"Amoeboid" and "amoeba" are often used interchangeably, even by biologists, and especially refer to a creature which moves by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to the specific genus Amoeba. The genus Amoeba and amoeboids in general both derive their names from the ancient Greek word for change.
Shape and structure
Food sources vary in amoeboids. They may consume bacteria or other protists. Some are detritivores and eat dead organic material. They extend a pair of pseudopodia around food. They fuse to make a food vacuole which then fuses with a lysosome to add digestive chemicals. Undigested food is expelled at the cell membrane.
Amoebas use pseudopodia to move and feed. They are powered by flexible microfilaments near the membrane. Microfilaments are at least 50% of the cytoskeleton. The other parts are more stiff and are composed of intermediate filaments and microtubules. These are not used in amoeboid movement, but are stiff skeletons on which organelles are supported or can move on.
The shells of amoebas are often composed of calcium. The proteins or materials are synthesised in the cell and exported just outside the cell membrane.
Multicellular amoebas: Slime molds
Amoebas seem to have connections with two phyla (now defunct) composed of multicellular organisms of the lineage of fungus-like protists, the so-called slime molds. These two defunct phyla were the Myxomycota (i.e., plasmodial slime molds, now classified in the taxon Myxogastria), and Acrasiomycota (i.e. cellular slime molds, now divided into the taxa Acrasida and Dictyosteliida). These two phyla use amoeboid movement in their feeding stage. The former is basically a giant multinucleate amoeba, while the latter lives solitary until food runs out; in which a colony of these functions as a unit. Myxomycete slime molds use amoeboid gametes, as well.
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.
As amoebas themselves are polyphyletic and subject to some imprecision in definition, the term "amoeboid" does not provide identification of an organism, and is better understood as description of locomotion.
When used in the broader sense, the term can include many different groups. One source includes 97 different genera. Others include far fewer.
Amoeboids with pseudopods supported by regular arrays of microtubules were called actinopods, whereas those that weren't were called rhizopods, which were further subdivided into lobose, filose, and reticulose amoebae. In contrast to "rhizopods", where most of their morphologies can be mapped to modern classification systems, "actinopods" appear to be extensively polyphyletic. Actinopods are divided into radiolaria and heliozoa (itself a polyphyletic grouping).
Finally, there was also a strange group of giant marine amoeboids, the xenophyophores, that did not fall into any of these categories.
More modern classifications are based upon cladistics. It has been stated that most amoeboid are now grouped in Amoebozoa or Rhizaria (though Nucleariida are opisthokonta, more closely related to animals than other amoeba). In contexts where "amoeboid" is defined more loosely, there are many amoeboid species that are in the Excavata clade.
Phylogenetic analyses place these genera into the following groups (not all of these are considered amoeboid (or "amoebas") by all sources):
|Chromalveolate||Heterokont: Hyalodiscus, Labyrinthula
|Adelphamoeba, Astramoeba, Cashia, Dinamoeba, Flagellipodium, Flamella, Gibbodiscus, Gocevia, Hollandella, Iodamoeba, Malamoeba, Nollandia, Oscillosignum, Paragocevia, Parvamoeba, Pernina, Pontifex, Protonaegleria, Pseudomastigamoeba, Rugipes, Striamoeba, Striolatus, Subulamoeba, Theratromyxa, Trienamoeba, Trimastigamoeba, Vampyrellium|
Pathogenic interactions with other organisms
Some amoeboids can infect other organisms pathogenically (causing disease):
- Entamoeba histolytica is the cause of amoebiasis, or amoebic dysentery.
- Naegleria fowleri (the "brain-eating amoeba") is a fresh-water-native species that can be fatal to humans if introduced through the nose.
- Acanthamoeba can cause amoebic keratitis and encephalitis in humans.
- Balamuthia mandrillaris is the cause of (often fatal) granulomatous amoebic meningoencephalitis
- "amoeboid". Memidex (WordNet) Dictionary/Thesaurus. Retrieved 2 December 2010.
- Eric Bapteste, Henner Brinkmann, Jennifer A. Lee, Dorothy V. Moore, Christoph W. Sensen, Paul Gordon, Laure Duruflé, Terry Gaasterland, Philippe Lopez, Miklós Müller & Hervé Philippe (2001). "The analysis of 100 genes supports the grouping of three highly divergent amoebae: Dictyostelium, Entamoeba, and Mastigamoeba". Proceedings of the National Academy of Sciences 99 (3): 1414–1419. doi:10.1073/pnas.032662799. PMC 122205. PMID 11830664.
- "The Amoebae". Archived from the original on 12 April 2009. Retrieved 2 May 2009.
- Jan Pawlowski: The twilight of Sarcodina: a molecular perspective on the polyphyletic origin of amoeboid protists. Protistology, Band 5, 2008, S. 281–302. (pdf, 570 kB)
- Pawlowski J, Burki F (2009). "Untangling the phylogeny of amoeboid protists". J. Eukaryot. Microbiol. 56 (1): 16–25. doi:10.1111/j.1550-7408.2008.00379.x. PMID 19335771.
- Park, J. S.; Simpson, A. G. B.; Brown, S.; Cho, B. C. (2009). "Ultrastructure and Molecular Phylogeny of two Heterolobosean Amoebae, Euplaesiobystra hypersalinica gen. Et sp. Nov. And Tulamoeba peronaphora gen. Et sp. Nov., Isolated from an Extremely Hypersaline Habitat". Protist 160 (2): 265–283. doi:10.1016/j.protis.2008.10.002. PMID 19121603.
- The Amoebae website brings together information from published sources.
- Amoebas are more than just blobs
- Sun Animacules and Amoebas
- Molecular Expressions Digital Video Gallery: Pond Life - Amoeba (Protozoa) Some good, informative Amoeba videos.
- Amoebae: Protists Which Move and Feed Using Pseudopodia at the Tree of Life web project
- Microworld: world of amoeboid organisms - Arcella.nl