tomont (Dickerson & Dawe, 1995; Ewing & Kocan, 1992) (Fig. 15.1). The theront contacts the host epithelium or gill tissue and penetrates between cells to enter the epidermis (Ewing, Kocan, & Ewing, 1985; Kozel, 1986). Fish hosts can be immunized against \u201cIch\u201d using the surface immo- bilization antigen of the ciliate (Buchmann, Sigh, Nielsen, & Dalgaard, 2001; Wang & Dickerson, 2002; Xu, Klesius, & Panangala, 2006). While malachite green and formalin have long been effective treatments, both have carcinogenic prop- erties. Malachite green has already been banned on fish farms in some countries. The search for other effective treatment compounds has included sodium percarbonate , garlic extract , triazinone , and crude extracts of plants (Buchmann, Jensen, & Kruse, 2003; Ekanem, Obiekezie, Kloas, & Knopf, 2004; Schmahl et al., 1989). Apostomes have been reported as commensal symbionts, primarily from crustaceans (Bradbury, 1996; Chatton & Lwoff, 1935a). Their life cycles are complex and varied (Fig. 3.1). One group, represented by species of Hyalophysa and Gymnodinioides , is termed exuviotrophic because they excyst to feed on the exuvial fluids in the host moult, often increasing their body volume 60-fold before they encyst , divide, and disperse to find another host (Bradbury, 1966a; Grimes, 1976; Landers, Confusione, & Defee, 1996). A sec- ond group, represented by Terebrospira , burrows through the endocuticle of the host shrimp and ingests the dissolved products (Bradbury, Clamp, & Lyon, 1974; Debaisieux, 1960). A third group, represented by Vampyrophrya , ingests tissues of the host calanoid copepod , either when it is injured or ingested (Grimes & Bradbury, 1992). A fourth group, represented by Collinia , lives endoparasiti- cally in the body fluids of euphausiid crustaceans (Capriulo & Small, 1986; Lindley, 1978), and can cause mass mortalities of their hosts (Gómez- Gutiérrez, Peterson, De Robertis, & Brodeur, 2003; Gómez-Gutiérrez, Peterson, & Morado, 2006). Two other, highly unusual members of the subclass are the pilisuctorids Conidophrys and Askoella , which attach to the setae of the host crustacean (Bradbury, 1975; Mayén-Estrada & Aladro-Lubel, 2004), and the cyrtocarid Cyrtocaryum , which lives in the digestive caeca of polychaete annelids (Fauré-Fremiet & Mugard, 1949b). A final example are the chromidinid apostomes , which were studied by Chatton and Lwoff, and have been recently reported from the kidneys of Japanese cephalopods (Furuya, Ota, Kimura, & Tsuneki, 2004). Astomes are obligate commensal symbionts, found typically in the digestive tract of annelids (de Puytorac, 1994g). Cépède (1910) and de Puytorac (1954) stand as the substantial 20th century monographic works on this group. Despite these intensive investigations with reports from Europe (Cépède; de Puytorac), North America (Bush, 1934; Powders, 1970), and Africa (de Puytorac & Dragesco, 1969a, 1969b; Ngassam, 1983), we still do not know how these ciliates are transmitted from one host to the next. They display a variety of cel- lular differentiations, such as hooks and suckers , to maintain their position in the intestine (de Puytorac, 1994g). The distribution of species of Maupasella , Anoplophrya , and Metaradiophrya along the diges- tive tract of their host worm Allolobophrya savigni is correlated with pH : each species apparently preferring a region characterized by a different pH (de Puytorac & Mauret, 1956). Cepedietta species are found in the intestine of salamanders , and their prevalence is inversely related to alti- tudes below 1,400 m, perhaps explained by tem- perature variations (Powders, 1970). There is yet no experimental evidence on how astomes feed. However, it is likely that they use receptor-mediated endocytosis , perhaps at the parasomal sacs , as has been demonstrated for Tetrahymena (Nilsson & van Deurs, 1983) and Paramecium (Allen, Schroeder, & Fok, 1992; Ramoino et al., 2001). While we can only speculate at the moment on the feeding habits and preferences of astomes , there is no doubt that most free-living oligohy- menophoreans are bacterivorous, down-stream filter feeders . The cilia of the paroral or undu- lating membrane typically are used to filter the water from the feeding current created by the oral polykinetids (Fenchel, 1980a, 1980b), although species without well-developed paroral cilia, such as Glaucoma species, may use the innermost oral polykinetid as the filter (Fenchel & Small, 1980). Hymenostomes , such as species in the genera Colpidium , Glaucoma , and Tetrahymena , can ingest a variety of bacterial species, which vary in how well they support growth (Dive, 1973; Taylor, 1979; Taylor & Berger, 1976; Taylor, Gates, & Berger, 1976). Colpidium (or Dexiostoma ) and other hymenostomes may also supplement their diet with small detrital parti- cles (Posch & Arndt, 1996). Tetrahymena may be a poor competitor in relation to Colpidium or Paramecium (Long & Karel, 2002). This may explain the selection for histophagous and endoparasitic feeding strategies in some Tetrahymena species (Corliss, 1972c; Roque, de Puytorac, & Savoie, 1971), although glaucomids , such as Espejoia , have also adopted histophagy , feeding on and in the gelatinous matrices of egg masses of aquatic insects and molluscs (Fryd- Versavel, Iftode, & Wilbert, 1975). Cannibalism has also evolved in Tetrahymena with species like Tetrahymena vorax and Tetrahymena patula able to respond to secretions from prey species and develop into large-mouthed or macrostome predators able to feed on smaller Tetrahymena species (Buhse, 1967; Corliss, 1973; Williams, 1960, 1961). Furthermore, macrostome forms of T. vorax appear to be highly selective feeders, pre- ferring to ingest T. thermophila over latex beads and microstome forms of T. vorax (Grønlien, Berg, & Løvlie, 2002). Peniculine feeding preferences range from bacte- rivory to mixotrophy . The peniculine Paramecium feeds on a variety of bacterial species, although some bacterial species may be toxic (Curds & Vandyke, 1966). Paramecium may also supplement 15.2 Life History and Ecology 293 294 15. Subphylum 2. INTRAMACRONUCLEATA: Class 9. OLIGOHYMENOPHOREA its diet by ingesting detrital particles (Posch & Arndt, 1996). Paramecium bursaria typically hosts endosymbiotic Chlorella species ( see below ), and these influence the emphasis on bacterivory: in the dark P. bursaria relies on bacterivory , but in the light the predominant nutritional source derives from the photosynthetic products of its symbionts (Weis, 1974). However, Berk, Parks, and Ting (1991) observed that light itself may enhance ingestion rates since mixotrophic P. bursaria fed faster than aposymbiotic individuals. As an aside, Chlorella -bearing P. bursaria are not ingested by Didinium as rapidly as apochlorotic individuals, suggesting that metabolites from the consortium may discourage predation (Berger, 1980). On the other hand, species of the peniculine Frontonia are typically not bacterivorous, but flourish on chryso- phytes , cryptophytes , chlorophytes , diatoms , and even testate amoebae (Dias & D\u2019Agosto, 2006; Skogstad et al., 1987). Carnivorous peniculines include the giant Neobursaridium , first classified as a heterotrich because of its large size and con- vergently arranged somatic cilia that appeared like an adoral zone (Dragesco & Tuffrau, 1967; Nilsson, 1969), and Lembadion , which can adjust its size to the size of its prey, such as Colpidium and Paramecium (Fyda, 1998; Kopp & Tollrian, 2003). Peritrichs , such as Vorticella , Epistylis , and Zoothamnium , are very efficient downstream filter feeders (Fenchel, 1980a, 1980b; Sleigh & Barlow, 1976).