a broad consensus has formed supporting the Ecdysozoa and in 2011 the Darwin-Wallace Medal
Darwin-Wallace Medal
The Darwin–Wallace Medal is a medal awarded by the Linnean Society of London for "major advances in evolutionary biology". Historically, the medals have been awarded every 50 years, beginning in 1908...
was awarded for the discovery of the New Animal Phylogeny consisting of the Ecdysozoa, the Lophotrochozoa, and the Deuterostomia.
External links
- UCMP-Ecdysozoa introduction
- http://www.palaeos.com/Kingdoms/Animalia/Ecdysozoa.html
- http://www.nematodes.org/tardigrades/Tardigrades_and_Ecdysozoa.html
- http://chuma.cas.usf.edu/~garey/articulata.html
- http://chuma.cas.usf.edu/~garey/essential.html
- http://www.nematomorpha.net