# Paramecium caudatum
*Paramecium caudatum* is a species of large, free-living freshwater ciliate, a single-celled eukaryote covered in cilia and carrying a dimorphic nuclear apparatus of a bean-shaped macronucleus and a spherical micronucleus[1](https://sites.google.com/view/devagiridigitalmuseum/specimens/invertebrates/porifera/paramecium-caudatum). First named by Ehrenberg in 1834, it is one of the most familiar members of the genus *Paramecium* and a long-standing model organism for cell biology, genetics, and ecotoxicology[2](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?command=show&id=5885&mode=node) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC11334263/).
| Key fact | Detail |
|---|---|
| Scientific name | *Paramecium caudatum* Ehrenberg 1834; NCBI Taxonomy ID 5885, within Ciliophora, Oligohymenophorea, Peniculida[2](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?command=show&id=5885&mode=node) |
| Cell size | 170–310 μm in length, among the larger *Paramecium* species[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC11334263/) |
| Nuclear dimorphism | A transcriptionally silent germline micronucleus and a polyploid somatic macronucleus in the same cytoplasm[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC11334263/) |
| Genome sizes | Somatic macronuclear genome about 30 Mb; germline micronuclear genome about 1,600 to 5,500 Mb[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10876655/) |
| DNA elimination | More than 98% of the germline genome is removed during somatic genome development, the most extreme known among eukaryotes[5](https://pubmed.ncbi.nlm.nih.gov/42810615/) |
| Gene count | 8,713 annotated protein-coding genes in strain 43c3d, the lowest among sequenced *Paramecium* congeners[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC11334263/) |
| Research uses | Model for cell biology, genetics, genome rearrangement, and ecotoxicity testing[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC11334263/) • [4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10876655/) |
## Structure and life cycle
Like all ciliates, *P. caudatum* carries two types of nuclei in each cell: a germline micronucleus, which divides mitotically but stays transcriptionally silent during vegetative growth, and a somatic macronucleus, which is polyploid, constantly transcriptionally active, and divides amitotically[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC11334263/). The cell surface is uniformly ciliated, with cilia used for both locomotion and food collection, and an oral groove on the ventral surface leads to the buccal cavity; nutrition is holozoic, mainly bacterial[1](https://sites.google.com/view/devagiridigitalmuseum/specimens/invertebrates/porifera/paramecium-caudatum) • [6](https://www.artis.nl/en/artis-micropia/featured-at-artis-micropia/paramecium-caudatum).
Reproduction is asexual by transverse binary fission, with sexual processes by conjugation[1](https://sites.google.com/view/devagiridigitalmuseum/specimens/invertebrates/porifera/paramecium-caudatum). During conjugation the parental macronucleus of *P. caudatum* fragments into some 40 to 50 pieces; these fragments do not degenerate immediately but persist until about the eighth cell cycle after conjugation, with their DNA degraded at about the fifth cell cycle, after a new macronucleus has fully formed[7](https://pubmed.ncbi.nlm.nih.gov/15354289/).
## An extreme germline genome
**Genome contrast.** *P. caudatum* has an unexpectedly large germline genome, estimated at approximately 1,600 to 5,500 Mb, compared with 108 Mb in *P. tetraurelia*, 157 Mb in *Tetrahymena thermophila*, and about 500 Mb in *Oxytricha trifallax*; only about two percent of this germline DNA is retained in the somatic genome after sexual processes[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10876655/). A recent study describes this as the most extreme DNA elimination known among eukaryotes, removing more than 98% of the germline genome, and presents a near-chromosome-level macronuclear genome assembly that is exceptionally compact, with ultra-short untranslated regions and high gene density[5](https://pubmed.ncbi.nlm.nih.gov/42810615/).
The somatic macronuclear genome is small by comparison, about 30 Mb versus roughly 70 Mb in *P. tetraurelia*, because *P. caudatum* diverged from the *P. aurelia* complex before the two most recent whole-genome duplications and therefore carries fewer homologous genes[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10876655/). Gene counts reflect this: strain 43c3d has 8,713 protein-coding genes, the lowest among sequenced congeners, against a high of 49,951 in *P. sonneborni*[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC11334263/). Despite the smaller macronuclear genome, the enormous difference between germline and somatic genome sizes makes *P. caudatum* a likely source of insight into genome rearrangement[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10876655/).
**Genetic tools.** For years, genetic engineering techniques had not been reported in *P. caudatum*, hampering such investigations[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10876655/). In 2024, researchers established [RNA interference](https://www.edgepedia.org/rna-interference) by feeding and protein localization by microinjection of fluorescent or antibody-tagged fusion constructs in the species, achieving expected phenotypes for both housekeeping genes (ND7 and RPB1) and developmentally regulated genes[8](https://www.nature.com/articles/s42003-024-05906-2). Using these methods they showed that *P. caudatum* PiggyMac, a domesticated piggyBac transposase, is essential for sexual development and is likely an active transposase directly involved in DNA cleavage[8](https://www.nature.com/articles/s42003-024-05906-2).
## Ecology, symbionts, and species status
*P. caudatum* occurs commonly in still and moving fresh water, together with *P. multimicronucleatum*, *P. aurelia*, and *P. bursaria*[9](https://www.sciencedirect.com/science/article/pii/S0091679X08617586). Its nuclei can host intracellular bacteria: in 1890 the microbiologist Waldemar Hafkine described microorganisms in the nuclei of *P. caudatum* and named them Holospora, now a recognized genus of intranuclear symbionts[10](https://academic.oup.com/femsec/article/94/7/fiy076/4987202).
Molecular analyses using ARDRA and RAPD fingerprinting have rejected the idea that *P. caudatum* is a complex of sibling species, concluding that it is a single species[11](https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1463-6409.2000.00031.x). Even so, sequence comparisons of mitochondrial genomes find substantial divergence between some isolates, with mean synonymous divergence of about 2.0 between *P. caudatum* and the isolate *P. caudatum*-C026, implying these two may represent separate species[12](https://pmc.ncbi.nlm.nih.gov/articles/PMC6505448/).
## Use as a model organism
**Classical and modern research.** *P. caudatum* was the organism with which the Russian ecologist G. F. Gause tested the hypothesis of competitive exclusion in the 1930s, showing that *P. caudatum* and *P. aurelia* occupying the same ecological niche could not coexist[13](https://www.scielo.br/j/alb/a/zfd6K9GVPxz4pT63gbyb7Vy/?lang=en). Its large cell size, ease of laboratory culture, and short cell cycle have kept it in use for studies of behavior, population growth, and cell biology[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC11334263/) • [13](https://www.scielo.br/j/alb/a/zfd6K9GVPxz4pT63gbyb7Vy/?lang=en).
**Ecotoxicology.** Ciliates have been used in toxicity testing since the 1950s, when *P. caudatum* was exposed to copper, mercury, cadmium, zinc, nickel, cobalt, and chromium[13](https://www.scielo.br/j/alb/a/zfd6K9GVPxz4pT63gbyb7Vy/?lang=en). Along with *Tetrahymena thermophila*, it is among the most tested ciliate species in heavy-metal ecotoxicology[14](https://www.sciencedirect.com/science/article/pii/S014765132030508X). Typical endpoints include lethal concentration, population growth, generation time, behavior, and morphology; for example, in acute tests with the herbicide diuron, the 6-hour LC50 for pure diuron was 64.6 ± 3.3 mg/L, and chronic pesticide exposure significantly reduced population growth, generation number, and biomass[15](https://pubmed.ncbi.nlm.nih.gov/26890484/). The species has also been used to assess nanoparticle clearance, with cells clearing an estimated 0.7 fg of contaminant material per cell over a 5 mm distance in a 6-hour experiment while showing few short-term adverse effects[16](https://pmc.ncbi.nlm.nih.gov/articles/PMC6586663/).
## References
1. [Devagiri Digital Zoology Museum: Paramecium caudatum](https://sites.google.com/view/devagiridigitalmuseum/specimens/invertebrates/porifera/paramecium-caudatum)
2. [NCBI Taxonomy Browser: Paramecium caudatum](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?command=show&id=5885&mode=node)
3. [Long et al., Paramecium Genetics, Genomics, and Evolution, Annual Review of Genetics (2023)](https://pmc.ncbi.nlm.nih.gov/articles/PMC11334263/)
4. [Gao et al., Application of RNA interference and protein localization ... in the emerging model protozoan Paramecium caudatum, Communications Biology (2024)](https://pmc.ncbi.nlm.nih.gov/articles/PMC10876655/)
5. [Transcriptional reprogramming and epigenetic regulation in Paramecium caudatum, Journal of Genetics and Genomics](https://pubmed.ncbi.nlm.nih.gov/42810615/)
6. [ARTIS-Micropia: Paramecium caudatum](https://www.artis.nl/en/artis-micropia/featured-at-artis-micropia/paramecium-caudatum)
7. [Delayed degradation of parental macronuclear DNA in programmed nuclear death of Paramecium caudatum (2004)](https://pubmed.ncbi.nlm.nih.gov/15354289/)
8. [Gao et al., Communications Biology (2024), full article](https://www.nature.com/articles/s42003-024-05906-2)
9. [Methods in Paramecium Research, Methods in Cell Biology, Volume 4 (1970)](https://www.sciencedirect.com/science/article/pii/S0091679X08617586)
10. [Preeria caryophila comb. nov., FEMS Microbiology Ecology (2018)](https://academic.oup.com/femsec/article/94/7/fiy076/4987202)
11. [Stoeck et al., ARDRA and RAPD-fingerprinting reject the sibling species concept for Paramecium caudatum, Zoologica Scripta (2000)](https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1463-6409.2000.00031.x)
12. [Population Genetics of Paramecium Mitochondrial Genomes, Genome Biology and Evolution](https://pmc.ncbi.nlm.nih.gov/articles/PMC6505448/)
13. [Vilas-Boas, Senra and Dias, Ciliates in ecotoxicological studies: a minireview, Acta Limnologica Brasiliensia (2020)](https://www.scielo.br/j/alb/a/zfd6K9GVPxz4pT63gbyb7Vy/?lang=en)
14. [Ciliates as model organisms for the ecotoxicological risk assessment of heavy metals: a meta-analysis, Ecotoxicology and Environmental Safety (2020)](https://www.sciencedirect.com/science/article/pii/S014765132030508X)
15. [Mansano et al., Effects of diuron and carbofuran ... on Paramecium caudatum, Environmental Pollution (2016)](https://pubmed.ncbi.nlm.nih.gov/26890484/)
16. [On measuring nanoparticle toxicity and clearance with Paramecium caudatum](https://pmc.ncbi.nlm.nih.gov/articles/PMC6586663/)
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*Topic: Encyclopedia › Life and health › Microorganisms and fungi › Other microbial eukaryotes › Ciliates › Model ciliate taxa*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license