Cercozoa
Encyclopedia
The Cercozoa are a group of protist
s. They are sometimes described as a kingdom.
s and flagellate
s that feed by means of filose pseudopods. These may be restricted to part of the cell surface, but there is never a true cytostome
or mouth as found in many other protozoa. They show a variety of forms and have proven difficult to define in terms of structural characteristics, although their unity is strongly supported by genetic studies. Cercozoa are closely related to Foraminifera
and Radiolaria, amoeboids that usually have complex shells, and together with them form a supergroup called the Rhizaria
.
s, filose amoebae with shells of siliceous scales or plates, which are commonly found in soils, nutrient-rich waters, and on aquatic plants. Some other filose amoebae produce organic shells, including the tectofilosid
s and Gromia
. They were formerly classified with the euglyphids as the Testaceafilosia. This group is not monophyletic, but nearly all studied members fall in or near the Cercozoa, related to similarly shelled flagellates.
s, strange amoebae that form a reticulating net. They are set apart by the presence of chloroplast
s, which apparently developed from an ingested green alga. They are bound by four membranes and still possess a vestigial nucleus, called a nucleomorph. As such, they have been of great interest to researchers studying the endosymbiotic origins of organelles.
Chlorarachniophytes are sometimes considered Filosa, rather than Endomyxa, while groups such as Gromia
are considered Endomyxa.
s, which are common soil flagellates, and the Phaeodarea
, marine protozoa that were previously considered radiolarians. In addition, three groups that are traditionally considered heliozoa
ns belong here: the Heliomonadida, Desmothoracida, and Gymnosphaerida.
In addition two groups of parasites, the Phytomyxea
and Ascetosporea
, and the shelled amoeba Gromia
may be basal Cercozoa, although some trees place them closer to the Foraminifera.
The spongomonads have been included here, but more recently have been considered Amoebozoa
.
Some other small groups of protozoans are considered Cercozoa but are of uncertain placement, and it is likely many obscure genera will turn out to be cercozoans with further study.
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s. They are sometimes described as a kingdom.
Characteristics
The group includes most amoeboidAmoeboid
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...
s and flagellate
Flagellate
Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...
s that feed by means of filose pseudopods. These may be restricted to part of the cell surface, but there is never a true cytostome
Cytostome
A cytostome or cell mouth is a part of a cell specialized for phagocytosis, usually in the form of a microtubule-supported funnel or groove. Food is directed into the cytostome, and sealed into vacuoles. Only certain groups of protozoa, such as the ciliates and excavates, have cytostomes. Such...
or mouth as found in many other protozoa. They show a variety of forms and have proven difficult to define in terms of structural characteristics, although their unity is strongly supported by genetic studies. Cercozoa are closely related to Foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...
and Radiolaria, amoeboids that usually have complex shells, and together with them form a supergroup called the Rhizaria
Rhizaria
The Rhizaria are a species-rich supergroup of unicellular eukaryotes. This supergroup was proposed by Cavalier-Smith in 2002. They vary considerably in form, but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods...
.
Filose (subphylum Filosa)
The best-known Cercozoa are the euglyphidEuglyphid
The euglyphids are a prominent group of filose amoebae that produce shells or tests from siliceous scales, plates, and sometimes spines. These elements are created within the cell and then assembled on its surface in a more or less regular arrangement, giving the test a textured appearance...
s, filose amoebae with shells of siliceous scales or plates, which are commonly found in soils, nutrient-rich waters, and on aquatic plants. Some other filose amoebae produce organic shells, including the tectofilosid
Tectofilosid
The tectofilosids or amphitremids are a group of filose amoebae with shells. These are composed of organic materials and sometimes collected debris, in contrast to the euglyphids, which produce shells from siliceous scales. The shell usually has a single opening, but in Amphitrema and a few other...
s and Gromia
Gromia
Gromia is a widespread genus of marine and freshwater amoeboids, closely resembling some foraminiferans. It produces an organic test, which is ovoid or lobed and may exceed one millimetre in size; this resembles a small grape, in that its centre is fluid-filled but lacks organic matter, which is...
. They were formerly classified with the euglyphids as the Testaceafilosia. This group is not monophyletic, but nearly all studied members fall in or near the Cercozoa, related to similarly shelled flagellates.
Reticulose (subphylum Endomyxa)
Another important group placed here are the chlorarachniophyteChlorarachniophyte
Chlorarachniophytes are a small group of algae occasionally found in tropical oceans. They are typically mixotrophic, ingesting bacteria and smaller protists as well as conducting photosynthesis. Normally they have the form of small amoebae, with branching cytoplasmic extensions that capture prey...
s, strange amoebae that form a reticulating net. They are set apart by the presence of chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
s, which apparently developed from an ingested green alga. They are bound by four membranes and still possess a vestigial nucleus, called a nucleomorph. As such, they have been of great interest to researchers studying the endosymbiotic origins of organelles.
Chlorarachniophytes are sometimes considered Filosa, rather than Endomyxa, while groups such as Gromia
Gromia
Gromia is a widespread genus of marine and freshwater amoeboids, closely resembling some foraminiferans. It produces an organic test, which is ovoid or lobed and may exceed one millimetre in size; this resembles a small grape, in that its centre is fluid-filled but lacks organic matter, which is...
are considered Endomyxa.
Ungrouped
Other notable cercozoans include the cercomonadCercomonad
Cercomonads are small flagellates, widespread in aqueous habitats and common in soils.-Characteristics:The cells are generally around 10 μm in length, without any shell or covering...
s, which are common soil flagellates, and the Phaeodarea
Phaeodarea
The Phaeodarea are a group of amoeboid protists. They are traditionally considered radiolarians, but in molecular trees do not appear to be close relatives of the other groups, and are instead placed among the Cercozoa...
, marine protozoa that were previously considered radiolarians. In addition, three groups that are traditionally considered heliozoa
Heliozoa
Heliozoa are phagotrophs. They are roughly spherical amoeboids with many stiff, microtubule-supported projections called axopods radiating outward from the cell surface. These give them the characteristic sun-like appearance for which they are named, and are variously used for capturing food,...
ns belong here: the Heliomonadida, Desmothoracida, and Gymnosphaerida.
Classification
The exact composition and classification of the Cercozoa are still being worked out. A general scheme is:| Class Chlorarachnea | Chlorarachniophyta |
| Class Proteomyxidea Proteomyxidea Proteomyxidea is a class of Rhizaria. Although it is known to be paraphyletic, further research is needed before its classification can be improved.... |
Gymnophryid Gymnophryid Gymnophryidae is a small family of amoeboids that lack shells and produce thin, reticulose pseudopods. These contain microtubules and have a granular appearance, owing to the presence of extrusomes, but are distinct from the pseudopods of Foraminifera... a, Heliomonadida, Desmothoracida, Gymnosphaerida, etc. |
| Class Sarcomonadea | Cercomonadida |
| Class Imbricatea Imbricatea Imbricatea is a class of Rhizaria characterised by silica scales. It is sometimes described as "Imbricatea/Silicofilosea", due to the similarity of those two groupings. Imbricatea is divided into the orders Euglyphida and Thaumatomonadida.... / Silicofilosea |
Euglyphida and Thaumatomonadida |
| Class Thecofilosea Thecofilosea Thecofilosea is a class of Rhizaria.... |
Tectofilosida and Cryomonadida Cryomonadida Cryomonadida is a group of heterotrophic protists, that belong to the Cercozoa.-Characteristics:Members of the Cryomonadida are single-celled organisms that are surrounded by a shell comprising layers of organic material. They possess two unequally long flagella, and a single nucleus with a... |
| Class Phaeodarea Phaeodarea The Phaeodarea are a group of amoeboid protists. They are traditionally considered radiolarians, but in molecular trees do not appear to be close relatives of the other groups, and are instead placed among the Cercozoa... |
|
| Class Ebridea | Ebridea |
In addition two groups of parasites, the Phytomyxea
Phytomyxea
The Phytomyxea are a group of protists that are parasites of plants. A more common name for them is the plasmodiophorids, but this does not always include Phagomyxa. They typically develop within plant cells, causing the infected tissue to grow into a gall or scab...
and Ascetosporea
Ascetosporea
The Ascetosporea are a group of protists that are parasites of animals, especially marine invertebrates. There are two groups, the haplosporids and paramyxids, which are not particularly similar morphologically but consistently group together on molecular trees, which place them near the base of...
, and the shelled amoeba Gromia
Gromia
Gromia is a widespread genus of marine and freshwater amoeboids, closely resembling some foraminiferans. It produces an organic test, which is ovoid or lobed and may exceed one millimetre in size; this resembles a small grape, in that its centre is fluid-filled but lacks organic matter, which is...
may be basal Cercozoa, although some trees place them closer to the Foraminifera.
The spongomonads have been included here, but more recently have been considered Amoebozoa
Amoebozoa
The Amoebozoa are a major group of amoeboid protozoa, including the majority that move by means ofinternal cytoplasmic flow. Their pseudopodia are characteristically blunt and finger-like,...
.
Some other small groups of protozoans are considered Cercozoa but are of uncertain placement, and it is likely many obscure genera will turn out to be cercozoans with further study.
External links
- Tree of Life Cercozoa