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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 “trained” 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 “symbiotic” 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 – 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 – 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
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 
1–2 mm sec −1 , 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 −1 , 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 “photosynthetic” 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’s 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 −1 , 
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