Paramecium (also Paramoecium /ˌpærəˈmiːʃ(i)əm/ PARR-ə-MEE-sh(ee-)əm, /-siəm/, -⁠see-əm)[1] is a genus of unicellular ciliates, commonly studied as a representative of the ciliate group. Paramecia are widespread in freshwater, brackish, and marine environments and are often very abundant in stagnant basins and ponds. Because some species are readily cultivated and easily induced to conjugate and divide, it has been widely used in classrooms and laboratories to study biological processes. Its usefulness as a model organism has caused one ciliate researcher to characterize it as the "white rat" of the phylum Ciliophora.[2]

"Paramecium aurelia"
Paramecium aurelia
Scientific classification
(unranked): SAR
Infrakingdom: Alveolata
Phylum: Ciliophora
Class: Oligohymenophorea
Order: Peniculida
Family: Parameciidae
Genus: Paramecium
Müller, 1773
  • Paramecium aurelia
  • Paramecium biaurelia
  • Paramecium buetschlii
  • Paramecium bursaria
  • Paramecium calkinsi
  • Paramecium caudatum
  • Paramecium chlorelligerum
  • Paramecium decaurelia
  • Paramecium dodecaurelia
  • Paramecium duboscqui
  • Paramecium grohmannae
  • Paramecium jenningsi
  • Paramecium multimicronucleatum
  • Paramecium nephridiatum
  • Paramecium novaurelia
  • Paramecium octaurelia
  • Paramecium pentaurelia
  • Paramecium polycaryum
  • Paramecium primaurelia
  • Paramecium putrinum
  • Paramecium quadecaurelia
  • Paramecium schewiakoffi
  • Paramecium septaurelia
  • Paramecium sexaurelia
  • Paramecium sonneborni
  • Paramecium tetraurelia
  • Paramecium tredecaurelia
  • Paramecium triaurelia
  • Paramecium undecaurelia
  • Paramecium woodruffi

Historical background

Muller paramecium aurelia
Paramecia, illustrated by Otto Müller, 1773
Joblot chausson
"Slipper animalcule", illustrated by Louis Joblot, 1718

Paramecia were among the first ciliates to be seen by microscopists, in the late 17th century. They were probably known to the Dutch pioneer of protozoology, Antonie van Leeuwenhoek, and were clearly described by his contemporary Christiaan Huygens in a letter of 1678.[3] In 1718, the French mathematics teacher and microscopist Louis Joblot published a description and illustration of a microscopic poisson (fish), which he discovered in an infusion of oak bark in water. Joblot gave this creature the name "Chausson", or "slipper", and the phrase "slipper animalcule" remained in use as a colloquial epithet for Paramecium, throughout the 18th and 19th centuries.[4] The name "Paramecium" – constructed from the Greek παραμήκης (paramēkēs, "oblong") – was coined in 1752 by the English microscopist John Hill, who applied the name generally to "Animalcules which have no visible limbs or tails, and are of an irregularly oblong figure".[5] In 1773, O. F. Müller, the first researcher to place the genus within the Linnaean system of taxonomy, adopted the name Paramecium, but changed the spelling to Paramœcium. C. G. Ehrenberg, in a major study of the infusoria published in 1838, restored Hill's original spelling for the genus name, and most researchers have followed his lead.[6]


A diagram of Paramecium caudatum

Species of Paramecium range in size from 50 to 330 micrometres (0.0020 to 0.0130 in) in length. Cells are typically ovoid, elongate, foot- or cigar-shaped. The body of the cell is enclosed by a stiff but elastic membrane (pellicle), uniformly covered with simple cilia, hairlike organelles which act like tiny oars to move the organism in one direction. Nearly all species have closely spaced spindle-shaped trichocysts embedded deeply in the cellular envelope (cortex) that surrounds the organism. Typically, an anal pore (cytoproct) is located on the ventral surface, in the posterior half of the cell. In all species, there is a deep oral groove running from the anterior of the cell to its midpoint. This is lined with inconspicuous cilia which beat continuously, drawing food inside the cell.[7] Paramecia live mainly by heterotrophy, feeding on bacteria and other small organisms. A few species are mixotrophs, deriving some nutrients from endosymbiontic algae (chlorella) carried in the cytoplasm of the cell.[8]

Osmoregulation is carried out by contractile vacuoles, which actively expel water from the cell to compensate for fluid absorbed by osmosis from its surroundings.[9] The number of contractile vacuoles varies from one, to many, depending on species.[7]


A Paramecium propels itself by whiplash movements of the cilia, which are arranged in tightly spaced rows around the outside of the body. The beat of each cilium has two phases: a fast "effective stroke", during which the cilium is relatively stiff, followed by a slow "recovery stroke", during which the cilium curls loosely to one side and sweeps forward in a counter-clockwise fashion. The densely arrayed cilia move in a coordinated fashion, with waves of activity moving across the "ciliary carpet", creating an effect sometimes likened to that of the wind blowing across a field of grain.[10]

The Paramecium spirals through the water as it progresses. When it happens to encounter an obstacle, the "effective stroke" of its cilia is reversed and the organism swims backward for a brief time, before resuming its forward progress. This is called the avoidance reaction. If it runs into the solid object again, it repeats this process, until it can get past the object.[11]

It has been calculated that a Paramecium expends more than half of its energy in propelling itself through the water.[12] This ciliary method of locomotion has been found to be less than 1% efficient. This low percentage is nevertheless close to the maximum theoretical efficiency that can be achieved by an organism equipped with cilia as short as those of the members of Paramecium.[13]

Gathering food

Paramecia feed on microorganisms like bacteria, algae, and yeasts. To gather food, the Paramecium makes movements with cilia to sweep prey organisms, along with some water, through the oral groove, and inside the mouth opening. The food passes through the cell mouth into the gullet. When enough food has accumulated at the gullet base, it forms a vacuole in the cytoplasm, which then begins circulating through the cell. As it moves along, enzymes from the cytoplasm enter the vacuole to digest the contents; digested nutrients then pass into the cytoplasm, and the vacuole shrinks. When the vacuole, with its fully digested contents, reaches the anal pore, it ruptures, expelling its waste contents to the environment.[14][15]


Some species of Paramecium form mutualistic relationships with other organisms. Paramecium bursaria and Paramecium chlorelligerum harbour endosymbiotic green algae, from which they derive nutrients and a degree of protection from predators such as Didinium nasutum.[16][17] Numerous bacterial endosymbionts have been identified in species of Paramecium.[18] Some intracellular bacteria, known as Kappa particles, give Paramecia that have them the ability to kill other strains of Paramecium that lack Kappa.[18]


The genome of the species Paramecium tetraurelia has been sequenced, providing evidence for three whole-genome duplications.[19]

In some ciliates, like Stylonychia and Paramecium, only UGA is decoded as a stop codon, while UAG and UAA are reassigned as sense codons, coding for the amino acid, Glutamic acid.[20]


The question of whether paramecia exhibit learning has been the object of a great deal of experimentation, yielding equivocal results. However, a study published in 2006 seems to show that Paramecium caudatum may be trained, through the application of a 6.5 volt electric current, to discriminate between brightness levels.[21] This experiment has been cited as a possible instance of cell memory, or epigenetic learning in organisms with no nervous system.[22] However, another study in 2017 suggested that the paramecium can only learn to associate bright side of its swimming medium to electric current and not the dark side.[23] The same study suggested a molecular mechanism for learning in paramecium.[24][23]

Reproduction and sexual phenomena

Like all ciliates, Paramecium has a dual nuclear apparatus, consisting of a polyploid macronucleus, and one or more diploid micronuclei. The macronucleus controls non-reproductive cell functions, expressing the genes needed for daily functioning. The micronucleus is the generative, or germline nucleus, containing the genetic material that is passed along from one generation to the next.[25]

Paramecium reproduces asexually, by binary fission. During reproduction, the macronucleus splits by a type of amitosis, and the micronuclei undergo mitosis. The cell then divides transversally, and each new cell obtains a copy of the micronucleus and the macronucleus.[2]

Fission may occur spontaneously, in the course of the vegetative cell cycle. Under certain conditions, it may be preceded by self-fertilization (autogamy),[26] or it may follow conjugation, a sexual phenomenon in which Paramecium of compatible mating types fuse temporarily and exchange genetic material. During conjugation, the micronuclei of each conjugant divide by meiosis and the haploid gametes pass from one cell to the other. The gametes of each organism then fuse to form diploid micronuclei. The old macronuclei are destroyed, and new ones are developed from the new micronuclei.[25]

Autogamy or conjugation can be induced by shortage of food at certain points in the Paramecium life cycle.[27]


In the asexual fission phase of growth, during which cell divisions occur by mitosis rather than meiosis, clonal aging occurs leading to a gradual loss of vitality. In some species, such as the well studied Paramecium tetraurelia, the asexual line of clonally aging paramecia loses vitality and expires after about 200 fissions if the cells fail to undergo autogamy or conjugation. The basis for clonal aging was clarified by transplantation experiments of Aufderheide.[28] When macronuclei of clonally young paramecia were injected into paramecia of standard clonal age, the lifespan (clonal fissions) of the recipient was prolonged. In contrast, transfer of cytoplasm from clonally young paramecia did not prolong the lifespan of the recipient. These experiments indicated that the macronucleus, rather than the cytoplasm, is responsible for clonal aging. Other experiments by Smith-Sonneborn,[29] Holmes and Holmes,[30] and Gilley and Blackburn[31] demonstrated that, during clonal aging, DNA damage increases dramatically (also reviewed by Bernstein and Bernstein).[32] Thus, DNA damage in the macronucleus appears to be the cause of aging in P. tetraurelia. In this single-celled protist, aging appears to proceed as it does in multicellular eukaryotes, as described in DNA damage theory of aging.

Meiosis and rejuvenation

When clonally aged P. tetraurelia are stimulated to undergo meiosis in association with either conjugation or automixis, the progeny are rejuvenated, and are able to have many more mitotic binary fission divisions. During either of these processes the micronuclei of the cell(s) undergo meiosis, the old macronucleus disintegrates and a new macronucleus is formed by replication of the micronuclear DNA that had recently undergone meiosis. There is apparently little, if any, DNA damage in the new macronucleus. These findings suggest that clonal aging is due, in large part, to a progressive accumulation of DNA damage (see DNA damage theory of aging); and that rejuvenation is due to the repair of this damage in the micronucleus during meiosis. Meiosis appears to be an adaptation for DNA repair and rejuvenation in these paramecia.[33]

Video gallery

Paramecium bursaria, a species with symbiotic algae

Paramecium putrinum

Paramecium binary fission

Paramecium caudatum in conjugation

List of species

  • Paramecium africanum Dragesco, 1970
  • Paramecium aurelia complex (includes biological species, P. primaurelia, P. biaurellia, etc.)
  • Paramecium bursaria (Ehrenberg, 1831) Focke, 1836
  • Paramecium calkinsi Woodruff, 1921
  • Paramecium caudatum Ehrenberg, 1834 (marine, brackish and freshwater)
  • Paramecium chlorelligerum Kahl, 1935
  • Paramecium duboscqui Chatton & Brachon, 1933
  • Paramecium jankowskii Dragesco, 1972
  • Paramecium jenningsi Diller & Earl, 1958
  • Paramecium nephridiatum Gelei, 1925
  • Paramecium polycaryum Woodruf, 1923
  • Paramecium pseudotrichium Dragesco, 1970
  • Paramecium putrinum Claparède & Lachmann, 1859
  • Paramecium schewiakoffi Fokin, Przybos, Chivilevc, §, 2004
  • Paramecium sonneborni Aufderheide, Daggett & Nerad, 1983
  • Paramecium ugandae Dragesco, 1972
  • Paramecium wichtermani, Mohammed and Nashed, 1968–1969,
  • Paramecium woodruffi Wenrich, 1928


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External links


Animalcule ("little animal", from Latin animal + the diminutive suffix -culum) is an older term for a microscopic animal or protozoan. The concept appears to have been proposed at least as early as around 30 BC, as evidenced by this translation from Marcus Varro's Rerum Rusticarum Libri Tres:

"Note also if there be any swampy ground, both for the reasons given above, and because certain minute animals, invisible to the eye, breed there, and, borne by the air, reach the inside of the body by way of the mouth and nose, and cause diseases which are difficult to be rid of."Some better-known animalcules include:

Actinophrys, and other heliozoa, called sun animalcules

Amoeba, called Proteus animalcule

Noctiluca scintillans, commonly called the sea sparkles

Paramecium, called slipper animalcules

Rotifers, called wheel animalcules

Stentor, called trumpet animalcules

Vorticella, and other peritrichs, called bell animalculesThe term was also used by Anton van Leeuwenhoek, the 17th-century preformationist and the discoverer of microorganisms, to describe them.The word appears in adjectival form in the "Major-General's Song", in which Major-General Stanley sings, "I know the scientific names of beings animalculous..."


Autogamy, or self-fertilization, refers to the fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a reproductive mechanism employed by many flowering plants. However, species of protists have also been observed using autogamy as a means of reproduction. Flowering plants engage in autogamy regularly, while the protists that engage in autogamy only do so in stressful environments.

Avoidance reaction

Avoidance reaction is a term used in the description of the movement of paramecium. This helps the cell avoid obstacles and causes other objects to bounce off of the cell's outer membrane. The paramecium does this by reversing the direction in which its cilia beat. This results in stopping, spinning or turning, after which point the paramecium resumes swimming forward. If multiple avoidance reactions follow one another, it is possible for a paramecium to swim backward, though not as smoothly as swimming forward.Avoidance reaction occurs when the cell hits an obstruction, providing an anterior, mechanical stimulus:

- The cell will then reverse.

- It will then stop and rotate.

- Now facing a new direction, the cell will move off in that direction.

This process will continue until the cell is able to negotiate its way around the obstruction.

Movement of Paramecium cells is caused by control of calcium ions inside the cell and membrane potentials. The simplest explanation for the avoidance reaction is that membrane potential controls the influx of calcium ions, which regulates the beat frequency and angles of cilia on the surface of the cell.


Chilomonas is a genus of cryptophytes, including the species Chilomonas paramecium. Chilomonas is a protozoa (heterotroph). Chilomonas is golden brown and has two flagella.


Chlorovirus, also known as Chlorella virus, is a genus of giant double-stranded DNA viruses, in the family Phycodnaviridae. This genus is found globally in freshwater environments where freshwater microscopic algae serve as natural hosts. There are currently 19 species in this genus including the type species Paramecium bursaria Chlorella virus 1.Chlorovirus was initially discovered in 1981 by Russel H. Meintz, James L. Van Etten, Daniel Kuczmarski, Kit Lee, and Barbara Ang while attempting to culture Chlorella-like alga. During the attempted process viral particles were discovered in the cells 2 to 6 hours after being initially isolated, followed by lysis after 12 to 20 hours. This virus was initially called HVCV (Hydra viridis Chlorella virus) since it was first found to infect Chlorella-like algae.Though relatively new to virologists and thus not extensively studied, one species, Chlorovirus ATCV-1, commonly found in lakes, has been recently found to infect humans. New studies focusing on effects of infection in mouse model are currently emerging as well.


The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different undulating pattern than flagella. Cilia occur in all members of the group (although the peculiar Suctoria only have them for part of their life-cycle) and are variously used in swimming, crawling, attachment, feeding, and sensation.

Ciliates are an important group of protists, common almost anywhere there is water — in lakes, ponds, oceans, rivers, and soils. About 3,500 species have been described, and the potential number of extant species is estimated at 30,000. Included in this number are many ectosymbiotic and endosymbiotic species, as well as some obligate and opportunistic parasites. Ciliate species range in size from as little as 10 µm to as much as 4 mm in length, and include some of the most morphologically complex protozoans.In most systems of taxonomy, "Ciliophora" is ranked as a phylum, under either the kingdom Protista or Protozoa. In some systems of classification, ciliated protozoa are placed within the class "Ciliata," (a term which can also refer to a genus of fish). In the taxonomic scheme proposed by the International Society of Protistologists, which eliminates formal rank designations such as "phylum" and "class", "Ciliophora" is an unranked taxon within Alveolata.

Contractile vacuole

A contractile vacuole (CV) is a sub-cellular structure (organelle) involved in osmoregulation. It is found predominantly in protists and in unicellular algae. It was previously known as pulsatile or pulsating vacuole


The Holosporaceae are a family of bacteria, formerly included in the order Rickettsiales, but now raised to their own order, the Holosporales. The member Holospora is an intracellular parasite found in the unicellular protozoa Paramecium.The genera Caedibacter, Holospora, Lyticum, Odyssella, Pseudocaedibacter, Pseudolyticum and Tectibacter

are Incertae sedis.

Kappa organism

In biology, Kappa organism or Kappa particle refers to inheritable cytoplasmic symbionts, occurring in some strains of the ciliate Paramecium. Paramecium strains possessing the particles are known as "killer paramecia". They liberate a substance also known as paramecin into the culture medium that is lethal to Paramecium that do not contain kappa particles. Kappa particles are found in genotypes of Paramecium aurelia syngen 2 that carry the dominant gene K.Kappa particles are Feulgen-positive and stain with Giemsa after acid hydrolysis. The length of the particles is 0.2–0.5μ.While there was initial confusion over the status of kappa particles as viruses, bacteria, organelles, or mere nucleoprotein, the particles are intracellular bacterial symbionts called Caedibacter taeniospiralis. Caedibacter taeniospiralis contains cytoplasmic protein inclusions called R bodies which act as a toxin delivery system.


The Litostomatea are a class of ciliates. The group consists of three subclasses: Haptoria, Trichostomatia and Rhynchostomatia. Haptoria includes mostly carnivorous forms such as Didinium, a species of which preys primarily on the ciliate Paramecium. Trichostomatia (trichostomes) are mostly endosymbionts in the digestive tracts of vertebrates. These include the species Balantidium coli, which is the only ciliate parasitic in humans. The group Rhynchostomatia includes two free-living orders previously included among the Haptoria, but now known to be genetically distinct from them, the Dileptida and the Tracheliida.


Myzocytosis (from Greek: myzein, (μυζεῖν) meaning "to suck" and kytos (κύτος) meaning "container", hence referring to "cell") is a method of feeding found in some heterotrophic organisms. It is also called "cellular vampirism" as the predatory cell pierces the cell wall and/or cell membrane of the prey cell with a feeding tube, the conoid, sucks out the cellular content and digests it.

Myzocytosis is found in Myzozoa and also in some species of Ciliophora (both comprise the alveolates). A classic example of myzocytosis is the feeding method of the infamous predatory ciliate, Didinium, where it is often depicted devouring a hapless Paramecium. The suctorian ciliates were originally thought to have fed exclusively through myzocytosis, sucking out the cytoplasm of prey via superficially drinking straw-like pseudopodia. It is now understood that suctorians do not feed through myzocytosis, but actually, instead, manipulate and envenomate captured prey with their tentacle-like pseudopodia.


Parameciidae are a family of ciliates in the order Peniculida; the body has differentiated anterior and posterior ends and is bounded by a hard but elastic pellicle. The family contains only the genus Paramecium, as well as the genus incertae sedis Physanter.

Paramecium aurelia

Paramecium aurelia are unicellular organisms belonging to the genus Paramecium of the phylum Ciliophora. They are covered in cilia which help in movement and feeding.Paramecium can reproduce sexually, asexually, or by the process of endomixis. Paramecium aurelia demonstrate a strong “sex reaction” whereby groups of individuals will cluster together, and emerge in conjugant pairs. This pairing can last up to 12 hours, during which the micronucleus of each organism will be exchanged. In Paramecium aurelia, a cryptic species complex was discovered by observation. Since then, some have tried to decode this complex using genetic data.

Paramecium bursaria

Paramecium bursaria is a species of ciliates that has a mutualistic endosymbiotic relationship with green algae called Zoochlorella. The algae live inside the Paramecium in its cytoplasm and provide it with food, while the Paramecium provides the algae with movement and protection. P. bursaria is 80-150 μm long, with a wide oral groove, two contractile vacuoles, and a single micronucleus as well as a single macronucleus. P. bursaria is the only species of Paramecium that forms symbiotic relationships with algae, and it is often used in biology classrooms both as an example of a protozoan and also as an example of symbiosis.

A transcriptome sequence is determined.

Paramecium caudatum

Paramecium caudatum is a species of unicellular organisms belonging to the genus Paramecium of the phylum Ciliophora. They can reach 0.25mm in length and are covered with minute hair-like organelles called cilia. The cilia are used in locomotion and feeding.

Paramecium sonneborni

Paramecium sonneborni is a species of unicellular organisms belonging to the genus Paramecium of the phylum Ciliophora. It was first isolated in Texas and named after Tracy M. Sonneborn. It is a member of the Paramecium aurelia species complex.


Phycodnaviridae is a family of large (100–560kb) double stranded DNA viruses that infect marine or freshwater eukaryotic algae. Viruses within this family are similar morphologically and possess an icosahedral capsid (polyhedron with 20 faces). There are currently 33 species in this family, divided among 6 genera. This family belongs to a super-group of large viruses known as nucleocytoplasmic large DNA viruses. Recently, there is evidence that specific strains of Phycodnaviridae may infect humans rather than just algal species, as was previously believed. Most genera under this family enter the cell of the host by cell receptor endocytosis and replicate in the nucleus. Phycodnaviridae play important ecological roles by regulating the growth and productivity of their algal hosts. Algal species such Heterosigma akashiwo and the genus Chrysochromulina can form dense blooms which can be damaging to fisheries, resulting in losses in the aquaculture industry. "Heterosigma akashiwo virus" (HaV) has been suggested for use as a microbial agent to prevent the recurrence of toxic red tides produced by this algal species. Furthermore, Phycodnaviridae cause death and lysis of freshwater and marine algal species, liberating organic carbon, nitrogen and phosphorus into the water, providing nutrients for the microbial loop.


Polyglycylation is a form of posttranslational modification of glutamate residues of the carboxyl-terminal region tubulin in certain microtubules (e.g., axonemal) originally discovered in Paramecium, and later shown in mammalian neurons as well.


A trichocyst is an organelle found in certain ciliates and dinoflagellates.A trichocyst can be found in tetrahymena and along cila pathways of several metabolic systems.

It is also a structure in the cortex of certain ciliate and flagellate protozoans consisting of a cavity and long, thin threads that can be ejected in response to certain stimuli. Trichocysts may be widely distributed over an organism or restricted to certain areas (e.g., tentacles, papillae, around the mouth). There are several types. Mucoid trichocysts are elongated inclusions that may be ejected as visible bodies after artificial stimulation. Filamentous trichocysts in Paramecium and other ciliates are discharged as filaments composed of a cross-striated shaft and a tip. Toxicysts (in Dileptus and certain other carnivorous protozoans) tend to be localized around the mouth. When discharged, a toxicyst expels a long, nonstriated filament with a rodlike tip, which paralyzes or kills other microorganisms; this filament is used to capture food and, presumably, in defense.

The functional significance of other trichocysts is uncertain, although those of Paramecium apparently can be extruded for anchorage during feeding.



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