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1979; Lynn, 1980) do not form postcili- odesmata , while redescriptions of the reichenowel- lid Balantidioides suggest that it has affinities to the spirotrichs (Foissner et al., 1982). While SSrRNA gene sequences support placement of Phacodinium among the spirotrichs (Shin et al., 2000), these same gene sequences confirm the heterotrich affinities of Peritromus (Rosati, Modeo, Melai, Petroni, & Verni, 2004), Chattonidium , (Modeo et al., 2006), and Condylostomides (Schmidt, Foissner, Schlegel, & Bernhard, 2007). In conclusion, we now recognize one order within the class, the Order Heterotrichida with characters of the class, and eight families: Family Blepharismidae [but see Aescht & Foissner, 1998], Family Chattonidiidae Family Climacostomidae , Family Condylostomatidae , Family Maristentoridae , Family Peritromidae , Family Spirostomidae , and Family Stentoridae (see Chapter 17. Ciliate Taxa ). 6.1 Taxonomic Structure 131 132 6. Subphylum 1. POSTCILIODESMATOPHORA: Class 2. HETEROTRICHEA \u2013 Once Close to the Top They are distinguished primarily by features of the oral region and variations in their overall body form. A number of works have treated different genera in detail: Spirostomum (Repak & Isquith, 1974); Blepharisma (Repak, Isquith, & Nabel, 1977); Stentor (Foissner & Wölfl, 1994); and a recent report on the rare genus Copemetopus (Al-Rasheid, 2001). We should not forget the classic works on Stentor , the majestic \u201cking of the ciliates\u201d, by Tartar (1961) and on Blepharisma , the light-sensitive pro- tozoon by Giese and collaborators (1973). Had\u17ei (1951) is the classic work on the folliculinids . 6.2 Life History and Ecology Because of their typically large size, heterotrichs can be conspicuous members of microbial foodwebs and have a widespread distribution. Heterotrichs have been recorded from freshwater lakes in subtropical Florida (Beaver & Crisman, 1989b), Antarctica (Kepner, Wharton, & Coats, 1999), Europe (Finlay, 1982), and high altitude lakes in South America (Woelfl & Geller, 2002), and streams in Europe (Madoni & Ghetti, 1980). They are found in a variety of marine habi- tats, including anaerobic sediments in Europe (Fenchel & Finlay, 1990a), the marine sublittoral in Europe (Agamaliev, 1971; Azovsky & Mazei, 2003; Kovaleva & Golemansky, 1979; Mazei & Burkovsky, 2003) and even deep marine habitats (Fenchel et al., 1995) and hydrothermal vents (Small & Gross, 1985; Bergquist et al., 2007). Heterotrichs are often dominant members of the low diversity ciliate communities of hypersaline habitats across the globe \u2013 in Europe (Esteban & Finlay, 2004), Africa (Yasindi, Lynn, & Taylor, 2002), Arabia (Al-Rasheid, Nilsson, & Larsen, 2001; Elloumi et al., 2006), and Australia (Post, Borowitzka, Borowitzka, Mackay, & Moulton, 1983). They are occasionally found in soils (Buitkamp, 1977; Foissner, 1998a; Griffiths, 2002). Most species are free-swimming, but some, such as Stentor , have the ability to use a holdfast to temporarily attach to the substrate (Fauré- Fremiet, 1984). A few species of Stentor secrete a mucoid sheath and all species of folliculinids secrete a lorica in which they can retract to avoid predation. The substances for these external coverings originate from extrusomes (Bussers, 1984; Mulisch & Hausmann, 1983), and in the folliculinids may contain chitin fibrils (Mulisch, Herth, Zugenmaier, & Hausmann, 1983). Substrates to which heterotrichs attach include inorganic substrates and macrophytes. Folliculinids attach to the integument of various invertebrates (Matthews, 1968; Fernández-Leborans & Córdoba, 1997), and may cause the skeletal eroding band or brown band diseases of scleractinian corals (Antonius, 1999; Cróquer et al., 2006). Maristentor is found on corals, but does not appear to cause disease (Lobban et al., 2002). Some genera, like Fabrea , are strictly marine or brackish water forms, which can attain abundances of 10 5 l \u22121 (Elloumi et al., 2006; García & Niell, 1993). Stentor species can reach more than 10 3 l \u22121 in some lakes in the southern hemisphere, perhaps due to the absence of larger microcrustacean predators (James, Burns, & Forsyth, 1995; Laybourn-Parry, Perriss, Seaton, & Rohozinski, 1997). Dispersal generally occurs by swimming, but cysts may also be involved (see below). Kusch (1998) has demon- strated clear evidence of relatively high gene flow among populations of Stentor separated by as much as 400 km. Genera in the Family Folliculinidae are typically marine although the fresh-water species Folliculina boltoni has been recorded from Europe (Penard, 1919), North America (Hamilton, 1952), and South America (Dioni, 1972) while Ascobius lentus has been recorded recently in European freshwaters (Mulisch, Heep, Sturm, & Borcherding, 1998). Folliculinids are dispersed in part by the movements of their host, but the proter or anterior daughter differentiates at cell division as a \u201cmouthless swarmer \u201d stage that is adapted for dispersal. Heterotrichs are omnivorous, upstream filter feed- ers (Fenchel, 1980a), showing little preference for prey species. Bacteria , autotrophic and heterotrophic flagellates , and ciliates are ingested, with some prey species proving more nutritious than others (Rapport, Berger, & Reid, 1972; Repak, 1983, 1986). Heterotrichs may change the shape of the oral region (Liebsch, 1976) and the spacing between the cilia of the oral polykinetids (Rickards & Lynn, 1985) in response to physiological states and prey types. When smaller food items become scarce, heterot- richs can become cannibalistic (Foissner & Wölfl, 1994; Giese, 1973; Pierce, Isquith, & Repak, 1978) and have also been known to ingest smaller metazoans (Foissner & Wölfl; Tartar, 1961). In an unusual turn of the tables, it appears that Mirofolliculina limnoriae , an epibiont on the wood-boring isopods of the genus Limnoria , may outcompete its host for food and hinder host dispersal, suggesting it can be considered an ectoparasite (Delgery, Cragg, Busch, & Morgan, 2006). Heterotrichs harbor a variety of endosymbionts : bacteria can be found in the cytoplasm and in the macronucleus (Fokin, Schweikert, Brummer, & Görtz, 2005; Görtz, 1983; Görtz & Wiemann, 1987). The bacterial endosymbionts do not appear to be harmful; in fact, some bacteria may be essential sym- bionts (Hufschmid, 1984). A variety of Chlorella spe- cies provide their Stentor and Climacostomum hosts with the \u201cby-products\u201d of photosynthesis (Fernández- Leborans & Zaldumbide, 1983; Kawakami, 1984; Reisser, 1984; Woelfl & Geller, 2002), and may com- pete with bacterial endosymbionts for the host cyto- plasmic niche (Hufschmid, 1984). Laybourn-Parry et al. (1997) determined that Stentor amethystinus could contribute almost 70% of the total plankton photosyn- thesis in some Australian lakes. Heterotrichs themselves are prey for other ciliates and metazoans. Stentor has mechanoreceptors dis- tributed on its cell surface that may enable response to predator contact (Wood, 1989). When contact is made with toxicyst-bearing litostome ciliates, like Dileptus (see Chapter 9 ), Blepharisma (Harumoto et al., 1998; Miyake, Harumoto, Salvi, & Rivola, 1990), Climacostomum (Masaki et al., 1999), and Stentor (Miyake, Harumoto, & Iio, 2001) induce a massive release of their pigmentocysts , respectively containing the pigments blepharismin , climacostol , and stentorin , which have proved lethal to this predator. However, the pigment does not inhibit predation by the heterotrich Climacostomum on its heterotrich relative Blepharisma (Terazima & Harumoto, 2004). Pigmented heterotrichs also exhibit light-sensitive behavior (Giese, 1973). Their photophobic response appears as a ciliary