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can also be hypersymbionts , clasping scyphidiid peritrichs that in their turn are ectosymbionts of gastropods (Botes, Van As, Basson, & Van As, 2001b; Peters, Van As, Basson, & Van As, 2004). The tips of these arms overlap in a structure called the bouton , whose substructure carries vesicles and microtubules similar to those of the scopula (Bradbury & Clamp, 1991). Crustaceans and fishes are by far the most commonly reported hosts of peritrichs . There are several comprehensive reviews of the epibionts of crustaceans (Fernández-Leborans & Tato-Porto, 2000c; Morado & Small, 1995). Briefly, peritrichs , typically sessiline and loricate forms, have been reported on every major group of crustaceans for almost every place their hosts can be found: on cladocerans (Green, 1974; Regali-Seleghim & Godinho, 2004); on ostracods (Griffiths & Evans, 1994; Matthes, 1990); on copepods (Basson & Van As, 1991; Nagasawa, 1988; Regali-Seleghim & Godinho, 2004; Valbonesi & Guglielmo, 1988) whose fecundity may not be decreased (Xie, Sanderson, Frost, & Magnuson, 2001); on mysids (Fernández-Leborans, 2003); on amphipods (Clamp, 1990, 1991; Fenchel, 1965b; Fernández- Leborans, Arndt, & Gabilondo, 2006; Jankowski, 1997); on isopods (Cook, Chubb, & Veltkamp, 1998; Ólafsdóttir & Svavarsson, 2002); and on decapods (Clamp, 1992; Fernández-Leborans & Gabilondo, 2006; Mayén-Estrada & Aladro-Lubel, 2002; Sprague & Couch, 1971). The prevalence and intensity of infection in aquaculture opera- tions have been correlated with water quality : Zoothamnium increased and Cothurnia decreased as ectosymbionts of prawns as the water quality decreased (Hudson & Lester, 1992). Formalin treatment of cultured marine shrimps reduced peri- trich infections (Bell, Arume, & Lightner, 1987). Peritrich symbionts of fishes , dominated by the mobiline trichodinids , are primarily restricted to the skin and gills (Lom, 1995; Lom & Laird, 1969), although sessiline peritrichs have also been reported (Chernyshova, 1976; Lom, 1966, 1973a, 1995; Fitzgerald, Simco, & Coons, 1982). Reviews of these fish parasites , listing or briefly describing numerous species, have been provided by Hoffman (1988), Basson and Van As (1989), Van As and Basson (1989), and Lom (1995). Lom (1958) provided a uniform approach to the characteriza- tion of mobiline peritrich species, which is now the standard approach for measuring the denti- cles in the adhesive disk (but see also Van As & Basson, 1989). Numerous surveys of the skin and gills of marine and freshwater fish demonstrate that trichodinids are particularly widespread parasites, being found in Europe (Arthur & Lom, 1984a; Dobberstein & Palm, 2000; Gaze & Wootten, 1998; Kazubski, 1991), Asia (Xu, Song, & Warren, 2002), Africa (Al-Rasheid, Ali, Sakran, Baki, & Ghaffar, 2000; Van As & Basson, 1992), North America , including the Caribbean (Arthur & Lom, 1984b; Arthur, Cone, Cusack, Barker, & Burt, 2004; Li & Desser, 1983), and the Pacific Ocean (Stein, 1979). Trichodinids have also been found in the urinary system (Basson, 1989) and intestine (Basson, Van As, & Fishelson, 1990) of fishes . There are rare reports of epidemic trichodinosis in natural fish populations, perhaps under stress (Do Huh, Thomas, Udomkusonsri, & Noga, 2005). However, Trichodina species typically have been recorded in aquaculture operations worldwide, parasitizing farmed (Arthur & Margolis, 1984; Basson & Van As, 1994; Lom, 1994; Özer, 2000; Urawa, 1992) and ornamental (Hoffman, 1988; Thilakaratne, Rajapaksha, Hewakopara, Rajapakse, & Faizal, 2003) fishes , and causing the disease trichodinosis . Triazinone at a dose of 50 µg ml \u22121 was effective at reducing parasitemia on several fish species (Schmahl, Mehlhorn, & Taraschewski, 1989). Peniculines , to our knowledge, have been reported as symbionts on only two occasions. Maguire and Belk (1967) reported Paramecium in snails while Singh and Dash (1992) reported an infection of Paramecium in the urinary tract of a patient on dialysis. However, the cytological evidence showing a seemingly small ciliate with a 15.2 Life History and Ecology 291 292 15. Subphylum 2. INTRAMACRONUCLEATA: Class 9. OLIGOHYMENOPHOREA posterior contractile vacuole and a somewhat pyri- form body shape, suggests that it might have been a hymenostome. Few hymenostomes have been reported as symbi- onts, with species in the three genera, Tetrahymena , Ophryoglena , and Ichthyophthirius being the most commonly reported. Tetrahymena species have been reported infecting natural populations of platyhelminthes (Wright, 1981), gastropod molluscs (Kazubski, 1964; Kozloff, 1956), and a variety of kinds of insects , such as black flies (Batson, 1983; Lynn, Molloy, & Lebrun, 1981), mosqui- toes (Barros et al., 2006; Clark & Brandl, 1976; Corliss & Coats, 1976; Egerter & Anderson, 1985; Jerome et al., 1996), chironomids (Corliss, 1960b; Corliss, Berl, & Laird, 1979; Golini & Corliss, 1981), and megalopterans (Batson, 1985), and aquarium or aquacultured fishes (Astrovsky et al., 2002; Ferguson, Hicks, Lynn, Ostland, & Bailey, 1987; Hatai et al., 2001; Hoffman et al., 1975; Imai, Tsurimaki, Goto, Wakita, & Hatai, 2000). Tetrahymena has infected a broader array of organ- isms in experimental situations (Thompson, 1958), and it is likely that it is only a facultative parasite in nature (Corliss, 1960b, 1972c). Nevertheless, in some situations, these ciliates may cause signifi- cant mortality of their insect hosts (Barros et al.; Grassmick & Rowley, 1973; Zaritsky, Ben-Dov, Zalkinder, & Barak, 1992). Could they be used as biocontrol agents for these vectors of important human diseases? The relationship between the predator-host mosquito and its ciliate symbiont is intimate enough to have evolved a predator-induced chemical signal that causes the free-living \u201csymbi- onts\u201d to transform as parasites, ultimately killing their host (Washburn et al., 1988). As a taxonomic aside, this tetrahymenid associated as a cuticular cyst on mosquitoes (Corliss & Coats, 1976; Egerter & Anderson, 1985) has been assigned to the genus Lambornella , although recent molecular evidence suggests that this genus rank is probably not justi- fied (Strüder-Kypke et al., 2001). Ophryoglena species are typically characterized as histophagous on moribund and dead aquatic organisms (Mugard, 1949), although they have been found infecting living bivalves (Karatayev, Burlakova, Molloy, Volkova, & Volosyuk, 2002; Molloy, Lynn, & Giamberini, 2005) and insects (Gaino & Rebora, 2000) (Fig. 15.1). There is now evidence that parasitic Ophryoglena have been very recently introduced to Ireland with their bivalve hosts, likely by humans from mainland Europe (Burlakova et al., 2006). Undoubtedly the most infamous ophryoglenine ciliate is Ichthyophthirius multifiliis , the causative agent of white spot disease of fishes. There is an extensive literature on this ciliate (see Dickerson & Dawe, 1995; Matthews, 2005). \u201cIch\u201d is apparently attracted to host fish by serum factors in their mucus (Buchmann & Nielsen, 1999; Haas, Haberl, Hofmann, Kerschensteiner, & Ketzer, 1999), and can cause significant mortalities in aquaculture operations (e.g., Munderle, Sures, & Taraschewski, 2004). To our knowledge, only one, natural, mass mortality has ever been reported dur- ing which over 18 million killifish in Lake Titicaca were killed (Wurtsbaugh & Tapia, 1988). However, \u201cIch\u201d infections can impact the swimming speed of eels and, while not killing them, may affect their ability to reach spawning sites in early spring (Münderle, Sures, & Taraschewski, 2004). The dispersal or theront stage of the life cycle arises by multiple palintomic divisions from an encysted