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net photosynthesis. The sym-
bionts can be a facultative association between 
a Chlorella and Coleps or Holophrya (formerly 
Prorodon ) (Christopher & Patterson, 1983) or the 
Holophrya may actually sequester chloroplasts of 
prey (Blackbourn, Taylor, & Blackbourn, 1973). 
As with other mixotrophs , symbiont-bearing 
Coleps had growth rates higher than aposymbiotic 
forms when food resources were scarce (Stabell, 
Andersen, & Klaveness, 2002). Symbiotic bacteria
have been reported in Urotricha ovata (de Puytorac 
& Grain, 1972). 
 As conspicuous components of microbial food 
webs , it is not surprising that prostomes are them-
selves eaten by other organisms. A variety of 
predatory ciliates, such as Dileptus , Lagynophrya , 
Lacrymaria , Didinium , and Favella (Jürgens, 
Skibbe, & Jeppesen, 1999; Seravin & Orlovskaja, 
1977; Stoecker & Evans, 1985), prey upon pros-
tomes , such as Coleps and Balanion . Even though 
Tiarina may feed upon the autotrophic dino-
flagellate Dinophysis , when the dinoflagellate is 
heterotrophic, the table is literally turned and 
predator becomes prey (Hansen, 1991)! Prostomes 
are also eaten by the rotifers Brachionus (Mohr, 
Gerten, & Adrian, 2002) and Keratella (Jack & 
Gilbert, 1993). Microcrustacean zooplankton are 
also predators of prostomes : in fresh water by 
the cladocerans Daphnia and Bosmina (Jack & 
Gilbert, 1993; Wickham & Gilbert, 1991) and the 
 copepod Cyclops (Wickham, 1995); and in marine 
environments by the copepod Acartia (Olsson, 
Granéli, Carlsson, & Abreu, 1992). Even the jel-
lyfish Aurelia preferred the prostome Urotricha
over dinoflagellates (Stoecker, Michaels, & Davis, 
1987b). Balanion is able to avoid predation by 
marine copepods by escape jumping in response 
to fluid mechanical signals generated by the preda-
tor’s feeding apparatus (Jakobsen, 2001), and 
some Urotricha species may have a similar ability 
(Tamar, 1979). 
 Finally, Hiller and Bardele (1988) make the impor-
tant point that the stages in the life cycle of some 
prostomes, such as the histophagous Holophrya
(formerly Prorodon ), can be so different in size and 
shape that one might consider them to be different 
species. Thus, careful and continuous observation is 
necessary to confirm that this is in fact not the case. 
 13.3 Somatic Structures 
 Prostomes are generally ovoid to ovoid elongate in 
shape, but they can range from nanociliates (i.e., 
< 20 µm in body length), as some Urotricha species 
do, to several hundreds of micrometers in length, as 
some Holophrya (formerly Prorodon ) species do 
(Fig. 13.2). They are typically not highly contractile,
although their bodies are flexible and, in histophagous
and parasitic forms, able to penetrate the tissues of 
hosts. A few species, such as Vasicola , can form 
a lorica (Dragesco et al., 1974). Since the species 
diversity of this class is not great, there have also 
been relatively few studies of the ultrastructure 
on this group. Species in the genera Balanion , 
Bursellopsis , Coleps , Cryptocaryon , Holophrya
Fig. 13.2. Stylized drawings of representative genera from orders in the Class PROSTOMATEA . A member of the 
Order Prostomatida – Apsiktrata . Members of the Order Prorodontida . Urotricha , Coleps , and Holophrya (formerly 
Prorodon )
(formerly Prorodon ), Placus , and Urotricha have 
been investigated. 
 The plasma membrane of prostomes rarely 
exhibits a glycocalyx , although a very thin and 
inconsistent layer can be observed in some micro-
graphs of Holophrya (formerly Prorodon ) (Hiller, 
1993a). The underlying alveoli are often swollen 
and can be occupied partially with smaller par-
ticles in Holophrya (formerly Prorodon ) (Hiller, 
1993a), bacteria in Placus (Grain et al., 1979), 
completely with dense material in both Holophrya
(de Puytorac, 1964) and Cryptocaryon (Colorni 
& Diamant, 1993; Matthews et al., 1993), and 
with calcium carbonate skeletal plates in Coleps
(Fauré-Fremiet, André, & Ganier, 1968b). The 
fine structure of these skeletal plates differs in 
morphology among Coleps species (Huttenlauch, 
1985). The epiplasm is conspicuous in larger forms 
with a fine layer observed in Holophrya (formerly 
Prorodon ) (Hiller, 1993a) and a thicker layer 
recorded in Cryptocaryon (Colorni & Diamant, 
1993; Matthews et al., 1993) and Placus (Grain 
et al., 1979). Longitudinal microtubules have been 
observed to lie above the epiplasm in the cortical 
ridges of Holophrya (formerly Prorodon ) species, 
a feature demonstrated also in oligohymenophoreans 
(Hiller, 1993a). 
 The prostome somatic monokinetid can now 
be characterized definitively as bearing a slightly 
divergent postciliary ribbon at triplet 9, an anteri-
orly directed and typically short kinetodesmal fibril 
at triplets 5, 6, 7, and a radially oriented transverse 
ribbon at triplet 4 or 5 (Hiller, 1993a). The kine-
todesmal fibril may be bifurcated at its beginning 
in some species (Fig. 13.3) (Hiller, 1993a; Lynn, 
1991). The transverse ribbon microtubules typi-
cally extend upward and laterally into the cortical 
ridge and the postciliary microtubular ribbons may 
not overlap (Fig. 13.4). However, in Placus , the 
transverse ribbons extend downward and laterally 
Fig. 13.3. Schematics of the somatic kinetids of the 
Class PROSTOMATEA . ( a ) Monokinetid of Coleps . ( b ) 
Dikinetid of Coleps . ( c ) Dikinetid of Bursellopsis . ( d ) 
Monokinetid of Bursellopsis (from Lynn, 1981, 1991) 
Fig. 13.4. Somatic cortex of a typical prostome inter-
preted based on the somatic cortex of Bursellopsis . 
(Modified after Hiller, 1993a.)
due to the position of the somatic kinetosomes 
near the tops of the cortical ridges in this genus 
(Grain et al., 1979). The kinetids at the anterior of 
somatic kineties may be dikinetids whose posterior 
kinetosome bears the same fibrillar associates in 
similar orientation to those of the somatic monoki-
netids . However, the anterior kinetosome bears a 
single postciliary microtubule and a tangentially 
oriented transverse ribbon adjacent to triplets 3, 
4, 5 (Hiller, 1993a; Lynn, 1991). Hiller (1993a) 
has suggested a transformation series deriving 
 prostome and colpodean dikinetids from a com-
mon ancestor with this somatic dikinetid structure. 
Microtubules have been observed underlying the 
kinetosomes of prostomes , originating from dense 
material at the kinetosome base and extending 
anteriorly in Holophrya (formerly Prorodon ), the 
so-called prekinetosomal microtubules (Hiller). 
Their association with the kinetosome has not been 
demonstrated in Coleps (Lynn, 1985). Rudberg and 
Sand (2000) have provided the only study of the 
electrophysiology of prostomes . They observed a 
unique biphasic membrane potential that correlates 
with the alternating periods of linear and circular 
swimming of Coleps . 
 Mucocysts , typically elongate ovoid, have been 
observed in the somatic cortex of prostomes (Hiller, 
1991, 1993a; Huttenlauch, 1987; Matthews et al., 
1993). Colorni and Diamant (1993) observed a 
bizarre phenomenon in Cryptocaryon : vesicles 
within the alveoli appear to fuse with the plasmale-
mma, presumably releasing material to the outside 
and leaving the cell’s surface covered with numer-
ous “pores”. Mitochondria , often present in the cell 
cortex, are the typical forms with tubular cristae. 
 As with other ciliates, prostomes have mineral 
elements in the cytoplasm. Like those in the alveoli 
of Coleps species, the mineral concretions in the 
endoplasm have been demonstrated to be calcium 
carbonate (André & Fauré-Fremiet, 1967; Rieder, 
Ott, Pfundstein, & Schoch, 1982). 
 There is typically a contractile vacuole in the 
posterior of the cell, although some larger forms 
can have about 70 independently functioning vacu-