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Rhabdostyla , Vorticella , and Trichodina (Figs. 4.3, 
4.6, Table 4.1), are a speciose assemblage that is 
not strongly supported by molecular phylogenies. 
The somatic kinetids are generally similar to those 
of the previous two classes. However, the somatic 
kinetids of the subclass Peniculia are more similar 
to those of other groups, like the hypotrichs , and 
the somatic kinetids of the trochal girdle of the 
subclass Peritrichia are highly divergent (Fig. 4.7; 
see Chapter 15 ). It is really only the paroral and 
the three adoral polykinetids that all these genera 
share, affirming the 20th century view that oral features
are indeed indicative of common ancestry! 
Fig. 4.5. Scanning electron micrographs of ciliate diversity. A–B Class ARMOPHOREA . Metopus ( A ) and 
Nyctotherus ( B). C–G Class LITOSTOMATEA . The haptorians Didinium ( C ) and Dileptus ( D ) and the trichostomes 
Isotricha ( E ), Entodinium ( F ), and Ophryoscolex ( G ). H Class COLPODEA . Colpoda . I Class PROSTOMATEA . 
Coleps . (Micrographs courtesy of E. B. Small.)
 4.2 Life History and Ecology 
 The life history of a typical ciliate would include 
an asexual or vegetative cycle during which growth 
and cell division occur, a sexual cycle during which 
the exchange of genetic material occurs between 
 conjugants , and a cryptobiotic cycle during which 
the organism would typically form a resting cyst 
(Fig. 4.8). These life histories, however, are diverse 
and undoubtedly adaptive. The cyst forms are 
diverse, stimulated by a variety of conditions to 
both encyst and excyst (Bussers, 1984; Corliss 
& Esser, 1974), and a complex set of physiological
changes, for example, “switched on” by gene 
expression, accompany the development of the 
 cryptobiotic state (Gutiérrez, Martín-González, & 
Matsusaka, 1990). One adaptive variation involves 
the presence or absence of the cryptobiotic cycle 
Fig. 4.6. Scanning electron micrographs of ciliate diversity. A , B , D , E , G–I Class OLIGOHYMENOPHOREA . 
The peritrichs Rhabdostyla ( A ), Vorticella with its helically contracted stalk ( B ), and Trichodina with its suction 
disk ( D , E). The peniculines Paramecium ( G , ventral on left and dorsal on right) and Lembadion ( H ). The hymenos-
tome Glaucoma ( I ). C , F Class PHYLLOPHARYNGEA . The cyrtophorian Trithigmostoma ( C ) and the suctorian
Podophrya ( F ). (Micrographs courtesy of E. B. Small, A. H. Hofmann, and C. F. Bardele.)
4.2 Life History and Ecology 99
100 4. Phylum CILIOPHORA – Conjugating, Ciliated Protists with Nuclear Dualism
and, related to this, differences in the survivability 
of the non-encysted stages (Jackson & Berger, 
1985a, 1985b). Often, the starving trophont trans-
forms into a highly motile form, the theront, 
which may be adapted both for dispersal and very 
long survival (Fig. 4.8) (Fenchel, 1990; Nelsen & 
DeBault, 1978). 
 A common adaptive variation is the differentiation 
of macrostome cannibal forms – ciliates that differ-
entiate a new oral apparatus large enough to ingest 
their microstomatous siblings (de Puytorac, 1984b) 
(Fig. 4.8). This transformation is often induced by 
starvation, like the theront transformation mentioned 
above. More dramatic examples of adaptation are 
found in symbiotic forms, especially parasitic ones 
(Bradbury, 1996). Ichthyophthirius , the parasite of 
fish gills and epithelium, apparently lacks a typical 
resting cyst stage. Instead, it grows to a consider-
able size as a trophont on the fish host, then drops 
off the host and becomes a tomont in a reproductive 
cyst. The tomont divides to produce thousands of 
tomites , which may reside for some time within 
the cyst before breaking out to find the next host. 
Finally, hyperparasites or hyperpredators can be 
Fig. 4.7. Schematics of somatic kinetids of genera representative of each class in the Phylum Ciliophora. ( a ) Loxodes
– Class KARYORELICTEA ; ( b ) Blepharisma – Class HETEROTRICHEA ; ( c , d ) Protocruzia ( c ), Euplotes ( d ) 
– Class SPIROTRICHEA ; ( e ) Metopus – Class ARMOPHOREA ; ( f ) Balantidium – Class LITOSTOMATEA ; ( g)
Chilodonella – Class PHYLLOPHARYNGEA ; ( h ) Obertrumia – Class NASSOPHOREA ; ( i) Colpoda – Class 
 COLPODEA ; ( j ) Plagiopyla – Class PLAGIOPYLEA ; ( k ) Holophrya – Class PROSTOMATEA ; ( l) Tetrahymena
– Class OLIGOHYMENOPHOREA ; ( m ) Plagiotoma – Class SPIROTRICHEA . Kd – kinetodesmal fibril; Pc – post-
ciliary microtubular ribbon; T – transverse microtubular ribbon (from Lynn, 1981, 1991)
found among the apostome oligohymenophoreans : 
Phtorophrya insidiosa is an apostome that attacks 
other apostomes, which are themselves symbionts 
on the cuticle of crustaceans (see Fig. 3.1). 
 Ciliates are heterotrophic, exhibiting a wide 
range of feeding behaviours, and occupying a 
diversity of ecological niches (Dragesco, 1984b; 
Finlay & Fenchel, 1996). As noted above, some 
can transform to feed on their siblings, which in 
the vast majority of cases are suspension feeders 
(Fenchel, 1980a, 1980b). The “particles” removed 
from suspension can be very small, like viruses
and bacteria , moderately-sized, like various kinds 
of unicellular algae, and relatively large, like other 
ciliates. The varieties of specific prey chosen by 
ciliates in the different classes are detailed in 
Fig. 4.8. Life cycle stages of ciliates. A microstome trophont , typically feeding on bacteria, grows from the tomite
stage until it roughly doubles in size to become a dividing tomont . This vegetative or asexual cycle can repeat itself 
as long as food is present. If food becomes limiting the ciliate may transform to a macrostome trophont , which is 
a cannibal form that can eat tomites and smaller microstome trophonts or other ciliates. If food is limiting or other 
stressful environmental circumstances prevail, the ciliate may form a cyst or may transform into a theront , a rapidly 
swimming dispersal stage. If the theront does not find food, it too may encyst . In unusual circumstances, when food 
is depleted and a complementary mating type is present, the ciliates may fuse together as conjugants and undergo the 
sexual process of conjugation
4.2 Life History and Ecology 101
102 4. Phylum CILIOPHORA – Conjugating, Ciliated Protists with Nuclear Dualism
the following chapters. Bacterivorous ciliates are 
 particularly important in maintaining the “quality” of 
effluent from sewage treatment plants as they can 
reduce bacterial densities ten-fold by their feeding 
(Curds & Cockburn, 1970a, 1970b; Foissner, 
1988a; Madoni, 2003) and may even consume 
viruses (Pinheiro et al., 2007). 
 In addition to being symbionts in other organisms 
(Bradbury, 1996; Fernández-Leborans & Tato-
Porto, 2000a, 2000c; Levine, 1972; Song, 2003), a 
variety of other organisms can use ciliates as their 
host (Ball, 1969). These endosymbionts of ciliates 
can range from bacteria living in the micronucleus 
(Görtz, 1983, 1996; Hovasse, 1984b) to various spe-
cies of algae (Hovasse, 1984a; Lobban et al., 2002; 
Reisser, 1986). 
 Ciliates are distributed globally in a diversity of 
habitats where they function as important trophic 
links in a variety of food webs (Adl, 2003; Finlay 
& Fenchel, 1996; Foissner, 1987; Pierce & Turner, 
1992; Sanders & Wickham, 1993). They are found 
in the world’s oceans, in the plankton (Edwards & 
Burkhill, 1995; Lynn & Montagnes, 1991; Pierce 
& Turner, 1993; Strom, Postel, & Booth, 1993), on 
ocean shores (Agamaliev, 1971; Al-Rasheid, 1999d; 
Dragesco, 1965; Kovaleva & Golemansky, 1979), 
in ocean depths (Fenchel et al., 1995; Hausmann, 
Hülsmann, Polianski, Schade, & Weitere, 2002; 
Silver, Gowing, Brownlee, & Corliss, 1984), and 
associated with sea ice (Lee & Fenchel, 1972; Song 
& Wilbert, 2000b). They are found in a variety of 
“land-locked” waters, including freshwater bodies,