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Abstract Ciliates in this class were considered 
in the 19th and to the mid-20th centuries to be the 
ancestral kind of ciliate – almost perfectly radi-
ally symmetrical and prostomatous. They are now 
 considered to be quite derived, and careful micro-
scopic analyses have exposed some differentiated oral 
ciliature in one of the included orders. Prostomateans 
are commonly represented in both freshwater and 
marine habitats where they can achieve significant 
abundances under bloom conditions, sometimes 
exceeding 30,000 l −1 , and they are suspected there-
fore of being able to control prey populations, 
which sometimes may be toxic red tide algae. 
Cryptocaryon irritans is a prostome that causes a 
disease of the skin of marine fishes. The somatic 
monokinetids of prostomes do not distinguish 
them from several other classes, although their 
somatic kineties are often organized in paratenes. 
They are distinguished by their oral structures: a 
paroral that almost encircles the anterior end and 
a set of brush or brosse polykinetids that abut near 
the oral region. Stomatogenesis is merotelokinetal. 
Although these ciliates conjugate, we do not know 
the genetical basis of their mating system(s). 
Keywords Cryptocaryoniasis, Coleps , histophagy 
 This class and the next, Class PLAGIOPYLEA , 
each include the smallest numbers of species and 
genera among classes of ciliates. Yet, their distinctive 
morphologies, their cortical ultrastructure, their 
genetic divergence, and their very different ecological
roles differentiate them among ciliates. Members of 
the Class PROSTOMATEA are typically ovoid to 
cylindroid in body shape. Moreover, they are some 
of the smallest ciliates: <20 µm for some Urotricha
species to moderate-sized for species of proro-
dontids , which typically do not exceed 500 µm in 
length. Body ciliation is typically holotrichous 
often with a single or multiple caudal cilia. The 
cytostome is apical to subapical in position. Most 
species are free-living microphages. However, a 
number of prorodontids are at least facultative his-
tophages (Czapik, 1965), and one species, the fish 
ectoparasite Cryptocaryon irritans , has recently 
been transferred to this class based on the sequence 
of its small subunit (SSU) rRNA gene (Wright & 
Colorni, 2002). 
 The focus of attention on the role of ciliates 
in the microbial loop has revealed that small pro-
stomateans can be extremely abundant. Coleps
hirtus reached densities exceeding 60,000 cells l −1
in rice fields (Madoni, 1986) while prostoma-
tids collectively can reach over 100,000 cells l −1
(Müller, 1989). The marine prostomatid Tiarina 
fusus has been observed in bloom conditions to 
exceed 30,000 l −1 (Dale & Dahl, 1987b). Ecologists 
have brought these ciliates into the laboratory 
and established long-term cultures of Coleps
(Klaveness, 1984) and Balanion (Müller, 1991) on 
the flagellate Rhodomonas . Isolates of Urotricha
grew well on the flagellate Cryptomonas (Weisse 
& Montagnes, 1998). Both species have been effi-
ciently counted using flow cytometry and nucleic 
acid stains (Lindstrom, Weisse, & Stadler, 2002). 
Weisse (2006) has argued that Urotricha species 
and other planktonic ciliates are excellent eco-
physiological models to explore the role of protists 
in microbial food webs . Parasitologists have 
 Chapter 13 
 Subphylum 2. 
PROSTOMATEA – Once Considered 
Ancestral, Now Definitely Derived 
established standardized methods for maintaining
C. irritans in culture using experimental hosts 
(Burgess & Matthews, 1994a; Dan, Li, Lin, Teng, 
& Zhu, 2006), and initial attempts at in vitro cul-
ture show promise (Yambot & Song, 2004). 
 The prostomes have also occupied a pivotal posi-
tion in our conceptions of evolutionary diversifica-
tion within the phylum. The name of the class, 
 PROSTOMATEA , is derived from the Greek pro
meaning before and the Greek stoma meaning mouth. 
It refers therefore to the apical to subapical position 
of the cytostome, which is surrounded by a simple 
 circum oral ring of ciliated dikinetids. Corliss (1979) 
argued that the apical to subapical cytostome and 
simple oral ciliature made prostomes the “most prim-
itive” group of ciliates next to the karyorelicteans. 
While we are still convinced that the karyorelicteans 
represent descendants of an early diverging clade 
of ciliates, the prostomes are now conceived as a 
fairly derived group whose oral features have become 
secondarily simplified (Bardele, 1989; Hiller, 1993b). 
These conclusions based on ultrastructural research 
have been confirmed by rRNA gene sequences that 
show prostomes to be closely associated with the 
 colpodeans , plagiopyleans , and oligohymenopho reans 
(Baroin-Tourancheau et al., 1992; Fleury, Delgado, 
Iftode, & Adoutte, 1992; Lynn, Wright, Schlegel, & 
Foissner, 1999; Stechmann, Schlegel, & Lynn, 1998). 
 Small and Lynn (1981) established the class based 
on both the unusual nature of the presumed trans-
verse microtubular ribbons that arose from the oral 
dikinetids in prostomes (de Puytorac & Grain, 1972) 
and features of the somatic monokinetids, which 
were very similar to those of the oligohymenopho-
reans (Rodrigues de Santa Rosa, 1976). Subsequent 
ultrastructural study by Huttenlauch and Bardele 
(1987) demonstrated that these presumed transverse 
ribbons were actually postciliary microtubular rib-
bons, oriented into a “transverse”-like position 
during stomatogenesis. Thus, the cytopharynx of 
prostomes is lined by postciliary microtubules aris-
ing from oral dikinetids, a situation certainly found 
in most of the classes of this subphylum – the Class 
 LITOSTOMATEA being the notable exception. 
Aside from these two features, stomatogenesis in 
prostomes appears to involve a clockwise migration 
of the oral dikinetids to form the circumoral crescent 
or ring characteristic of the class (Hiller, 1993b). 
 To close these introductory remarks, we must 
draw the reader’s attention to a nomenclatural 
change that has complicated our subsequent dis-
cussion. Foissner, Berger, and Kohmann (1994) 
discovered that for years ciliates assigned to the 
genus Prorodon ought properly to have been 
assigned to the genus Holophrya based on the 
features of its type species, Holophrya ovum
Ehrenberg, 1831, which exhibits brosse kineties . 
Furthermore, they concluded that Prorodon ought 
to include only species with a slit-like oral region 
and a brosse that extends the length of the body 
(Foissner et al., 1994), based on its type species, 
Prorodon niveus Ehrenberg, 1833. Foissner et al. 
(1994) established a new genus Apsiktrata based 
on Urotricha gracilis , a Urotricha species with-
out a brosse , and a new family Apsiktratidae , to 
include those prostomes without a brosse . We have 
assumed throughout this chapter that ciliates identi-
fied as Prorodon , Pseudoprorodon , and Holophrya
prior to Foissner et al. actually ought to have been 
identified as Holophrya , Prorodon , and Apsiktrata , 
respectively. Throughout the chapter, we have used 
the new names but followed these in parentheses 
by the genus name used in the literature cited. 
 13.1 Taxonomic Structure 
 Corliss (1979) assigned the prostome ciliates to 
a single order in the Subclass Gymnostomatia . 
He included three suborders, the Archistomatina , 
 Prostomatina , and Prorodontina . Morphological 
and ultrastructural analysis of the archistomatines , 
which includes only the Family Buetschliidae , 
argues for a closer relationship with ciliates in 
the Order Vestibuliferida (see Chapter 9. Class
LITOSTOMATEA ; see also parsimony analysis of 
de Puytorac, Grain, & Legendre, 1994). Following 
earlier ultrastructural studies, Small and Lynn 
(1985) concluded that the cytopharyngeal structure 
of prostomes was a rhabdos , and