Holozoa
Encyclopedia
Holozoa is a group of organisms that includes animal
s and their closest single-celled
relatives, but excludes fungi.
Holozoa is also an old name for the tunicate
genus
Distaplia.
Because Holozoa is a clade
including all organisms more closely related to animals than to fungi, some authors prefer it to recognizing paraphyletic groups such as Choanozoa
, which mostly consists of Holozoa minus animals.
Perhaps the best-known holozoans, apart from animals, are the choanoflagellate
s, which strongly resemble the collar cells of sponges, and so were theorized to be related to sponges even in the 19th century. Proterospongia
is an example of a colonial choanoflagellate that may shed light on the origin of sponges.
The affinities of the other single-celled holozoans only began to be recognized in the 1990s.
A group of mostly parasitic species called Icthyosporea or Mesomycetozoea
is sometimes grouped with other species in Mesomycetozoa (note the difference in the ending). The amoeboid genera Ministeria
and Capsaspora
may be united in a group called Filasterea
by the structure of their thread-like pseudopod
s. Along with choanoflagellates, filastereans may be closely related to animals, and one analysis grouped them together as the clade Filozoa
.
period, about 600 million years ago.
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s and their closest single-celled
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
relatives, but excludes fungi.
Holozoa is also an old name for the tunicate
Tunicate
Tunicates, also known as urochordates, are members of the subphylum Tunicata, previously known as Urochordata, a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata. While most tunicates live on the ocean floor, others such...
genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Distaplia.
Because Holozoa is 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...
including all organisms more closely related to animals than to fungi, some authors prefer it to recognizing paraphyletic groups such as Choanozoa
Choanozoa
Choanozoa is the name of a phylum of protists that belongs to the line of opisthokonts....
, which mostly consists of Holozoa minus animals.
Perhaps the best-known holozoans, apart from animals, are the choanoflagellate
Choanoflagellate
The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals...
s, which strongly resemble the collar cells of sponges, and so were theorized to be related to sponges even in the 19th century. Proterospongia
Proterospongia
Proterospongia is a genus of choanoflagellate, notable for its formation of colonies. As a colony-forming choanoflagellate, Proterospongia is of interest to scientists studying the mechanisms of intercellular signaling and adhesion present prior to the origin of animals.-Physical...
is an example of a colonial choanoflagellate that may shed light on the origin of sponges.
The affinities of the other single-celled holozoans only began to be recognized in the 1990s.
A group of mostly parasitic species called Icthyosporea or Mesomycetozoea
Mesomycetozoea
The Mesomycetozoea are a small group of protists, mostly parasites of fish and other animals.-Significance:...
is sometimes grouped with other species in Mesomycetozoa (note the difference in the ending). The amoeboid genera Ministeria
Ministeria
Ministeria is a small single-celled eukaryote which feeds on bacteria. Although it is an opisthokont, it seems not to fit in any of the established opisthokont lineages such as choanoflagellates or animals. It might form a group with Capsaspora, another opisthokont of uncertain placement....
and Capsaspora
Capsaspora
Capsaspora is a single-celled eukaryote which is a symbiont in the haemolymph of the tropical freshwater snail Biomphalaria glabrata.- Taxonomy :...
may be united in a group called Filasterea
Filasterea
Filasterea is a grouping that includes Ministeria and Capsaspora....
by the structure of their thread-like pseudopod
Pseudopod
Pseudopods or pseudopodia are temporary projections of eukaryotic cells. Cells that possess this faculty are generally referred to as amoeboids. Pseudopodia extend and contract by the reversible assembly of actin subunits into microfilaments...
s. Along with choanoflagellates, filastereans may be closely related to animals, and one analysis grouped them together as the clade Filozoa
Filozoa
The Filozoa are a monophyletic grouping within the Opisthokonta. They include animals along with their nearest unicellular relatives ....
.
Fossil record
The oldest fossils of holozoans are animals, which date back to the EdiacaranEdiacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
period, about 600 million years ago.