amoeboid

Foraminiferan (Ammonia tepida)

Amoeboids are single-celled life-forms characterized by an irregular shape.^[1]

"Amoeboid" and "amoeba" are often used interchangeably even by biologists^[2], and especially refers to a creature moving by using pseudopods. 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.

Structure

The superclass Rhizopoda has naked and shelled amoebas. They tend to ingest food and creep toward food. They move using Pseudopodia, which are bulges of cytoplasm. Naked amoebas use all of their body for pseudopod movement. Usually half is used in shelled amoebas; the other half grips the shell.

Amoebas breathe using their entire cell membrane that is constantly immersed in water. Because they live in water, they have no problem keeping it wet. But it does have one drawback. Excess water often crosses into the cytosol. All have one contractile vacuole that expels excess water. This contraction is powered by contracting cytosol.

The nuclei of amoebas do not have chromosomes. They also persist during cell division. The nuclear membrane pinches in two during telophase. Amoebas also do not go through meiosis. Amoebas have one or more nucleoli.

The food scources vary in rhizopoda. Some consume bacteria. Others are detritivores and eat dead organic material. Still others eat other protists. 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. Undigestied 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 macrotubules. 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 ar other materials. The proteins or materials are synthesised in the cell and exported just outside the cell membrane. The shells are stiff and flexible, but not too much of either trait.

Amoebas seem to have connections with two phyla of the lineage funguslike protists. The two phyla are myxomycota (plasmodial slime molds), and acrasiomycota (cellular slime molds). These two phyla use amoeboid movement in their feeding stage. One is basically a giant multinucleate amoeba, while the other lives solitary until food runs out; in which a colony of these functions as a unit. Myxomycotes use amoeboid gametes, as well. They definitely have a connection.

Diversity

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 the following groups: Acanthamoeba, Acrasis, Adelphamoeba, Amoeba, Astramoeba, Balamuthia, Cashia, Chaos, Clydonella, Dactylamoeba, Dientamoeba, Dinamoeba, Discamoeba, Echinamoeba, Endamoeba, Entamoeba, Filamoeba, Flabelulla, Flagellipodium, Flamella, Gephyramoeba, Gibbodiscus, Glaeseria, Gocevia, Gruberella, Gyromitus, Hartmannella, Heteramoeba, Hollandella, Histomonas, Hyalodiscus, Hydramoeba, Hyperamoeba, Iodamoeba, Korotnevella, Labyrinthula, Learamoeba, Leptomyxa, Lingulamoeba, Macropharyngomonas, Malamoeba, Mastigamoeba, Mastigella, Mastigina, Mayorella, Metachaos, Micronuclearia, Monopylocystis, Naegleria, Neoparamoeba, Neovahlkampfia, Nollandia, Nuclearia, Oscillosignum, Paragocevia, Paramoeba, Paratetramitus, Paravahlkampfia, Parvamoeba, Pelomyxa, Pernina, Pfiesteria, Polychaos, Pontifex, Phreatamoeba, Platyamoeba, Protoacanthamoeba, Protonaegleria, Psalteriomonas, Pseudomastigamoeba, Plaesiobystra, Rhizamoeba, Rosculus, Rugipes, Saccamoeba, Sappinia, Sawyeria, Stachyamoeba, Stereomyxa, Striamoeba, Striolatus, Stygamoeba, Subulamoeba, Tetramitus, Thecamoeba, Theratromyxa, Trichamoeba, Trichosphaerium, Trienamoeba, Trimastigamoeba, Unda, Vahlkampfia, Vampyrella, Vampyrellium, Vannella, Vexillifera, and Willaertia.^[3]

Classification

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.

Most amoeboid are now grouped in Amoebozoa or Rhizaria.^[4]

Amoebozoa

• Lobose pseudopods (Lobosea): 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.

Rhizaria

• Filose pseudopods (Filosa): 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 also includes vampyrellids.

• Reticulose pseudopods (Endomyxa): 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.

Other

The term "amoeboid" has in the past has sometimes been used in a more broad sense, including certain groups not currently included in Amoebozoa or Rhizaria:

• The Percolozoa, includes protists that can transform between amoeboid and flagellate forms.

• Nucleariids appear to be close relatives of animals and fungi.

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) primary amoebic meningoencephalitis

References

External links

• 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.
• Joseph Leidy's Amoeba Plates
• Amoebae: Protists Which Move and Feed Using Pseudopodia at the Tree of Life web project