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ameba or amoeba (both: əmēˈbə), common name for certain one-celled organisms belonging to the phylum Sarcodina of the kingdom Protista. Amebas were previously classified as members of the animal kingdom. Most amebas are very small (from 5 to 20 microns in diameter) and contain a single nucleus. A. proteus averages 0.25 mm in length. Members of the genus Pelomyxa, however, may be well over a millimeter (up to 8 mm) in diameter and may contain hundreds of nuclei. The giant ameba Gromia sphaerica, found on the ocean floor, may reach 1.5 in. (40 mm) in diameter.

Amebas constantly change the shape of their bodies as a result of the phenomenon known as ameboid movement, involving the formation of temporary extensions (pseudopodia, or false feet) of the body. Pseudopodia, used in locomotion and feeding, may be rounded at the tip (lobopodia), pointed (filopodia), branched and fused together (rhizopodia), or somewhat rigid and pointed (axopodia).

Although simple in form, amebas are very successful organisms and are found abundantly in a variety of habitats all over the world. Amebas live in freshwater, the oceans, and in the upper layers of the soil, and many have adapted to a parasitic life on the body surface of aquatic animals or in the internal organs of both aquatic and terrestrial animals. Few animals escape invasion by some type of ameba. Some are harmless, but others are pathogenic and cause serious diseases; e.g., Entamoeba histolytica causes amebic dysentery, which is fatal if untreated, and Naegleria fowleri causes primary amebic meningoencephalitis, which is rare but usually fatal unless treated promptly. The many genera of amebas were given their common name because of their resemblance to the genus Amoeba (order Amoebida), which includes several large, common species of which the freshwater Amoeba proteus is the most familiar.

The term ameba is sometimes also used to refer to other unicellular protists (e.g., slime molds) that have ameboid features such as pseudopodia. Other ameboid protozoans of the phylum Sarcodina include the marine radiolarians, which form silicate skeletons; their freshwater counterparts, the heliozoans; and the shell-bearing foraminiferans.

Digestion and Respiration

In a process known as phagocytosis, amebas engulf their prey, or particles of appropriate size, with their pseudopodia, forming food vacuoles. Digestive enzymes, manufactured and secreted by the organism, are then poured into these vacuoles, and the particles are digested. Useful compounds are subsequently absorbed into the ameba's body. Useless residues remain in the vacuoles and are ultimately expelled (egested) as the vacuole comes in contact with the membrane at the body surface. Amebas can distinguish food (e.g., algae, diatoms, bacteria, and other protozoans) from other material and use different tactics in approaching different food. Freshwater amebas take up water constantly through the process of osmosis, and water content is regulated with a pulsating contractile vacuole. Marine amebas lack a contractile vacuole. Respiration is by diffusion of gases through the cell membrane.


Under favorable conditions amebas divide by binary fission (splitting) to produce two daughter amebas, the nucleus dividing by mitosis. When an ameba is divided artificially, the portion containing the nucleus forms a new cell membrane and continues as a whole animal, while the other portion lives only as long as its present food supply lasts, ultimately dying, since it cannot ingest food or reproduce. If conditions are unfavorable, e.g., in the absence of food and water, amebas secrete a firm protective covering and encyst until conditions are again favorable to active division.
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Any protozoon moving by means of protoplasmic flow. In their entirety, the ameboid protozoa include naked amebas, those enclosed within a shell or test, as well as more highly developed representatives such as the heliozoians, radiolarians, and foraminiferans. Ameboid movement is accomplished by pseudopods—cellular extensions which channel the flow of protoplasm. Pseudopods take varied forms and help distinguish among the different groups. A lobe-shaped extension or lobopod is perhaps the simplest type of pseudopod. The shapelessness and plasticity of these locomotory organelles impart an asymmetric, continually changing aspect to the organism. Other, more developed, representatives have pseudopodial extensions containing fibrous supporting elements (axopods) or forming an extensive network of anastomosing channels (reticulopods). Though involved in locomotion, these organelles are also functional in phagocytosis—the trapping and ingesting of food organisms (usually bacteria, algae, or other protozoa) or detritus. See Foraminiferida, Phagocytosis, Radiolaria

Amebas range from small soil organisms, such as Acanthamoeba (20 micrometers), to the large fresh-water forms Amoeba proteus (600 μm; see illustration) and Pelomyxa (1 mm, or more). Some types, such as Amoeba, are uninucleate; others are multinucleate. Reproduction is by mitosis with nuclear division preceding cytoplasmic division to produce two daughters. Multinucleate forms have more unusual patterns of division, since nuclear division is not immediately or necessarily followed by cytoplasmic division. Transformation of the actively feeding ameba into a dormant cyst occurs in many species, particularly those found in soil or as symbionts. The resting stages allow survival over periods of desiccation, food scarcity, or transmission between hosts. See Reproduction (animal)

Phase-contrast photomicrograph of Amoeba proteus , a large fresh-water amebaenlarge picture
Phase-contrast photomicrograph of Amoeba proteus, a large fresh-water ameba

Amebas are found in a variety of habitats, including fresh-water and marine environments, soil, and as symbionts and parasites in body cavities and tissues of vertebrates and invertebrates. Because of their manner of locomotion, amebas typically occur on surfaces, such as the bottom of a pond, on submerged vegetation, or floating debris. In soil, they are a significant component of the microfauna, feeding extensively on bacteria and small fungi. Amebas in marine habitats may be found as planktonic forms adapted for floating at the surface (having oil droplets to increase bouyancy and projections to increase surface area), where they feed upon bacteria, algae, and other protozoa. Several species of amebas may be found in the human intestinal tract as harmless commensals (for example, Entamoeba coli) or as important parasites responsible for amebic dysentery (E. histolytica).

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.


(invertebrate zoology)
The common name for a number of species of naked unicellular protozoans of the order Amoebida; an example is a member of the genus Amoeba.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


(US), ameba
any protozoan of the phylum Rhizopoda, esp any of the genus Amoeba, able to change shape because of the movements of cell processes (pseudopodia). They live in fresh water or soil or as parasites in man and animals
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005