The Ciliated Protozoa

Phylum Ciliophora

Denis Lynn, University of Guelph

The ciliated protozoa are the most conspicuous and easily recognized groups of protists. They are conspicuous because they are usually motile and quite large, ranging from an average of about 50 micrometers up to 4 mm in size. They are easily recognized because they are propelled through the water by hundreds to thousands of cilia that cover their bodies, aligned in long files or kineties.

Ciliates are related to two other major groups of protists - the dinoflagellates and apicomplexans - because all three groups possess membrane-bound sacs or alveoli beneath their plasma membrane. Ciliates are distinguished from other alveolates on the basis of three major characters.
  1. Ciliates have two kinds of nuclei and they are thus called heterokaryotic. The macronucleus as its name suggests is typically larger with sometimes thousands of copies of the genome. It is transcriptionally active (i.e., makes messenger RNA), and so controls cell functions and physiology. The micronucleus is smaller, usually diploid, and is the germ-line reserve, similar to the sperm and egg cell nuclei of multicellular organisms.

  2. Ciliates also have a complex fibrillar system - the infraciliature - below the cell surface. The locomotor units or kinetids are composed of a cilium, a basal body or kinetosome surrounded by a kinetodesmal fibril, a postciliary microtubular ribbon, and a transverse microtubular ribbon. The arrangement of these latter three components distinguishes different major groupings or classes of ciliates.

  3. (3) Ciliates undertake sex in a process called conjugation, which is typically the temporary fusion of the two partners. The two partners usually have to be from complementary mating types and once they have fused they exchange gametic nuclei, which are equivalent to the nuclei of our sperm and eggs. After this exchange, the two partners separate. The gametic nuclei fuse and the zygotic nucleus divides. The nuclei produced develop into a new macronucleus and a new micronucleus.

The ciliates are predominantly heterotrophic, meaning that they must consume other organisms, usually other protists, to live and reproduce. Some large ciliates, like Stentor, can even eat smaller multicellular invertebrates like rotifers! They are found virtually everywhere there is water or a water film - ponds, puddles, soil, beach sands, arctic ice, alpine streams, desert sands, and deep ocean vents. Just as many species of ciliates are found as symbionts associated with invertebrates and vertebrates. Some ciliates, themselves, serve as hosts for other protists as well as bacteria and viruses.

For more information on ciliates, check out the web sites below