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(prō'təzō`ən), informal term for the unicellular heterotrophs of the kingdom ProtistaProtista
or Protoctista
, in the five-kingdom system of classification, a kingdom comprising a variety of unicellular and some simple multinuclear and multicellular eukaryotic organisms.
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. Protozoans comprise a large, diverse assortment of microscopic or near-microscopic organisms that live as single cells or in simple colonies and that show no differentiation into tissues. Formerly classified in the animal kingdom, they are now generally divided into five protist phyla: MastigophoraMastigophora
, phylum of unicellular heterotrophic protozoans of the kingdom Protista. Most of the approximately 1,500 species of Mastigophora are propelled by one or more flagella, and members of the group are sometimes referred to as flagellates.
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 (the flagellates), SarcodinaSarcodina,
the largest phylum (11,500 living species and 33,000 fossil species) of protozoans). It comprises the amebas and related organisms; which are all solitary cells that move and capture food by means of pseudopods, flowing temporary extensions of the cell.
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 (the amebas), CiliophoraCiliophora
, phylum in the kingdom Protista consisting of the ciliates, or ciliophores, complex freshwater or saltwater protozoans that swim by the coordinated beating of their cilia—short, hairlike structures that cover the cell surface.
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 (the ciliates), OpalinidaOpalinida
, phylum of unicellular heterotrophic organisms of the kingdom Protista. The opalinids are all intestinal parasites of small vertebrates, such as frogs, toads, and fish.
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, and SporozoaSporozoa
, phylum of unicellular heterotrophic organisms of the kingdom Protista. Unlike most other protozoans, sporozoans have no cilia or flagella. All species are parasitic and have elaborate life cycles, often requiring more than one host.
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. Most are motile, and most ingest food, as do animals, rather than produce it themselves, as do plants. The 26,000 living species are cosmopolitan in distribution; they are found in freshwater and at all depths in the ocean; some live in soil. Some are parasites in the bodies of humans or other animals, sometimes causing diseases.

Cellular Structure and Function

The various forms have in common a unicellular structure consisting of a mass of cytoplasm with one or more nuclei (see cellcell,
in biology, the unit of structure and function of which all plants and animals are composed. The cell is the smallest unit in the living organism that is capable of integrating the essential life processes. There are many unicellular organisms, e.g.
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). Like all cells, they are bounded by a thin cell membrane; in addition, most have a tough outer membrane called a pellicle, which maintains their form. Despite their small size and lack of organization into multicellular systems, protozoans carry on all the metabolic functions of animals. Organelles, or intracellular structures, carry out a variety of functions, such as digestion, excretion, respiration, and coordination of movement; some protozoans are much more complex in their internal structure than are the cells of multicellular animals.


Some protozoans have complex digestive systems and feed on large food particles, such as other microorganisms. The food is digested by means of enzymes and the wastes transported to the cell surface or stored in vacuoles (bubblelike spaces in the cytoplasm). Others have no digestive system and absorb dissolved organic matter through the cell membrane.


Respiration is accomplished by the diffusion of dissolved gases through the cell membrane. Oxygen diffuses into the cell, where it oxidizes food molecules, producing energy and the organic molecules used for the building and maintenance of the cell. Carbon dioxide and water, the waste products of this oxidation, diffuse out of the cell.


Reproduction is usually asexual, occurring mostly by cell division, or binary fission; some forms reproduce asexually by budding or by the formation of spores (reproductive cells that give rise to a new organism without fertilization). In certain groups sexual reproduction sometimes also occurs. In these instances, cell division is preceded by the fusion of two individuals or, in ciliates, by conjugation and exchange of nuclear material.


See H. Curtis, The Marvelous Animals (1968); T. Jahn, How to Know the Protozoa (2d ed. 1978); J. J. Lee, S. H. Hunter, and E. C. Bovee, An Illustrated Guide to the Protozoa (1985); M. Sleigh, Protozoa and Other Protists (1989).


Also called protozoon any of various minute unicellular organisms formerly regarded as invertebrates of the phylum Protozoa but now usually classified in certain phyla of protoctists. Protozoans include flagellates, ciliates, sporozoans, amoebas, and foraminifers
References in periodicals archive ?
The findings of present study suggested the baseline information for these most prevalent gastrointestinal and blood protozoan parasites.
Concerning standard levels of protozoans in drinking water, EPA's MCLG recommended the proper way to control pathogenic protozoans is using an effective water treatment technique, such as reverse osmosis or ozonation (17).
Prevalence of nematodes, cestodes and protozoans in male and female individuals related to Education Department of Swat, Pakistan.
Pace and Orcutt, 1981), protozoans are seen as a significant portion of the microzooplankton community (Beaver and Crisman, 1990) and an important link in the food chain, performing a key role in energy flow and acting as important mineralizing agents of limiting essential nutrients such as phosphorus and nitrogen (Azam et al.
Notably, phylogenetic comparison of 479 bp of the mitochondrial cytochrome b gene derived from protozoan cyst-like structures with known sequences of avian hematozoa found 99%-100% homology of parasites from both outbreaks with the avian malaria parasites (Haemoproteus spp.
We therefore studied the occurrence of helminth and protozoan infections in adult patients throughout KZN from stool samples obtained from regional laboratory services.
The opalinid protozoan, Protoopalina mitotica has been reported previously from the New Mexico spadefoot (Spea multiplicata) from El Paso County, Texas (Delvinquier et al.
Indirect effects in each of these two-consumer food webs were produced by strong competitive interactions between pairs of protozoans.
Our laboratory studies of what happens to colloidal iron oxides ingested by protozoans show that protozoan grazers can greatly accelerate the production of dissolved and chemically reactive iron from colloidal iron oxides in seawater.
Three monogenean species (Gyrodactylus turnbulli Harris 1986;, Gyrodactylus VonNordmann, Dactylogyrus extensus Mueller and Van Cleave 1932); six protozoans (Trichodina Ehrenberg 1831, Chilodonella Stand, 1928, Ichthyophthirius multifiliis Fouquet, 1876, Piscinoodinium pillulare Schaperclaus 1954, Tetrahymena sp.
The quick keys to the identification of protozoans, helminths, and arthropods are helpful for distinguishing pseudoparasites from harmful ones.