Amoeboids are unicellular lifeforms that mainly consist of contractile vacuoles, a nucleus, and cytoplasm as their basic structure. They move and feed by means of temporary cytoplasmic projections, called pseudopods (false feet). They have appeared in a number of different groups. Some cells in multicellular animals may be amoeboid, for instance human white blood cells, which consume pathogens. Many protists also exist as individual amoeboid cells, or take such a form at some point in their life-cycle. The most famous such organism is Amoeba proteus; the name amoeba is variously used to describe its close relatives, other organisms similar to it, or the amoeboids in general.
Morphological categories
Amoeboids may be divided into several morphological categories based on the form and structure of the pseudopods. Those where the pseudopods are supported by regular arrays of microtubules are called actinopods, and forms where they are not are called rhizopods, further divided into lobose, filose, and reticulose amoebae. There is also a strange group of giant marine amoeboids, the xenophyophores, that do not fall into any of these categories.
Lobose pseudopods
Lobose pseudopods are blunt, and there may be one or several on a cell, which is usually divided into a layer of clear ectoplasm surrounding more granular endoplasm. Most, including Amoeba itself, move by the body mass flowing into an anterior pseudopod. The vast majority form a monophyletic group called the Amoebozoa, which also includes most slime molds. A second group, the Percolozoa, includes protists that can transform between amoeboid and flagellate forms.
Filose pseudopods
Filose pseudopods are narrow and tapering. The vast majority of filose amoebae, including all those that produce shells, are placed within the Cercozoa together with various flagellates that tend to have amoeboid forms. The naked filose amoebae comprise two other groups, the vampyrellids and nucleariids. The latter appear to be close relatives of animals and fungi.
Reticulose pseudopods
Reticulose pseudopods are cytoplasmic strands that branch and merge to form a net. They are found most notably among the Foraminifera, a large group of marine protists that generally produce multi-chambered shells. There are only a few sorts of naked reticulose amoeboids, notably the gymnophryids, and their relationships are not certain.
Actinopods
Actinopods are divided into the radiolaria and heliozoa. The radiolaria are mostly marine protists with complex internal skeletons, including central capsules that divide the cells into granular endoplasm and frothy ectoplasm that keeps them buoyant. The heliozoa include both freshwater and marine forms that use their axopods to capture small prey, and only have simple scales or spines for skeletal elements. Both groups appear to be polyphyletic.
However, amoeboids have appeared separately in many other groups, including various different lines of algae not listed above.
Subphylum Sarcodina
Sarcodina is a subphylum of the phylum Sarcomastigophora, of unicellular life forms that move by cytoplasmic flow. Some species use cytoplasmic extensions called pseudopodia for locomotion or feeding. The subphylum includes such protozoa as the common amoeba and the Foraminifera and Radiolaria. Most members of the subphylum reproduce asexually through fission, although some reproduce sexually. Sarcodina is sometimes subdivided into two classes - Rhizopoda and Actinopoda.
External links
The Amoebae website brings together information from published sources.
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