Ecdysozoa () is a group of
protostome
Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's me ...
animals,
including
Arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
a (
insects,
chelicerata
The subphylum Chelicerata (from New Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks, and mite ...
,
crustaceans
Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean g ...
, and
myriapods),
Nematoda, and several smaller
phyla Phyla, the plural of ''phylum'', may refer to:
* Phylum, a biological taxon between Kingdom and Class
* by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another
Phy ...
. They were first defined by Aguinaldo ''et al.'' in 1997, based mainly on
phylogenetic trees constructed using 18S
ribosomal RNA
Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from riboso ...
genes. A large study in 2008 by Dunn ''et al.'' strongly supported the Ecdysozoa as a
clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...
, that is, a group consisting of a common ancestor and all its descendants.
The group is also supported by
morphological characters, and includes all animals that grow by
ecdysis, moulting their
cuticle.
The group was initially contested by a significant minority of biologists. Some argued for groupings based on more traditional taxonomic techniques,
while others contested the interpretation of the molecular data.
Etymology
The name ''Ecdysozoa'' stems etymologically .
Characteristics
The most notable characteristic shared by ecdysozoans is a three-layered
cuticle (four in
Tardigrada) 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 and produce mostly
amoeboid sperm, and their
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
s do not undergo
spiral cleavage
In embryology, cleavage is the division of cells in the early development of the embryo, following fertilization. The zygotes of many species undergo rapid cell cycles with no significant overall growth, producing a cluster of cells the same size ...
as in most other protostomes. Ancestrally, the group exhibited sclerotized teeth within the foregut, and a ring of spines around the mouth opening, though these features have been secondarily lost in certain groups. A respiratory and circulatory system is only present in
onychophorans and arthropods (often absent in smaller arthropods like mites); in the rest of the groups, both systems are missing.
Phylogeny
The Ecdysozoa include the following phyla:
Arthropoda
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin ...
,
Onychophora,
Tardigrada,
Kinorhyncha
Kinorhyncha ( grc, κινέω, kīnéō, I move, ' "snout") is a phylum of small marine invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos. They are also called mud dragons. Modern species are or less, ...
,
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
Panarthropoda is a proposed animal clade containing the extant phyla Arthropoda, Tardigrada (water bears) and Onychophora (velvet worms). Panarthropods also include extinct marine legged worms known as lobopodians ("Lobopodia"), a paraphyletic ...
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 is a clade of ecdysozoan animals including the Scalidophora (Kinorhynchans, Loriciferans, Priapulids) and the Nematoida (nematodes, Nematomorphs). It may be paraphyletic, or may be a sister group to Panarthropoda. Or perhaps Pa ...
but they are more usually considered
paraphyletic
In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...
in representing the primitive condition from which the Panarthropoda evolved.
A modern consensus
phylogenetic tree for the protostomes is shown below.
It is indicated when approximately clades radiated into newer clades in millions of years ago (Mya); dashed lines show especially uncertain placements.
The phylogenetic tree is based on Nielsen ''et.al.'' with provisional placement of Loricifera from Hiroshi ''et al.''
Older alternative groupings
Articulata hypothesis
The grouping proposed by Aguinaldo ''et al.'' is almost universally accepted, replacing an older hypothesis that
Panarthropoda
Panarthropoda is a proposed animal clade containing the extant phyla Arthropoda, Tardigrada (water bears) and Onychophora (velvet worms). Panarthropods also include extinct marine legged worms known as lobopodians ("Lobopodia"), a 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, though later considered them unrelated.
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
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. That year marked 50 years after the joint p ...
was awarded to
James Lake for the discovery of the New Animal Phylogeny consisting of the Ecdysozoa, the
Lophotrochozoa
Lophotrochozoa (, "crest/wheel animals") is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence. The clade includes animals like annelids, molluscs, bryozoans, bra ...
, and the Deuterostomia.
Coelomata hypothesis
Before Aguinaldo's Ecdysozoa proposal, one of the prevailing theories for the evolution of the
bilateral animals
The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...
was based on the morphology of their
body cavities. There were three types, or
grades of organization: the
Acoelomata (no
coelom
The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, ...
), the
Pseudocoelomata (partial coelom), and the
Eucoelomata (true
coelom
The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, ...
). Adoutte and coworkers were among the first to strongly support the Ecdysozoa. With the introduction of molecular phylogenetics, the coelomate hypothesis was abandoned, although some molecular, phylogenetic support for the Coelomata continued until as late as 2005.
References
External links
UCMP-Ecdysozoa introduction* https://web.archive.org/web/20030313115747/http://www.palaeos.com/Kingdoms/Animalia/Ecdysozoa.html
* https://web.archive.org/web/20080618211750/http://www.nematodes.org/tardigrades/Tardigrades_and_Ecdysozoa.html
* https://web.archive.org/web/20100510081324/http://chuma.cas.usf.edu/~garey/articulata.html
* https://web.archive.org/web/20030313065540/http://chuma.cas.usf.edu/~garey/essential.html
* http://www.nematomorpha.net
{{Taxonbar, from=Q5176
Superphyla
Extant Cambrian first appearances