protozoa protozoa protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a paraphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. historically, protozoans were regarded as "one-celled animals", because they often possess animal-like behaviours, such as motility and predation, and lack a cell wall, as found in plants and many algae. this article is about the organism. for the infection, see protozoan infection. clockwise from top left: blepharisma japonicum, a ciliate; giardia muris, a parasitic flagellate; centropyxis aculeata, a testate (shelled) amoeba; peridinium willei, a dinoflagellate; chaos carolinense, a naked amoebozoan; desmarella moniliformis, a choanoflagellate when first introduced by georg goldfuss (originally spelled goldfuß) in 1818, the taxon protozoa was erected as a class within the animalia, with the word 'protozoa' meaning "first animals". in later classification schemes it was elevated to a variety of higher ranks, including phylum, subkingdom and kingdom, and sometimes included within the similarly paraphyletic protoctista or protista. the approach of classifying protozoa within the context of animalia was widespread in the 19th and early 20th century, but not universal. by the 1970s, it became usual to require that all taxa be monophyletic (derived from a common ancestor that would also be regarded as protozoan), and holophyletic (containing all of the known descendants of that common ancestor). the taxon 'protozoa' fails to meet these standards, and the practices of grouping protozoa with animals, and treating them as closely related, are no longer justifiable. the term continues to be used in a loose way to describe single-celled protists (that is, eukaryotes that are not animals, plants, or fungi) that feed by heterotrophy. traditional textbook examples of protozoa are amoeba, paramecium, euglena and trypanosoma. frederick chapman's the foraminifera: an introduction to the study of the protozoa (1902) despite awareness that the traditional protozoa was not a clade, a natural group with a common ancestor, some authors have continued to use the name, while applying it to differing scopes of organisms. in a series of classifications by thomas cavalier-smith and collaborators since 1981, the taxon protozoa was applied to certain groups of eukaryotes, and ranked as a kingdom. a scheme presented by ruggiero et al. in 2015, placed eight not closely related phyla within kingdom protozoa: euglenozoa, amoebozoa, metamonada, choanozoa sensu cavalier-smith, loukozoa, percolozoa, microsporidia and sulcozoa. this approach excludes several major groups traditionally placed among the protozoa, such as the ciliates, dinoflagellates, foraminifera, and the parasitic apicomplexans, which were moved to other groups such as alveolata and stramenopiles, under the polyphyletic chromista. the protozoa in this scheme were paraphyletic, because it excluded some descendants of protozoa. history of classification class protozoa, order infusoria, family monades by georg august goldfuss, c. 1844 the word "protozoa" (singular protozoon) was coined in 1818 by zoologist georg august goldfuss (=goldfuß), as the greek equivalent of the german urthiere, meaning "primitive, or original animals" (ur- ‘proto-’ + thier ‘animal’). goldfuss created protozoa as a class containing what he believed to be the simplest animals. originally, the group included not only single-celled microorganisms but also some "lower" multicellular animals, such as rotifers, corals, sponges, jellyfish, bryozoa and polychaete worms. the term protozoa is formed from the greek words πρῶτος (prôtos), meaning "first", and ζῶα (zôa), plural of ζῶον (zôon), meaning "animal". in 1848, with better microscopes and theodor schwann and matthias schleiden's cell theory, the zoologist c. t. von siebold proposed that the bodies of protozoa such as ciliates and amoebae consisted of single cells, similar to those from which the multicellular tissues of plants and animals were constructed. von siebold redefined protozoa to include only such unicellular forms, to the exclusion of all metazoa (animals). at the same time, he raised the group to the level of a phylum containing two broad classes of microorganisms: infusoria (mostly ciliates) and flagellates (flagellated protists and amoebae). the definition of protozoa as a phylum or sub-kingdom composed of "unicellular animals" was adopted by the zoologist otto bütschli—celebrated at his centenary as the "architect of protozoology". john hogg's illustration of the four kingdoms of nature, showing "primigenal" as a greenish haze at the base of the animals and plants, 1860 as a phylum under animalia, the protozoa were firmly rooted in a simplistic "two-kingdom" concept of life, according to which all living beings were classified as either animals or plants. as long as this scheme remained dominant, the protozoa were understood to be animals and studied in departments of zoology, while photosynthetic microorganisms and microscopic fungi—the so-called protophyta—were assigned to the plants, and studied in departments of botany. criticism of this system began in the latter half of the 19th century, with the realization that many organisms met the criteria for inclusion among both plants and animals. for example, the algae euglena and dinobryon have chloroplasts for photosynthesis, like plants, but can also feed on organic matter and are motile, like animals. in 1860, john hogg argued against the use of "protozoa", on the grounds that "naturalists are divided in opinion — and probably some will ever continue so—whether many of these organisms or living beings, are animals or plants." as an alternative, he proposed a new kingdom called primigenum, consisting of both the protozoa and unicellular algae, which he combined under the name "protoctista". in hoggs's conception, the animal and plant kingdoms were likened to two great "pyramids" blending at their bases in the kingdom primigenum. in 1866, ernst haeckel proposed a third kingdom of life, which he named protista. at first, haeckel included a few multicellular organisms in this kingdom, but in later work, he restricted the protista to single-celled organisms, or simple colonies whose individual cells are not differentiated into different kinds of tissues. despite these proposals, protozoa emerged as the preferred taxonomic placement for heterotrophic microorganisms such as amoebae and ciliates, and remained so for more than a century. in the course of the 20th century, the old "two kingdom" system began to weaken, with the growing awareness that fungi did not belong among the plants, and that most of the unicellular protozoa were no more closely related to the animals than they were to the plants. by mid-century, some biologists, such as herbert copeland, robert h. whittaker and lynn margulis, advocated the revival of haeckel's protista or hogg's protoctista as a kingdom-level eukaryotic group, alongside plants, animals and fungi. a variety of multi-kingdom systems were proposed, and the kingdoms protista and protoctista became established in biology texts and curricula. while most taxonomists have abandoned protozoa as a high-level group, cavalier-smith used the term with a different circumscription. in 2015, protozoa sensu cavalier-smith excluded several major groups of organisms traditionally placed among the protozoa (such as ciliates, dinoflagellates and foraminifera). this and similar concepts of protozoa are of a paraphyletic group which does not include all organisms that descended from protozoa. in this case, the most significant absences were of the animals and fungi. the continued use by some of the 'protozoa' in its old sense highlights the uncertainty as to what is meant by the word 'protozoa', the need for disambiguating statements such as "in the sense intended by goldfuß", and the problems that arise when new meanings are given to familiar taxonomic terms. some authors classify protozoa as a subgroup of mostly motile protists. others class any unicellular eukaryotic microorganism as protists, and make no reference to 'protozoa'. in 2005, members of the society of protozoologists voted to change its name to the international society of protistologists. by 1954, protozoa were classified as "unicellular animals", as distinct from the "protophyta", single-celled photosynthetic algae, which were considered primitive plants. in the system of classification published in 1964 by b. m. honigsberg and colleagues, , the phylum protozoa was divided according to the means of locomotion, such as by cilia or flagella. in the system of eukaryote classification published by the international society of protistologists in 2012, members of the old phylum protozoa have been distributed among a variety of supergroups. characteristics reproduction reproduction in protozoa can be sexual or asexual. most protozoa reproduce asexually through binary fission. many parasitic protozoa reproduce both asexually and sexually. however, sexual reproduction is rare among free-living protozoa and it usually occurs when food is scarce or the environment changes drastically. both isogamy and anisogamy occur in protozoa with anisogamy being the more common form of sexual reproduction. size protozoa, as traditionally defined, range in size from as little as 1 micrometre to several millimetres, or more. among the largest are the deep-sea–dwelling xenophyophores, single-celled foraminifera whose shells can reach 20 cm in diameter. the ciliate spirostomum ambiguum can attain 3 mm in length species cell type size in micrometres plasmodium falciparummalaria parasite, trophozoite phase 1–2 massisteria voersifree-living cercozoa cercomonad amoebo-flagellate 2.3–3 bodo saltansfree-living kinetoplastid flagellate 5–8 plasmodium falciparummalaria parasite, gametocyte phase 7–14 trypanosoma cruziparasitic kinetoplastid, chagas disease 14–24 entamoeba histolyticaparasitic amoeban 15–60 balantidium coliparasitic ciliate 50–100 paramecium caudatumfree-living ciliate 120–330 amoeba proteusfree-living amoebozoan 220–760 noctiluca scintillansfree-living dinoflagellate 700–2000 syringammina fragilissimaforaminifera amoeba up to 200000 habitat free-living protozoa are common and often abundant in fresh, brackish and salt water, as well as other moist environments, such as soils and mosses. some species thrive in extreme environments such as hot springs and hypersaline lakes and lagoons. all protozoa require a moist habitat; however, some can survive for long periods of time in dry environments, by forming resting cysts that enable them to remain dormant until conditions improve. feeding all protozoa are heterotrophic, deriving nutrients from other organisms, either by ingesting them whole by phagocytosis or taking up dissolved organic matter or micro-particles (osmotrophy). phagocytosis may involve engulfing organic particles with pseudopodia (as amoebae do), taking in food through a specialized mouth-like aperture called a cytostome, or using stiffened ingestion organelles parasitic protozoa use a wide variety of feeding strategies, and some may change methods of feeding in different phases of their life cycle. for instance, the malaria parasite plasmodium feeds by pinocytosis during its immature trophozoite stage of life (ring phase), but develops a dedicated feeding organelle (cytostome) as it matures within a host's red blood cell. paramecium bursaria, is one example of a variety of freshwater ciliates that host endosymbiont chlorophyte algae from the genus chlorella protozoa may also live as mixotrophs, combining a heterotrophic diet with some form of autotrophy. some protozoa form close associations with symbiotic photosynthetic algae (zoochlorellae), which live and grow within the membranes of the larger cell and provide nutrients to the host. the algae are not digested, but reproduce and are distributed between division products. the organism may benefit at times by deriving some of its nutrients from the algal endosymbionts or by surviving anoxic conditions because of the oxygen produced by algal photosynthesis. some protozoans practice kleptoplasty, stealing chloroplasts from prey organisms and maintaining them within their own cell bodies as they continue to produce nutrients through photosynthesis. the ciliate mesodinium rubrum retains functioning plastids from the cryptophyte algae on which it feeds, using them to nourish themselves by autotrophy. the symbionts may be passed along to dinoflagellates of the genus dinophysis, which prey on mesodinium rubrum but keep the enslaved plastids for themselves. within dinophysis, these plastids can continue to function for months. motility organisms traditionally classified as protozoa are abundant in aqueous environments and soil, occupying a range of trophic levels. the group includes flagellates (which move with the help of undulating and beating flagella). ciliates (which move by using hair-like structures called cilia) and amoebae (which move by the use of temporary extensions of cytoplasm called pseudopodia). many protozoa, such as the agents of amoebic meningitis, use both pseudopodia and flagella. some protozoa attach to the substrate or form cysts so they do not move around (sessile). most sessile protozoa are able to move around at some stage in the life cycle, such as after cell division. the term 'theront' has been used for actively motile phases, as opposed to 'trophont' or 'trophozoite' that refers to feeding stages. walls, pellicles, scales, and skeletons unlike plants, fungi and most types of algae, most protozoa do not have a rigid external cell wall, but are usually enveloped by elastic structures of membranes that permit movement of the cell. in some protozoa, such as the ciliates and euglenozoans, the outer membrane of the cell is supported