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(Jakobsen, Everett, & Strom, 2006). However, some pleurostomatids show a marked preference for more sedentary prey, such as colo- nial peritrichs and rotifers (Canella, 1951, 1954). Didinium nasutum and Paramecium have been the subjects of numerous studies exploring the relation- ship between a predator and its prey. Didinium can be \u201ctrained\u201d to feed on a variety of Paramecium species (Berger, 1980; Hewett, 1980a) and even Colpidium (Berger, 1979). Didinium consumes Paramecium bursaria that have zoochlorellae less efficiently than apochlorotic cells, indicating that the zoochlorellae may have a protective function for their host (Berger, 1980). To support continued growth, the best Paramecium prey must be reared on a mixture of wild bacteria (Burbanck & Eisen, 1960). The size of Didinium varies throughout the growth cycle (Salt, 1975). Size of Didinium also is related to the prey size: Didinium feeding on Colpidium are much smaller than those feeding on the larger Paramecium multimicronucleatum (Berger, 1979; Hewett, 1980a). Although the situa- tion is complex, Didinium tend to be most success- ful at capturing the size of prey on which they have been conditioned. However, large Didinium did have shorter handling times and encounter times regardless of prey size (Hewett, 1988), but capture rate is also influenced by predator density (Hewett, 1980b; Salt, 1974). The Didinium-Paramecium predator-prey system has also been used to model the stability of simple ecosystems . Luckinbill (1973) showed that populations of both species could be maintained for other 1 month if the Cerophyl culture medium was used at half-strength but thickened with methyl cellulose . Harrison (1995) reanalyzed the data from these experiments, con- cluding that the functional response curve might actually be of a more sigmoid, Type III form. Maly (1978) confirmed that spatial or temporal complex- ity is necessary to maintain the stable interaction between these two organisms. Litostomes are also hosts for a variety of symbionts, both as epibionts and endobionts. Endosymbiotic methanogens have already been mentioned above in our discussion of rumen ciliates . Non-methanogens of the genera Streptococcus and Ruminococcus can be found attached to the cell surface of rumen cili- ates , localizing in some cases around the cytostome (Imai & Ogimoto, 1978), while a Balantidium species infecting marine fish has epibiontic and endosymbiotic bacteria , some even residing in the macronucleus (Grim, 1993b). Haptorians , such as Askenasia , Didinium , Dileptus , Homalozoon , Lacrymaria , and Monodinium , harbor endosymbiotic Chlorella species (Foissner et al., 1999; Karpov, Goodkov, & Marinich, 1991). Undoubtedly the most famous \u201csymbiotic\u201d litostome is Myrionecta (= Mesodinium ) rubra . Ultrastructural studies dem- onstrated that this ciliate was in fact a consortium of a cryptophyte living inside the ciliate so that three nuclei are characteristic of this ciliate (Grain, de Puytorac, & Grolière, 1982; Hibberd, 1977). The crypto phyte symbiont is necessary for growth of M. rubra , which must have continued access to these prey populations for continuous and optimal growth (Hansen & Fenchel, 2006; Johnson & Stoecker, 2005). Johnson, Oldach, Delwiche, and Stoecker (2007) have demonstrated that the cryptophyte nuclei are essential for continued function of cryp- tophyte chloroplasts . They have termed this rela- tionship karyoklepty \u2013 nuclear stealing, since one cryptophyte nucleus may support the cytoplasm of several cryptophyte individuals whose nuclei have been lost from the consortium. They pose the ques- tion \u2013 is this truly a symbiosis? Myrionecta does have oral extrusomes , presum- ably toxicysts , that are carried in oral tentacles . 9.2 Life History and Ecology 195 196 9. Subphylum 2. INTRAMACRONUCLEATA: Class 3. LITOSTOMATEA These are presumably used to capture the sym- biont and other prey organisms, such as bacteria (Lindholm, Lindroos, & Mörk, 1988; Myung, Yih, Kim, Park, & Cho, 2006). Myrionecta is an extremely active swimmer, achieving rates of 1\u20132 mm sec \u22121 , enabling it to migrate vertically to take advantage of changing light levels (Lindholm, 1985). It is functionally a member of the phytoplankton . When it is in bloom, numbering over 10 5 l \u22121 , it can contribute over 70% of the total primary produc- tion , but typically contributes much less than that (Crawford, 1989; Stoecker, Putt, Davis, & Michaels, 1991). In these abundances it can cause non-toxic red tides . These \u201cphotosynthetic\u201d lito- stomes are technically mixotrophs . One of the most unusual mixotrophic litostomes is Perispira ovum . This ciliate has a preference for Euglena proxima , whose chloroplasts , mitochondria , and paramylon it sequesters. Although these intact organelles are surrounded by Perispira membranes and endoplas- mic reticulum, we do not yet know how much they contribute functionally to the predator\u2019s physiology (Johnson et al., 1995). Among the unicellular eukaryotes, litostomes , and particularly haptorians , are top predators in microbial food webs . Nevertheless, they are eaten by multicellular organisms, including copepods (Wickham, 1995), insect larvae (Addicott, 1974), and bivalve molluscs , such as mussels and scal- lops (Carver, Mallet, Warnock, & Douglas, 1996). Cyclotrichiids , as a group, are characterized by their ability to jump to avoid predators (Tamar, 1979). Escape speeds can exceed 100 body lengths per second, attaining speeds of up to 1.2 cm sec \u22121 , likely a speed record for ciliates (Fenchel & Hansen, 2006; Jakobsen, 2001). Myrionecta may be able to avoid predation because of its tremen- dous swimming ability, since it was hardly ingested by barnacle nauplii and some copepods (Jakobsen, 2001; Turner, Levinsen, Nielsen, & Hansen, 2001). However, ambush-feeding copepods , like Acartia tonsa , are able to consume Mesodinium species (Jakobsen, 2001). The endosymbiotic trichostomes can be preyed upon by the suctorian Allantosoma , which resides in the caecum and colon of the horse (Imai, 1979; Sundermann & Paulin, 1981). Ophryoscolecids can be parasitized by chytrid fungi (Lubinsky, 1955a, 1955b). Like other ciliates, litostomes have chemosen- sory abilities. Didinium is attracted to a heat-stable chemoattractant isolated from bacterial cultures, a behavior that presumably gets it to where its typical prey, Paramecium , might be found (Antipa, Martin, & Rintz, 1983). Litonotus lamella feeds on Euplotes crassus and modifies its behavior when placed in the Euplotes cell-free fluid: the predator decreases its creeping speed and modifies its turn- ing behavior so that it accumulates in regions of high prey density (Morelli, Ricci, & Verni, 1999). The swimming behavior of Didinium is apparently controlled in a fashion similar to other ciliates, with intracellular Ca 2+ concentrations influencing cili- ary beating mode (Pernberg & Machemer, 1995). Didinium also shows gravity-dependent swimming velocities, modulated by mechanically sensitive membrane channels , which keeps the cells sta- tionary in the gravity field (Bräucker, Machemer- Röhnisch, & Machemer, 1994). Myrionecta rubra has been reported to avoid being flushed from an estuary by dispersing away from the top of the water column on the ebb tide. Crawford and Purdie (1992) speculated that the major cue to this may be the turbulence generated by the shearing surface currents. This is supported by Fenchel and Hansen (2006) who have demonstrated that M. rubra can detect fluid flows and orient appropriately. Encystment is a common feature of most free- living litostomes , especially haptorians that are found in freshwater and soils . In contrast, the