Ciliophora is the name for a phylum of protists commonly called the ciliates. Ciliates are the most complex of cells, having an elaborate cytoskeleton, cilia and two different kinds of nuclei. Free‐living ciliates can be found in almost any habitat that has water – in soils, hot springs and Antarctic sea ice. Symbiotic species live as commensals in sea urchins or as parasites of lobsters and fish. Ciliate life histories can have specialised forms for dispersal and for resisting desiccation. Their cell cortex is supported by a complex framework of basal bodies or kinetosomes, microtubules and microfilaments. The kinetosomes form the central unit in an organellar structure called the kinetid, which is important to understanding phylogenetic relationships among ciliates. The pattern of fibres and microtubules in the kinetid identifies a ciliate to a major clade, along with the sequences of genes. These two features together identify 12 major clades or classes of ciliates.

Key Concepts:

  • Ciliates are characterised by three main features: they exhibit nuclear dimorphism; undergo conjugation as a sexual process; and typically have cilia at some stage in their life cycle.

  • Ciliates are the ‘top’ predators in microbial food webs, and were likely the major predatory group before the evolution of animals.

  • Parasitic ciliates can cause morbidity and death of animals, and are becoming particularly important in aquaculture operations.

  • Ciliates can have complex life cycles, including macrostome or cannibalistic stages, swarmers or dispersal stages and cyst or desiccation‐resistant stages.

  • The kinetid, an organellar complex in the cell cortex, is composed of at least one kinetosome and its cilium associated with two microtubular ribbons and a striated kinetodesmal fibril, whose patterned arrangement identifies a ciliate to a particular major clade or class.

  • Ciliate macronuclei divide in two ways, which suggest that macronuclear division evolved independently twice in the phylum: heterotrich ciliates divide their macronucleus principally using extramacronuclear microtubules while intramacronucleate ciliates divide their macronucleus with intramacronuclear microtubules.

  • Ciliates are divided into two major clades or subphyla and 12 classes based on features of the kinetid and sequences of genes.

Keywords: alveolate; ciliate; Euplotes; Paramecium; suctoria

Figure 1.

Life cycles of ciliated protozoa. (a) A generalised life cycle of a ciliate showing three major phases: (1) growth and reproduction during which the ciliate feeds and undergoes binary fission; (2) conjugation usually stimulated by starvation conditions, during which the ciliates undergo meiosis and exchange gametic nuclei before separating and (3) encystment and excystment during which the ciliate secretes a cyst wall about itself to survive harsh conditions, like desiccation or the absence of food. (b) The life cycle of the hymenostome Tetrahymena patula, which has a microstome phase that eats bacteria, a macrostome phase that eats microstomes and other smaller ciliates when the bacterial food supply is depleted, and a tomont or dividing phase that undergoes sequential binary fissions in a cyst to produce tomites. The tomites escape the cyst when bacteria are again abundant. (c) The life cycle of the hymenostome Ichthyophthirius multifiliis, the parasite that causes white‐spot disease of the skin of fishes. The theront, a small cell, burrows into the skin of a fish to become a phoront that begins feeding as a trophont stage. The trophont may reach over 1 mm in diameter at which time it falls off the fish on to the bottom to become a tomont. Here, the tomont divides sequentially by binary fission to produce sometimes 1000 tomites, which break out of the division cyst to become the next generation of theronts. Based on Lynn and Small .

Figure 2.

Generalised drawing of the ventral surface of a ciliated protozoon. The body can be divided into somatic and oral regions. The oral region illustrated here has two oral polykinetids and one paroral kinety, and adjacent to this are some specialised perioral kineties that aid in feeding. Refer to Figure for some illustrations of more oral structures. The somatic cortex is covered by a pellicle that includes the alveoli (see Figure ). The pellicle is part of the cortex, the outer portion of the cell in which are embedded the somatic mono‐ and dikinetids and the mitochondria. The cortex may be separated from the endoplasm by a filamentous layer. There are typically three openings to the outside: the cytostome or cell mouth through which food passes into the food vacuole; the cytoproct through which undigested food passes from the food vacuole, and the contractile vacuole pore out of which excess water and some wastes are passed from the contractile vacuole. Based on Lynn and Small .

Figure 3.

A patch of the cortex of a ciliate that shows the subcomponents of the pellicle and the kinetid. The microtubular and filamentous components of the cortex are elements of the cytoskeleton that provide support for the form of the ciliate. The anterior end of the ciliate is towards the left. Based on Lynn and Small .

Figure 4.

Transverse sections of the somatic kinetids of representative genera from the major classes of ciliates. (a, b) Subphylum Postciliodesmatophora. (a) Class Karyorelictea – Geleia. (b) Class Heterotrichea – Climacostomum. (c–p) Subphylum Intramacronucleata. (c, d) Class Spirotrichea – Protocruzia (c) and Stylonychia (d). (e, f) Class Colpodea – Sorogena (e) and Pseudoplatyophrya (f). (g, h) Class Phyllopharyngea – Hypocoma (g) and the suctorian Trichophrya (h). (i, j) Class Nassophorea – monokinetid (i) and dikinetid of Nassula (j). (k, l) Class Oligohymenophorea – dikinetid of Colpidium (k) and monokinetid of Ichthyophthirius (l). (m, n) Class Prostomatea – monokinetid (m) and dikinetid of Coleps (n). (o, p) Class Litostomatea – Lepidotrachelophyllum (o) and Isotricha (p). kd, kinetodesmal fibril; pc, post‐ciliary microtubular ribbon; t1 and t2, transverse microtubular ribbons. Based on Lynn .

Figure 5.

A phylogeny based on phylogenies derived from comparisons of the nucleotide sequences of small subunit rRNA genes. Two major alveolate phyla, the Apicomplexa and Dinozoa, serve as outgroups to root the ciliate portion of the tree.

Figure 6.

A demonstration of the diversity of oral structures of genera in the class Colpodea. Some of these ciliates represent oral features that are convergent on oral features of ciliates from other classes. (a) Colpoda with two oral polykinetids. (b) Hausmanniella also with two oral polykinetids. (c) Bardeliella with a left oral polykinetid that extends out over the anterior end as a series of rows of cilia. (d) Grossglockneria whose everted cytopharynx is used to pierce fungal cells and ingest their cytoplasm, like a suctorian's tentacle. (e, f) Bursaria (e) and Bursaridium (f) whose extensive series of left oral polykinetids placed them in the class Heterotrichea for many years. (g) Cyrtolophosis with its small number of oral polykinetids and small cell size was once considered a member of the class Oligohymenophorea. (h) Platyophrya with paroral and multiple left oral polykinetids. (i) Sagittaria similar to Platyophrya. (j) Rostrophrya with small polykinetids extending out on to the somatic surface was initially thought to be a member of the class Nassophorea. (k) Pseudochlamydonella whose ventral oral cavity and basket‐like cytopharynx are similar to those structures in the class Phyllopharyngea. (l) Sorogena with its simple, ring‐like anterior oral apparatus was initially classified as a member of the class Prostomatea. (m) Bryophrya shows some similarities to Rostrophrya and nassophoreans although it has a deep oral cavity. (n) Trihymena, a colpodean with a simple oligohymenophorean‐like oral apparatus. Based on Lynn and Foissner .



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Further Reading

Buchmann K, Lindenstrom T and Bresciani J (2001) Defence mechanisms against parasites in fish and the prospect for vaccines. Acta Parasitologica 46: 71–81.

Buchmann K, Sigh J, Nielsen CV and Dalgaard (2001) Host responses against the fish parasitizing ciliate Ichthyophthirius multifiliis. Veterinary Parasitology 100: 105–116.

Ferry T, Bouhour D, De Monbrison F et al. (2004) Severe peritonitis due to Balantidium coli acquired in France. European Journal of Clinical Microbiology and Infectious Diseases 23: 393–395.

Frankel J (1989) Pattern Formation. Ciliate Studies and Models. Oxford: Oxford University Press.

Hausmann K and Bradbury PC (eds) (1996) Ciliates. Cells as Organisms. Stuttgart: Gustav Fisher.

Jee BY, Kim KH, Park SI and Kim YC (2000) A new strain of Cryptocaryon irritans from the cultured olive flounder Paralichthys olivaceus. Diseases of Aquatic Organisms 43: 211–215.

Jones AR (1974) The Ciliates. London: Hutchinson University Library.

Parama A, Iglesias R, Alvarez MF et al. (2003) Philasterides dicentrarchi (Ciliophora, Scuticociliatida): experimental infection and possible routes of entry in farmed turbot (Scophthalmus maximus). Aquaculture 217: 73–80.

de Puytorac P (ed.) (1994) Infusoires Ciliés. Traité de Zoologie, vol II. Paris: Masson.

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Lynn, Denis H(Apr 2012) Ciliophora. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001966.pub3]