Amoebozoa

Amoebozoa is a large group of protists, with roughly 2,400 known species. These organisms are characterized by their blunt, finger-like pseudopods and mitochondria that have branching, tubular cristae. Older classification systems typically placed Amoebozoa as a phylum within the kingdoms Protista or Protozoa. Today, the International Society of Protistologists treats it as an unranked supergroup within Eukaryota. Genetic evidence confirms that Amoebozoa is a single, natural lineage (monophyletic). Phylogenetic studies show it is the sister group to Opisthokonta, the clade that includes fungi, animals, and about 300 species of single-celled eukaryotes. Because of this close relationship, Amoebozoa and Opisthokonta are sometimes combined into a higher taxon called Amorphea.

This group includes many well-known amoebae, such as *Amoeba*, *Chaos*, *Entamoeba*, and *Pelomyxa*. Species can be either naked or covered by a shell (testate), and some cells have flagella. Free-living forms are common in saltwater, freshwater, soil, moss, and leaf litter. Others live as parasites or symbionts, and a few cause disease in humans and other organisms. Although most amoebozoans are single-celled, the group also contains several types of slime molds. These have a multicellular stage: either a macroscopic plasmodium formed by repeated cell division without separation, or, in cellular slime molds, an aggregate of individual cells.

Size varies enormously. Some species are only 10–20 μm across, while others are among the largest protozoa. *Amoeba proteus*, which can reach 800 μm, is a common model organism in classrooms and labs. Multinucleate species like *Chaos* and *Pelomyxa* can be several millimeters long. Some multicellular slime molds, such as *Fuligo septica* (the “dog vomit” slime mold), can cover several square meters.

**Morphology**

Despite the group’s diversity, many members share common features. The cell is typically divided into a granular inner endoplasm and a clear outer ectoplasm. During movement, the endoplasm flows forward while the ectoplasm moves backward along the cell’s exterior. Many amoebozoans have a clear front and back and can move in a monopodial form, using the whole cell as a single pseudopod. Large pseudopods may produce smaller, temporary projections called subpseudopodia, used for locomotion or feeding. A cell can also extend several indeterminate pseudopods, through which the entire cell contents flow in the direction of travel. The cell mass pours into the leading pseudopod, and the others retract unless the organism changes course.

Most amoebozoans are naked (like *Amoeba* and *Chaos*) or covered with a loose coat of tiny scales (like *Cochliopodium* and *Korotnevella*). Members of the order Arcellinida, however, build rigid shells (tests) with a single opening for pseudopods. These tests can be secreted from organic material (as in *Arcella*) or assembled from collected particles cemented together (as in *Difflugia*).

The primary feeding method is phagocytosis: the cell surrounds food particles with pseudopods, seals them into vacuoles, and digests them. Some species have a posterior bulb called a uroid, which may collect waste and periodically detach. When food is scarce, most can form cysts, which can be carried through the air to new environments. In slime molds, these structures are called spores and are produced on stalked fruiting bodies (sporangia). Mixotrophic species that host symbiotic *Chlorella* algae inside their cytoplasm occur in Arcellinida and *Mayorella*.

Most Amoebozoa lack flagella and do not form microtubule-supported structures except during mitosis. Flagella do appear in the Archamoebae, and many slime molds produce biflagellate gametes. The flagellum is anchored by a cone of microtubules, hinting at a close relationship with opisthokonts. Mitochondria typically have branching tubular cristae, but Archamoebae, which live in low-oxygen habitats, have lost their mitochondria.

**Classification**

Molecular genetics places Amoebozoa as a sister group to animals and fungi, diverging after that lineage split from other eukaryotes. Strong similarities between Amoebozoa and Opisthokonta suggest they form a distinct clade. Thomas Cavalier-Smith proposed the name Unikonta for this branch, based on the idea of a common ancestor with a single emergent flagellum rooted in one basal body. While the close relationship is well supported, the uniciliate ancestor hypothesis is now considered unlikely. In the 2012 Revised Classification of Eukaryotes, Adl et al. proposed the name Amorphea for this clade.