The Ciliated Protozoa
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
beneath their plasma membrane. Ciliates are distinguished from
other alveolates on the basis of three major characters.
- 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.
- 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) 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