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can be 
up to 2,500 cells l −1 in freshwater (Barbieri & 
Orlandi, 1989) and up to 4,000 cells l −1 in marine 
coastal habitats (Leakey et al., 1993). Coleps
species can range from 1,000–8,000 cells l −1
(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, 
 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; Šimek, Macek, Pernthaler, 
Straškrabová, & Psenner, 1996), Urotricha (Šimek 
et al., 1996), and Holophrya (formerly Prorodon ) 
(Epstein & Shiaris, 1992). These ciliates graze 
from 60–160 bacteria h −1 and planktonic species 
may survive and grow exclusively on a bacte-
rial diet (Epstein & Shiaris; Šimek et al.). The 
majority of studies on prostome feeding have 
demonstrated that a variety of “algal” 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
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, Šimek, 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