(Schaeffer, 1916) Schaeffer, 1926
Polychaos dubium is a freshwater amoeboid and one of the larger species of protist. Like other amoebozoans, P. dubium moves by means of temporary projections called pseudopods. P. dubium reportedly has the largest genome of any organism known.
Polychaos dubium was previously known as Amoeba dubia. The author who named the species later recognized it as different from species of Amoeba, and so designated it the type species of the genus Polychaos. Unlike species of Amoeba, P. dubium lacks longitudinal ridges on its pseudopods.
A few characteristics distinguish Polychaos dubium from other species of Polychaos. The crystals floating in its cytoplasm take the shape of flat bipyramids, flat plates, or clustered platelets. The nucleus is ellipsoid in shape, has granules next to the membrane, and lacks an endosome. The cell is usually polypodal (has many pseudopodia), and the endoplasm and ectoplasm of the cytoplasm are clear. During rapid locomotion, P. dubium may become monopodial (present only one pseudopod),  but there are an average of 12 pseudopodia. 
Polychaos dubium may have the largest genome known for any organism, consisting of 670 billion base pairs of DNA, which is over 200 times larger than the human genome. The authors of one study, however, suggest treating that measurement with caution, because it was taken before the advent of modern genomic methods.
Ecology and range
- Schaeffer, Asa Arthur (1926a). "Taxonomy of the amebas, with descriptions of thirty-nine new marine and freshwater species". Papers from the Department of Marine Biology of the Carnegie Institution of Washington. 24: 115 pp. 12 plates.
- Schaeffer, Asa A. (1916). "Notes on the specific and other characters of Amoeba proteusPall. (Leidy), A. discoides spec. nov., and A. dubia spec. nov.". Archiv für Protistenkunde 37: 204–228.
- Laura Wegener Parfrey et al. (2008). "The Dynamic Nature of Eukaryotic Genomes". Molecular Biology and Evolution 25: 787–794. doi:10.1093/molbev/msn032. PMC 2933061. PMID 18258610.
- Lee, J.J., Hutner, S.H. & Bovee, E.C., ed. (1985). Illustrated Guide to the Protozoa. Lawrence, Kansas: Society of Protozoologists. pp. ix + 630 pages.
- "Polychaos dubium". micro*scope - version 6.0. Retrieved 2009-10-01.
- "Polychaos dubium". Protist Information Server. Retrieved 2009-10-01.
- McGrath, Casey, L, & Katz, Laura A. (2004). "Genome diversity in microbial eukaryotes". TRENDS in Ecology and Evolution 19 (1): 32–38. doi:10.1016/j.tree.2003.10.007. PMID 16701223.
- "Polychaos dubium". Amoebae on the Web. Retrieved 2009-10-01.
- Leidy, Joseph (1878). "Amoeba proteus". The American Naturalist 12 (4): 235–238. doi:10.1086/272082.