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An amoeba (also ameba, amœba or amoeboid) is a type of cell or organism which has the ability to alter its shape, primarily by extending and retracting pseudopods. Amoebae do not form a single taxonomic group, but are found in every major lineage of eukaryotic organisms (domain Eukaryota). Amoeoboid cells occur not only among the protozoa, but also fungi, algae and animals.
The best known amoeboid protists are the "giant amoebae" Chaos carolinense and Amoeba proteus, both of which are widely cultivated and studied in classrooms and laboratories. Other well known species include the so-called "brain-eating amoeba" Naegleria fowleri, the intestinal parasite Entamoeba histolytica, which causes amoebic dysentery, and the multicellular "social amoeba" Dictyostelium discoideum.
Amoebae move and eat by using pseudopods, which are bulges of cytoplasm formed by the coordinated action of actin microfilaments pushing out the plasma membrane that surrounds the cell. Groups of amoebae are distinguished by the appearance and internal structure of their pseudopods. Amoebozoan species typically have bulbous pseudopods, rounded at the ends and roughly tubular in cross-section (lobose). Cercozoan amoeboids, such as Euglypha and Gromia, have slender, thread-like (filose) pseudopods. Foraminiferan emit fine, branching pseudopods that merge with one another to form net-like (reticulose) structures. Some groups, such as the Radiolaria and the amoeboids loosely called Heliozoa, have stiff, needle-like, radiating actinopods supported from within by bundles of microtubules.
Free-living amoebae may be "testate" (enclosed within a hard shell), or "naked" (lacking any hard covering). Testate amoebae shells are composed various substances, including calcium, silica, chitin, or agglutinations of found materials like small grains of sand and the frustules of diatoms.
To regulate osmotic pressure, most freshwater amoebae have a contractile vacuole which expels excess water from the cell. This organelle is necessary because freshwater has a lower concentration of solutes (such as salt) than the amoeba's own internal fluids (cytosol). Because the surrounding water is hypotonic with respect to the contents of the cell, water is transferred across the amoeba's cell membrane by osmosis. Without a contractile vacuole, the cell would fill with excess water and, eventually, burst. Marine amoebae do not usually possess a contractile vacuole, because the concentration of solutes within the cell are in balance with the tonicity of the surrounding water.
The food sources of amoebae vary. Some amoebae are predatory and live by consuming bacteria and other protists. Some are detritivores and eat dead organic material. Amoebae typically ingest their food by phagocytosis, extending pseudopods to encircle and engulf live prey or particles of scavenged material. Amoeboid cells do not have a mouth or cytostome, and there is no fixed place on the cell at which phagocytosis normally occurs. Some amoebae also feed by pinocytosis, imbibing dissolved nutrients through vesicles formed within the cell membrane.
Some multicellular organisms have amoeboid cells only in certain phases of life, or use amoeboid movements for specialized functions. In the immune system of humans and other animals, amoeboid white blood cells pursue invading organisms, such as bacteria and pathogenic protists, and engulf them by phagocytosis.
Amoeboid stages also occur in the multicellular fungus-like protists, the so-called slime molds. Both the plasmodial slime molds, currently classified in the class Myxogastria, and the cellular slime molds of the groups Acrasida and Dictyosteliida, live as amoebae during their feeding stage. The cells of the former form a giant multinucleate amoeboid organism, while the cells of the latter live separately until food runs out, at which time the amoebae aggregate to form a multicellular migrating slug which functions as a single organism.
Amoebae are no longer classified together in a single group, since any taxon that included all of the traditional amoebae would be a polyphyletic group, containing many species that are not closely related.
In older classification systems, most amoebae were placed in the class or subphylum Sarcodina. Within the Sarcodina, amoebae were divided into morphological categories, on the basis of the form and structure of their pseudopods. Amoebae with pseudopods supported by regular arrays of microtubules were called actinopods, whereas those with unsupported pseudopods were classified as rhizopods. The Rhizopoda were further subdivided into lobose, filose, and reticulose amoebae, according to the morphology of their pseudopods.
When molecular (genetic) analyses confirmed that Sarcodina was a polyphyletic group, the old scheme was abandoned and the amoebae were dispersed among many other groups. The majority of traditional "Sarcodines" are now placed in two eukaryote supergroups (unranked taxa above the Kingdom level): Amoebozoa and Rhizaria. The rest have been distributed among the excavates, opisthokonts, and stramenopiles. Some, like the Centrohelida, have yet to be placed in any supergroup.
Whereas older classifications were based mainly on morphology, modern schemes are based upon cladistics. Phylogenetic analyses place amoeboid genera into the following groups:
|Chromalveolate||Heterokont: Hyalodiscus, Labyrinthula|
|Adelphamoeba, Astramoeba, Cashia, Dinamoeba, Flagellipodium, Flamella, Gibbodiscus, Gocevia, Hollandella, Iodamoeba, Malamoeba, Nollandia, Oscillosignum, Paragocevia, Parvamoeba, Pernina, Pontifex, Protonaegleria, Pseudomastigamoeba, Rugipes, Striamoeba, Striolatus, Subulamoeba, Theratromyxa, Trienamoeba, Trimastigamoeba, Vampyrellium|
Some amoebae can infect other organisms pathogenically, causing disease: