Amoebozoa is a group consisting of about 2,400 described species. It includes many of the amoeboid organisms, but not all. The word amoeba, or amoeboid organism, is a nonspecific term used to describe a cell that can change its shape by moving, extending, and retracting pseudopodia (or pseduopods). Pseudopods are projections of the cytoplasm that move by polymerization and depolymerization of actin.  They allow the cell to move and engulf food particles. Amoebozoan pseudopods are lobe- or tube-shaped, as opposed to the slender, hair-like pseudopodia of Rhizarians and other groups. Some amoebozoans also have flagella.

Most amoebozoan species are unicellular, amoeboid organisms, although some are multicellular or have a multicellular phase of their life cycle. Many slime molds, for example, are classified in this supergroup. Amoebozoans are heterotrophs and acquire food via phagocytosis. They lack cell walls. Some amoebae have a shell, or test, for protection. Others, the “naked” amoebae, lack a test.

Many amoebozoan species are free-living in water or soil; others are symbionts. These include the Entamoebas, which are animal parasites or commensals. Entamoeba histolytica is the most common cause of amoebic dysentery, which can be fatal. Other amoebozoans, such as Entamoeba coli and Endolimax nana, are human commensals and do not cause disease.

Recent molecular analyses place Supergroup Amoebozoa as the sister clade of Opisthokonta, which includes animals and fungi. Some scientists have proposed a higher-order clade, called Amorphea, which includes Amoebozoans and Opisthokonts.

Amoebae

The image that probably comes immediately to mind when you hear the word ‘amoeba’ is that of a lobose amoebozoan, with broad pseudopods and no test. We will examine two naked amoebae (Gymnamoebae) and two testate amoebae.

Amoeba proteus is a naked amoeba that is widespread in freshwater environments. It feeds on ciliates, other amoebas, and small animals such as rotifers.

Chaos carolinensis is a naked amoeba, also known as the giant amoeba. It can be large enough (1-5mm) to be seen with the naked eye. Chaos can have multiple nuclei.

Arcella vulgaris is a testate amoeba, with a shell composed of chitin. Its test is approximately hemispherical in shape and has an opening through which the pseudopodia extend. It has two nuclei.

Difflugia lobostoma is a testate amoeba that constructs its shell from debris scavenged from the environment, generally grains of sand, which the amoeba cements together. The test has an opening through which the pseudopodia extend.

Plasmodial Slime Molds

Plasmodial slime molds are essentially huge, multinucleate cells enclosed by a plasma membrane. They can be over a meter in diameter and are easily seen with the naked eye in their plasmodial form. They do not have plastids. They are heterotrophs, and may feed on bacteria, yeast, fungi, and decaying organic material.

Plasmodial slime molds form fruiting bodies in which spores are produced by meiosis. These are carried by wind and germinate to form gametes, which are either amoeboid or flagellated. Fusion of an amoeboid gamete with a flagellated gamete results in a diploid cell, which divides its nucleus mitotically many times. It does not complete cytokinesis, however, resulting in a large, multinucleate cell called a plasmodium.

Plasmodial slime molds can sense the environment through protein receptors. They have been investigated for their navigation abilities. Slime molds can “solve” mazes by finding the shortest path between food sources.

Cellular Slime Molds

Cellular slime molds are heterotrophic organisms that exist as individual amoeboid cells when food is plentiful. When food becomes scare, they begin to secrete a signaling molecule into the environment. This signals other amoebae in the vicinity to aggregate into a slug-like mass that can move. It eventually reorganizes into a fruiting body that produces spores.

Dictyostelium discoideum is an important model organism. It has been used for decades to studying cell signaling, taxis, programmed cell death (apoptosis), and the origin of multicellularity.