Ciliates are protozoa that have cilia, structures used in locomotion and food capture. The cilia, which also occur in some multicellular eukaryotic cells, have the same internal structure as the flagella, differing from them in terms of length (cilia are much shorter), number and beat.
Paramecium caudatum, a free-living protozoan, is an example of a very common ciliate that appears in freshwater ponds and lakes.
Paramecia are easily distinguished by their typical shape, similar to the sole of a slipper. The anterior (front) portion of the cell is rounded and the posterior portion slightly tapered. The outermost layer of the cell is less flexible than the amoebas, giving the cell a more constant shape. The entire cell is covered by small filiform projections (cilia), which, in addition to locomotion organs, also serve to guide food towards the oral pore.
In paramecium, food ingestion occurs through a well-defined indentation and constant location in the cell, called the oral sulcus or, even, the buccal funnel. The food driven by the coordinated ciliary beat, enters this recess, which is also ciliated, passes through the cytostoma (cyto = cell, stoma = mouth), located at the end of the oral sulcus, and enters the cytopharynx, a tubular structure that has a tuft of cilia.
Food is pushed to the end of the cytopharynx, inside the endoplasm. In this region, a digestive vacuole is formed, which detaches itself and begins to circulate through the interior of the cell, moved by cytoplasmic currents, distributing the digested food. Unusable remains are eliminated through a specific region of the cell, called cytopygeus or cytoprocto, or even anal pore. As in amoebas and other single-celled microorganisms, oxygen enters the cell through the cell membrane, while CO2 diffuses out. Used fluids collect in contractile vacuoles, which have fixed positions in paramecia, rather than appearing anywhere in the cell, as in amoebas.
Another characteristic of paramecia, and ciliates in general, is the presence of 2 distinct types of nucleus: the micro and macronucleus, as mentioned above. In paramecia there is a macro and a micronucleus per cell, but in other species of ciliates, the number of micronuclei per cell can vary, however, there is always a macronucleus. Asexual reproduction is by division or binary fission. The macro and micronucleus divide and a transverse groove forms, separating the cell in half.
The sexual process of reproduction of ciliates is conjugation. In this process, the temporary union of two individuals occurs, followed by degeneration of the macronucleus, meiotic division of the micronucleus, exchange of micronuclei between the conjugating individuals and fusion of the micronucleus that migrated with the stationary micronucleus. The micronucleus resulting from the fusion undergoes mitosis and the conjugating individuals separate, each of which undergoes a number of cell divisions, which varies from species to species, reestablishing the number of nuclei typical of the species. The macronucleus arises by differentiation from the micronucleus.
Diversity of Members
Ciliates may have cilia covering the entire cell, as in the Paramecium genus, or just in some regions. Most representatives of this phylum are aquatic, living in fresh or salt water. There are species that swim freely, others that live fixed to the substrate by a peduncle and others that form colonies. A very different group of ciliates is that of suctoria. These organisms have cilia only in the early stages of development, when adults, they attach to the substrate, lose their cilia and develop special structures called tentacles, which are used to capture food.
The main prey of suctories are other ciliates that, when touched by tentacles, are immobilized by tricocysts (oval or rod-shaped structures that act as a defense). Then, the prey’s cytoplasm is sucked up by the tentacles and digested in digestive vacuoles inside the succulent body.
In addition to free-living ciliates, there are parasitic forms, such as Balantidium coli, which lodge in the human intestine. This protozoan is acquired by drinking contaminated water.
(previous post) shows some representatives of the ciliates.
References:
Hausmann, Klaus, and Phyllis Clarke Bradbury, eds. Ciliates: cells as organisms. Stuttgart: Gustav Fischer, 1996.
Chalker, Douglas L., Eric Meyer, and Kazufumi Mochizuki. “Epigenetics of ciliates.” Cold Spring Harbor perspectives in biology 5, no. 12 (2013): a017764.
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