This website covers those groups of protists traditionally lumped together under the name “amoebae”. Protists are defined as a confederation of eukaryotic organisms that lack the tissue-level organization found in plants, animals, and fungi. The term “amoebae” is used for those protists that move and feed by means of cytoplasmic streaming, a unique mode of locomotion known as amoeboid movement. Amoebae are also characterized by their irregular shape and the absence of a constant body form. The name “amoeba” is derived from an ancient Greek word meaning “changing”. Cilia, if present, are usually restricted to developmental stages. A defining feature of amoeboid locomotion is the formation of pseudopodia, temporary cytoplasmic extensions.

Amoeboid protists comprise a large and highly diverse group with more than 15,000 described species. Most are free‑living, ubiquitous organisms that are widespread in nature and occur in nearly any moist or aquatic habitat, whether aerobic or anaerobic. Some amoeboids are endosymbiotic, including parasitic and pathogenic forms.

Many protists exhibit amoeboid movement and feeding, yet many of them are not closely related. Therefore, the term “amoebae” has no taxonomic value and is comparable to grouping organisms into categories such as “flying animals” or “four‑legged animals”.

Diversity
Traditionally, amoebae were divided into shelled forms (Shellacea) and naked amoeboids (Gymnamoebae). These names were once recognized taxonomic units used to classify these protozoa, but the introduction of new techniques, such as electron microscopy, has dramatically changed our understanding. An example is the common amoeba Korotnevella (Fig. 1).

In light microscopy the cell appears naked, without any visible covering (Fig. 1A), but electron microscopy reveals that it is densely coated with self‑secreted micro‑scales (Fig. 1B).

Some amoeboids, such as Cochliopodium species, possess larger micro‑scales that can be seen easily with a light microscope. A coating of micro‑scales is flexible and protects the slow‑moving cell against predators; this covering is called a tectum.

Other amoeboids have developed a thickened cell membrane known as a pellicle or theca. There are also amoebae that construct a shell, much like snails do. These shells, or tests, are built from collected environmental particles, self‑secreted scales or platelets, or a combination of both.

The greatest diversity of amoeboid organisms occurs within the phyla Amoebozoa and Cercozoa. They represent by far the largest biomass of any protist group in soils and are also abundant in marine habitats. Most species prey on bacteria and other protists.