Live Protozoa Specimens

Protozoa (in Greek proto = first and zoa = animal) are single-celled eukaryotes (organisms whose cells have nuclei) that show some characteristics usually associated with animals, most notably mobility and heterotrophy.

They are often grouped in the kingdom Protista together with the plant-like algae and fungus-like water molds and slime molds. In some newer schemes, however, most algae are classified in the kingdoms Plantae and Chromista, and in such cases the remaining forms may be classified as a kingdom Protozoa. The name is misleading, since they are not animals.

Protozoa have traditionally been divided on the basis of locomotion, although this is no longer believed to represent genuine relationships:

• Amoeboids (Sarcodina)
• Ciliates
• Flagellates
• Sporozoans: Apicomplexa, Myxozoa, Microsporidia

Most protozoans are too small to be seen with the naked eye – most are around 0.01-0.05 mm, although forms up to 0.5 mm are still fairly common – but can easily be found under a microscope. Protozoa are ubiquitous throughout aqueous environments and the soil, and play an important role in their ecology.

Protozoa occupy a range of trophic levels. As predators upon unicellular or filamentous algae, bacteria, and microfungi, protozoa play a role both as herbivores and as consumers in the decomposer link of the food chain. Protozoa also play a vital role in controlling bacteria population and biomass.

As components of the micro- and meiofauna, protozoa are an important food source for microinvertebrates. Thus, the ecological role of protozoa in the transfer of bacterial and algal production to successive trophic levels is important. Protozoa are also important as parasites and symbionts of multicellular animals.

• Amoeboids are cells that move or feed by means of temporary projections, called pseudopods (false feet). They 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 take such a form at some point in their life-cycle. The most famous such organism is Amoeba proteus; the name amoebae is variously used to describe its close relatives, other organisms similar to it, or the amoeboids in general. Traditionally the amoeboid protozoa are grouped together as the Sarcodina, variously ranked from class to phylum, with each of the above categories as a formal subtaxon.
• Ciliates are one of the most important groups of protists, common almost everywhere there is water – lakes, ponds, oceans, and soils, with many ecto- and endosymbiotic members, as well as some obligate and opportunistic parasites. Ciliates tend to be large protozoa, a few reaching 2 mm in length, and are some of the most complex in structure. The name ciliate comes from the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers. Cilia occur in all members of the group, although the peculiar suctoria only have them for part of the life-cycle, and are variously used in swimming, crawling, attachment, feeding, and sensation.
• Flagellates are cells with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the sperm cells of most phyla. Higher plants and fungi do not produce flagellate cells, but the closely related green algae and chytrids do. Many protists take the form of single-celled flagellates. They are found in most lines of eukaryotes, and it is likely that all surviving eukaryotes evolved from them.