Amoeboid

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Amoeboids are cells that move or feed by means of temporary projections, called pseudopods (false feet). They may also be called amoebae, but that term is frequently restricted to the amoeboid genus Amoeba and creatures very similar to it. Amoeboid cells have appeared in a number of different groups. Some cells in multicellular animals may be amoeboid, for instance our white blood cells, which consume pathogens. Many protists exist as individual amoeboid cells, or have such a form as part of their life-cycle. These are generally larger cells, some reaching several millimetres in size, and are common in all sorts of habitats.

Amoeboid protozoa may be organized into several broad categories based on what types of pseudopods they form. Those which produce axopods supported by microtubule bundles are called actinopods, and are traditionally divided into the radiolaria and heliozoa. Radiolaria are primarily marine plankton, and produce intricate mineral skeletons that are often found in the fossil record. They are distinguished by the presence of a central capsule that separates the inner portion of the cell, or endoplasm, from the frothy outer portion, or ectoplasm. Heliozoa only produce simple scales and spines, if they have any skeletal elements at all, and are found in both freshwater and marine environments.

Amoeboids that lack axopods are called rhizopods, and are traditionally divided into those with lobose, filose, and reticulose pseudopods. Both lobose and filose pseudopods arise from the same sort of underlying pressure system, though the former are blunt and the latter tapering in form. Such amoebae usually have a distinct layer of clear ectoplasm surrounding the granular endoplasm. Most move by the cell mass flowing into an anterior pseudopod, of which there may be one or several, but some crawl using relatively permanent pseudopods to support the cell like limbs, and others roll, the ectoplasm sliding around the outside of the cell like a tank tread.

The vast majority of lobose and filose amoebae, including Amoeba itself, appear to form a monophyletic group called the ramicristates. These include both naked forms and ones which produce simple shells, as well as most slime molds, multinucleate or multicellular forms of macroscopic size. Other groups of lobose amoeboids are the entamoebae, which include the pathogen responsible for amoebic dystentery, the pelobionts, which include the giant amoeba and have been considered as a candidate for the most primitive living eukaryotes, and the Heterolobosea, which alternate between amoeboid and flagellate forms. Other groups of filose amoeboids are the vampyrellids and nucleariids, small parasites on algae and fungi, and chlorarachniophytes, which have acquired chloroplasts.

Reticulose pseudopods are cytoplasmic strands that branch and merge to form a net. Most reticulose amoeboids belong to a single group, called the Granuloreticulosea, and include most notably the Foraminifera. These are marine plankton that form multichambered cells, and along with the radiolaria are the primary protozoa in the fossil record. Other small groups that produce reticulose pseudopods are the Gymnophrea, Komokiacea, and Biomyxa. There is also a strange group of giant marine rhizopods, the Xenophyophorea, which do not fit into any of these three categories.

Traditionally the amoeboid protozoa are grouped together as the Sarcodina, ranked variously from class to phylum. Each of the above categories would then be treated as a formal taxon. However, since these categories are all based on form rather than phylogeny, newer systems often separate out some amoeboid groups or abandon the Sarcodina altogether. The higher classification of amoeboids, like most protists, remains unstable.


References

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