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can be up to 2,500 cells l \u22121 in freshwater (Barbieri & Orlandi, 1989) and up to 4,000 cells l \u22121 in marine coastal habitats (Leakey et al., 1993). Coleps species can range from 1,000\u20138,000 cells l \u22121 (Barbieri & Orlandi). James et al. (1995) recorded a large Holophrya (formerly Prorodon ) species contributing over 70% to the biomass during the fall overturn. However, prostomes , being typi- Fig. 13.1. Life cycle of a typical prostome , Holophrya (formerly Prorodon ) showing the two typical phases in its life cycle . In the Starvation Cycle , the ciliate forms a resting cyst when deprived of food. In the Reproduction Cycle , the ciliate may feed for some time without divi- sion. The protomont then forms a division cyst in which the tomont undergoes palintomy to produce multiple tomites that either encyst in resting cysts if there is no food or begin feeding again. Perhaps lacking a resting cyst stage, the parasite of marine fishes , the prorodontid Cryptocaryon , has a life cycle similar to this, which shows remarkable convergence on the life cycle of the oligohy- menophorean Ichthyophthirius , a parasite of freshwater fishes (see Chapter 15 ) (from Hiller & Bardele, 1888) cally small bodied, contribute less to the average biomass although they are often widespread and numerically dominant in lakes (Beaver & Crisman, 1989a). Prostomes are encounter feeding or raptorial cil- iates . They have been reported to feed on a variety of prey from bacteria to filamentous green algae . Bacteria feeders include planktonic and benthic species in the genera Coleps (Madoni, Berman, Hadas, & Pinkas, 1990; \u160imek, Macek, Pernthaler, Stra\u161krabová, & Psenner, 1996), Urotricha (\u160imek et al., 1996), and Holophrya (formerly Prorodon ) (Epstein & Shiaris, 1992). These ciliates graze from 60\u2013160 bacteria h \u22121 and planktonic species may survive and grow exclusively on a bacte- rial diet (Epstein & Shiaris; \u160imek et al.). The majority of studies on prostome feeding have demonstrated that a variety of \u201calgal\u201d species support good growth. Algae consumed by both marine and freshwater prostomes of the genera Balanion , Bursellopsis , Coleps , Holophrya (for- merly Prorodon ), Tiarina , and Urotricha include the chlorophytes Chlamydomonas and Chlorella (Madoni et al., 1990), the cryptophytes Rhodomonas and Cryptomonas (Pedrós-Alió, Massana, & Latasa, 1995a; Klaveness, 1984; Müller, 1991; Weisse & Montagnes, 1998), the chrysophytes Mallomonas and Synura (Skogstad, Granskog, & Klaveness, 1987; Wilbert, 1986), and the dino- flagellates Akashiwa , Heterocapsa , Scrippsiella , and Dinophysis (Hansen, 1991; Jeong, Yoon, Kim, Yoo, & Seong, 2002; Nakamura & Hirata, 2006; Stoecker, Davis, & Provan, 1983). The marine prostome Tiarina has been implicated in control- ling growth of some of these red-tide and toxic algae (Jeong et al., 2002). Holophrya (formerly Prorodon ) species can also ingest filamentous green algae , like Spirogyra (Leipe, 1989), and other ciliates (Canella, 1951). Finally, some species of Holophrya (formerly Prorodon ) are histophages , which can be cultured in the laboratory on tissues from a variety of organisms, including the annelid Tubifex , fish , and mammals (Czapik, 1965; Hiller & Bardele, 1988; de Puytorac & Savoie, 1968), which they likely encounter as moribund or dead in the natural environment. Prostomes are rarely implicated in the deaths of animals: Székely and Bereczky (1992) reported an unusual case of Coleps killing the fry of three species of aquarium fishes . A real killer of marine fishes , especially in tropical and subtropical environ- ments, is Cryptocaryon irritans , a ciliate ectopara- site of fishes (Dickerson & Dawe, 1995). Reports of Cryptocaryon are increasing as the marine aqua- culture industry expands. Outbreaks and infections have been reported on a variety of fishes , including flounder Paralichthys olivaceus in Japan (Kaige & Miyazaki, 1985), sea perch Lates calcarifer , and grouper Epinephelus coioides (Yambot, Song, & Sung, 2003). With a life cycle convergent on that of Ichthyophthirius multifiliis , Cryptocaryon was placed in the Family Ichthyophthiriidae (Corliss, 1979). However, light microscopical (Diggles, 1997), ultrastructural (Colorni & Diamant, 1993), and molecular (Diggles & Adlard, 1995; Wright & Colorni, 2002) data now assign it unequivo- cally to the Class PROSTOMATEA . Trophonts of C. irritans tend to leave the host fish during the dark cycle of a photoperiod (Burgess & Matthews, 1994b). Trophonts and tomonts do not develop nor- mally at 34°C and anoxic conditions killed tomonts (Yoshinaga, 2001). However, tomonts could sur- vive hypoxia and were stimulated to develop and excyst when transferred to oxic conditions. Yoshinaga (2001) argued that this explains why outbreaks of cryptocaryoniasis in fish maricul- ture cages are often associated with the autumnal turnover of the water column. Several species of fish can acquire protective immunity to crypto- caryoniasis through controlled infections (Burgess & Matthews, 1995a; Yoshinaga & Nakazoe, 1997). It is interesting to note that this immunity does not protect these fish against ichthyophthiriasis (Burgess & Matthews, 1995a), supporting the distant phylogenetic relationship between the two species. Fish species vary in their susceptibility to cryptocaryoniasis . Susceptible fish can be suc- cessfully treated only with a rigorous regimen of repeated hyposaline baths (Rigos, Pavlidis, & Divanach, 2001) or by dietary supplements with the medium-chain fatty acid, caprylic acid (Hirazawa, Oshima, Hara, Mitsuboshi, & Hata, 2001). Based on rDNA sequence analysis, Yambot et al. (2003) provided evidence of marine and low- salinity variants of C. irritans . This suggests both that this may be a cryptic species complex and that treatments may need to be adapted to the genotype of the invading strain. The distributions of prostomes and their prey are spatially correlated in both vertical and horizontal 13.2 Life History and Ecology 261 262 13. Subphylum 2. INTRAMACRONUCLEATA: Class 7. PROSTOMATEA dimensions in aquatic habitats (Pedrós-Alió et al., 1995; Stoecker, Davis, & Anderson, 1984). In situ growth rates are typically about one doubling per day (Macek, \u160imek, Pernthaler, Vyhnálek, & Psenner, 1996). Growth can be strongly influenced by the strains of prey organism: a strain of Coleps grew much better on some isolates of Cryptomonas than on others (Klaveness, 1984), while Balanion preferred cryptophytes as prey (Müller & Schlegel, 1999). These preferences are presumably due to chemical signals . Coleps will not eat living ciliates, such as Blepharisma and Spirostomum , but will readily ingest them when they are dead (Seravin & Orlovskaja, 1977). While there are obvious preferences for prey, growth may equally be influenced by the strain of prostome predator doing the feeding. Different strains and species of Urotricha and Balanion grew significantly differently on the same strain of Cryptomonas (Weisse & Montagnes, 1998; Weisse et al., 2001). This niche separation extended also to physiological responses to temperature as dif- ferent Urotricha species and strains showed dif- ferent temperature optima for growth. At high temperatures, Urotricha farcta grew fastest while Urotricha castalia grew well at low temperatures (Weisse et al., 2001). Balanion species appear not to tolerate temperatures much in excess of 20°C (Müller, 1991; Weisse et al., 2001). Mixotrophic prostomes have been observed to be dominant in the metalimnion of stratified temperate lakes (Zingel & Ott, 2000) and near the oxic-anoxic boundary in smaller bodies of water (Finlay & Maberly, 2000) where there is sufficient light for