Protozoan

protozoan, organism, usually single-celled and heterotrophic (using organic carbon as a source of energy), belonging to any of the major lineages of protists and, like most protists, typically microscopic. All protozoans are eukaryotes and therefore possess a “true,” or membrane-bound, nucleus. They also are nonfilamentous (in contrast to organisms such as molds, a group of fungi, which have filaments called hyphae) and are confined to moist or aquatic habitats, being ubiquitous in such environments worldwide, from the South Pole to the North Pole. Many are symbionts of other organisms, and some species are parasites.

amoeba

Modern ultrastructural, biochemical, and genetic evidence has rendered the term protozoan highly problematic. For example, protozoan historically referred to a protist that has animal-like traits, such as the ability to move through water as though “swimming” like an animal. Protozoans traditionally were thought to be the progenitors of modern animals, but contemporary evidence has revealed that this is not the case for most protozoans. In fact, modern science has shown that the protozoans represent a very complicated grouping of organisms that do not necessarily share a common evolutionary history. This unrelated, or paraphyletic, nature of the protozoans has caused scientists to abandon the term protozoan in formal classification schemes. Hence, the subkingdom Protozoa is now considered obsolete. Today the term protozoan is used informally in reference to nonfilamentous heterotrophic protists.

Commonly known protozoans include representative dinoflagellates, amoebas, paramecia, and the malaria-causing Plasmodium.