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from research on conjugation in Dileptus , and so any generalizations from its features must be taken with caution. Three mating types , which show serial dominance , have been identified in this species (Miyake, 1996). Each mating type excretes a gamone that can attract cells of complementary mating type (Afon’kin & Yudin, 1987). The gamone is thought to be a small polypeptide (Parfenova, Afon’kin, Yudin, & Etingof, 1989). Homotypic pairs (i.e., pairs of cells of the same mating type) may form, but are usually not stable, possibly due to the cessation of the expression of adhesive molecules (Afon’kin, 1991). Intraclonal conjugation or selfing has been reported in Spathidium (Williams, 1980). Litostomes are generally characterized as under- going a preconjugation cell division , although this is not a universal trait (Xu & Foissner, 2004). The preconjugation division can be equal, but then yields cells that are smaller than trophic cells since no growth takes place prior to conjugation (Raikov, 1972). In entodiniomorphids , the preconjugation division can be unequal (e.g., Opisthotrichum ) and so produce macroconjugants and microconjugants (Raikov). Tavrovskaja (1974 in Miyake, 1996) has demonstrated that the gamones of Dileptus stimu- late the preconjugation cell division . Cell fusion usually occurs in the oral region, often involving some dedifferentiation of oral structures in haptorians (Xu & Foissner, 2004). Cell membranes of the two partners fuse, often over a considerable area (e.g., Didinium – Karadzhan, 1979; Dileptus – Golińska & Afon’kin, 1993; Homalozoon – Leipe & Hausmann, 1993). Thus, cytoplasmic organelles, as well as the migratory pronuclei , might be exchanged during the process, which may take several days. There are typically three maturation divisions during micronuclear meiosis in haptorians and trichostomes but only two in entodiniomorphids (Raikov, 1972; Xu & Foissner, 2004). In the vesti- buliferids , the micronuclear mitotic spindle is much enlarged and the division products remain swollen and spindle-like during conjugation . The migratory gametic nucleus of entodiniomorphids has portions of the telophase spindle attached so that it appears “spermatozoon-like” (Raikov, 1972). Following fertilization, the synkaryon of haptorians typically undergoes two or three divisions (Raikov, 1972; Serrano, Martín-González, & Fernández-Galiano, 1990; Xu & Foissner, 2004). Trichostomes typi- cally undergo one synkaryon division although one of these products may undergo a second division in Paraisotricha and Balantidium species, which is followed by fusion of these two division prod- ucts (Raikov, 1972). We do not know whether this feature is a homologous or convergent one in these two genera. Williams (1980) has observed clonal aging in the haptorian Spathidium . This aging process is observed as a reduction in daily fission rate. The time of greatest reduction is species specific and is reversed in part by intraclonal conjugation or encystment . 9.7 Other Features The free-living litostomes have been recorded in a variety of habitats, including anaerobic ones (Foissner, 1988a; Madoni & Sartore, 2003), from which they have been collected by an electromigra- tion apparatus (Wagener, Stumm, & Vogels, 1986). Some of the anaerobic species (e.g., Lacrymaria ) may harbor endosymbiotic methanogens , while 9.7 Other Features 207 208 9. Subphylum 2. INTRAMACRONUCLEATA: Class 3. LITOSTOMATEA other genera (e.g., Lagynophrya ) may harbor Chlorella species (Finlay & Maberly, 2000). Didinium and Spathidium have been recorded in waste treatment facilities where their prey is also abundant (e.g., Rivera et al., 1988). Myrionecta rubra appears to be sensitive to oil pollution in the marine environment when compared to other ciliates (Dale, 1988). Didinium appears to be more sensitive to copper stress than its prey Paramecium , a feature that has some impact on the stability of this predator-prey interaction (Doucet & Maly, 1990). If haptorians are generally more sensitive to toxicants than their prey, this may have significant impacts on the dynamics of microbial food webs . However, we know nothing yet of the generality of this susceptibility among haptorians or its impact on the stability of planktonic food webs, especially those dominated by protistan predators.