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distribution in turbulent water columns (Jonsson, 1989). Negative geotaxis may also enable these ciliates to move towards light, whose presence may be correlated with more abundant prey. Light may also enhance the digestion and growth rates of tintinnids , possibly by photooxidative breakdown of ingested food (Strom, 2001). In more stable aquatic habitats, where oxyclines become established, spirotrichs may distribute vertically both in the water column (Bernard & Fenchel, 1994; Fenchel et al., 1995) and in benthic microbial mats (Fenchel & Bernard, 1996), seeking a preferred oxygen pressure. The chemosensory behavior of benthic forms, such as Euplotes , has been interpreted as a form of the classical random walk , which enables populations to accumulate very quickly at point sources of attractants (Fenchel, 2004). A behaviour unique to the tintinnid choreotrichs is the construction of the lorica in which the cell resides and which may have some selective advan- tage for avoiding predation (Capriulo, Gold, & Okubo, 1982). The progeny of Favella constructs its hyaline lorica within 10 min of leaving the parental lorica (Laval-Peuto, 1977), while species with agglutinated loricae , like Tintinnopsis species, may take several hours to complete lorica construc- tion (Gold, 1979). The materials agglutinated to the lorica secretory material may vary with the envi- ronment in which the tintinnid is found and with the season (Bernatzky, Foissner, & Schubert, 1981; Gold, 1979). However, in field samples, there also appears to be some selectivity, both with respect to the size of the particles on the lorica relative to those in the environment and with respect to the quality of the particles (Gold & Morales, 1976; Rassoulzadegan, 1980). The basis of this selectiv- ity is not understood, and cannot yet be duplicated in the laboratory (Gold, 1979). Spirotrichs , like other ciliates, survive from days to weeks without food (Jackson & Berger, 1985b; Montagnes, 1996). If they are able to encyst , sur- vival can be extended to years, and this is espe- cially true of species found in soils and on mosses (Corliss & Esser, 1974). Both freshwater and marine choreo trichs and oligotrichs encyst ; tintin- nids form their cyst within the lorica (Jonsson, 1994; Müller & Wunsch, 1999; Paranjape, 1980; Reid & John, 1978). Encystment has been cor- related with tidal rhythms (Fauré-Fremiet, 1948b; Jonsson, 1994), and reductions in prey density or increases in predator density (Müller & Wunsch, 1999; Reid, 1987). Excystment may be stimulated by changes in temperature (Kim & Taniguchi, 1995; Kim, Suzuki, & Taniguchi, 2002; Müller, 2002; Paranjape), light (Kim & Taniguchi, 1995), algal exudates (Kamiyama, 1994), and soil extracts (Müller, Foissner, & Weisse, 2006). Cysts of plank- tonic species settle through the water column during the productive season. These cysts may ultimately reach the sediments from which they are resuspended by turbulent events and turn over cur- rents (Kim & Taniguchi, 1997; Müller & Wunsch, 1999; Reid, 1987). Excystment may be facilitated by an expanding excystment vacuole (Rawlinson & Gates, 1985), often through a specialized region of the cyst wall, such as the papula in oligotrich and choreotrich cysts (Kim & Taniguchi, 1995; Kim et al., 2002; Müller & Wunsch, 1999; Reid, 1987). The \u201ccystment cycle\u201d can be complex, vary seasonally, and is likely maintained in all cases by natural selection of correctly phased individuals. This may be especially important for the mainte- nance of the unusual tidal rhymicity of the cyst- ment cycle in the tide-pool oligotrich Strombidium 7.2 Life History and Ecology 155 156 7. Subphylum 2. INTRAMACRONUCLEATA: Class 1. SPIROTRICHEA oculatum (Jonsson; Montagnes, Wilson, Brooks, Lowe, & Campey, 2002). Oligotrich and tintinnid cysts are flask-shaped with a neck-like extension, which is sealed by a cap through which the ciliate exits the cyst (Jonsson, 1994; Kim et al., 2002; Müller and Wunsch, 1999; Reid, 1987; Reid & John, 1978). This characteristic cyst shape has lead Reid and John (1981) to suggest that tintinnids might be the enigmatic chitinozoa found in Proterozoic sedi- ments, dating the fossil record of ciliates to well over 700 million years ago. Further, Reid and John (1983) argued that cyst morphology may be phy- logenetically informative, noting that flask-shaped cysts are found in members of the now \u201cbasal\u201d Classes HETEROTRICHEA , SPIROTRICHEA , and ARMOPHOREA . Hypotrichs and stichotrichs have been the main focus of ultrastructural descriptions of encystment in spirotrichs . Walker and Maugel (1980) noted that resting cysts of hypotrichs and stichotrichs showed significant differences. Hypotrich cysts, termed non-kinetosome resorbing (NKR), are now characterized as retaining cilia and kinetosomes in the encysted cell (Foissner & Foissner, 1987; Walker & Maugel, 1980). Stichotrich cysts, termed kinetosome-resorbing (KR), are now characterized as dedifferentiating cilia and kinetosomes in the mature cyst (Foissner & Foissner, 1987; Grimes, 1973; Matsusaka, Nakamura, & Nagata, 1984; Walker & Maugel, 1980), a feature shared by at least the halteriid Meseres (Foissner, Müller, & Weisse, 2005a). Other studies have generally con- firmed these features, indicating also that the NKR cyst wall typically has only three layers (Greco, Bussers, van Daele, & Goffinet, 1990; Walker & Maugel) while the KR cyst wall typically has four layers (Calvo & De Miguel, 1995/1996; Delgado, Calvo, & Torres, 1987; Gutiérrez, Torres, & Perez- Silva, 1983; Walker, Maugel, & Goode, 1980). Extrusomes provide the materials for the cyst wall components (Grim & Monganaro, 1985; Walker & Maugel, 1980; Walker et al., 1980). Cyst wall con- situents include glycoproteins (Calvo & De Miguel, 1995/1996; Matsusaka & Hongo, 1984) and chitin (Foissner et al., 2005a; Greco et al., 1990; Rosati, Verni, & Ricci, 1984). In the highly unusual sti- chotrich Meseres , the surface of the resting cyst is coated by scale-like structures, called lepidosomes (Foissner, 2005b; Foissner et al., 2005a). 7.3 Somatic Structures The great diversity of body shapes in spirotrichs is correlated both with the taxonomic diversity of the group and its seven subclasses and the tremendous diversity of habitats in which these groups are dis- tributed. As with the heterotrichs (see Chapter 6 ), body shape generally correlates with benthic or planktonic habitat. Benthic forms, such as Phacodinium , hypotrichs , and stichotrichs , are typ- ically dorsoventrally flattened and in most species dorsoventrally differentiated, with cilia forming large compound cirri scattered on the ventral sur- face and widely spaced stiff bristles or dikinetids with more flexible cilia on the dorsal surface. Planktonic forms range from small spheroids to larger cone-shaped forms (Figs. 7.2\u20137.4). The cell surface typically does not exhibit a con- spicuous glycocalyx . In tintinnids and some stichot- richs , at least, it is replaced by the perilemma , not a true cell membrane, which covers even the cilia (Foissner & Pichler, 2006; Laval-Peuto, 1975). The plasma membrane is typically not underlain by a well-developed layer of cortical alveoli . The hypot- richs are an exception with their conspicuous silver- line system (Valbonesi & Luporini, 1995; Wise, 1965), which reflects the extensive development of cortical alveoli containing novel \u201csupport\u201d protein called plateins (Böhm & Hausmann, 1981;Hausmann & Kaiser, 1979; Kloetzel, 1991; Kloetzel, Hill, & Kosaka, 1992; Kloetzel et al., 2003). Cell form is likely maintained by cortical microtubules, either arising from kinetids or independently subtending a very fine epiplasmic layer (e.g., Fleury, 1991b; Grain,