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united the Classes 
Subphylum Rhabdophora , respectively distinguish-
ing ciliates in these two classes by the radial and 
tangential orientation of transverse microtubular 
ribbons on somatic monokinetids. Huttenlauch 
and Bardele (1987) undercut the entire conceptual 
unity of the Subphylum Rhabdophora by show-
ing that the “cytopharyngeal” microtubules of 
 prostomes were postciliary ribbons, and not trans-
verse ribbons. De Puytorac (1994c) assigned the 
 prostomes to the Subclass Prostomatia in the Class 
 NASSOPHOREA , and recognized two orders, the 
 Prorodontida and Prostomatida . However, there 
are few detailed similarities in morphology (see 
below and Chapter 11. Class NASSOPHOREA ) 
and in molecules to support a close relationship 
between nassophoreans and prostomes (Baroin et al.,
1992; Bernhard & Schlegel, 1998; Fleury et al., 
1992; Lynn et al., 1999; Stechmann et al., 1998). 
Thus, we follow Lynn and Small (2002) in retain-
ing a Class PROSTOMATEA in the Subphylum 
 Intramacronucleata . 
 The Class PROSTOMATEA is based on the 
structure of the somatic kinetids, which have a 
divergent postciliary ribbon, anteriorly extend-
ing kinetodesmal fibril , and a somewhat radially 
oriented transverse ribbon, and on the structure of 
the circumoral dikinetids , which, at least in some 
 prorodontids , have postciliary ribbons organized as 
ribbons of circumoral microtubules . We recognize 
two orders within this class, the Order Prorodontida 
and the Order Prostomatida . 
 The Order Prorodontida is characterized by 
having an oral region that is apical or slightly 
subapical. In some species (e.g., Balanion , 
Bursellopsis , Urotricha ), the cytostome is sur-
rounded by overlapping ribbons of circumoral 
microtubules derived from the postciliary ribbons 
of the circumoral dikinetids (Hiller, 1992, 1993b). 
 Toxicysts are typical and may be carried in oral 
palps , which are internal to the cilia of these oral 
dikinetids (e.g., Bardele, 1999; Fauré-Fremiet & 
André, 1965b). Finally, typically outside the cir-
cumoral dikinetids , there are three or more “oral” 
polykinetids that form the so-called brosse . The 
 brosse can be reduced to several dikinetids in 
Balanion (Bardele, 1999) or be composed of many 
units extending down the entire length of the 
body in Pleurofragma (see Lynn & Small, 2002). 
We include the following families: Balanionidae , 
 Colepidae , Holophryidae , Lagynidae , Placidae , 
 Plagiocampidae , Prorodontidae , and Urotrichidae . 
 The Order Prostomatida is characterized by hav-
ing a truly apical oral region, by having perioral 
somatic kineties that form conspicuous paratenes 
(see Dragesco, Iftode, & Fryd-Versavel, 1974), and 
by two negative characters – lack of a brosse and 
lack of toxicysts . Bardele (1999) argued that this 
order is not justified. He imagined the taxa assigned 
to it, such as Metacystis and Apsiktrata (formerly 
Holophrya ), as being at the end of an evolutionary 
process of adaptation to the planktonic habitat by 
ventrostomial ancestors of the prostomes: Balanion
has remnants of the brosse while the almost per-
fectly radially symmetrical prostomatids have lost 
the brosse completely. While we sympathize with 
this view, there are yet no detailed ultrastructural 
studies on a “true” prostomatid and no molecular 
data. Since other higher taxa are also defined by 
the absence of characters (e.g., absence of toxicysts 
in the Subclass Trichostomatia ) and have been 
subsequently supported by molecular data, we 
have retained the Order Prostomatida in which we 
include the following families: Apsiktratidae and 
 Metacystidae . 
 Finally, we have placed the Family Malaco-
phryidae incertae sedis in this class and await data 
from electron microscopy and gene sequencing to 
confirm its position. 
 There are no monographic works on this class. 
However, Foissner, Berger, and Schaumburg 
(1999) provide an excellent recent treatment in 
their monograph on the identification and ecology 
of limnetic plankton ciliates, while Foissner and 
Pfister (1997) have provided a key to some com-
mon Urotricha species. 
 13.2 Life History and Ecology 
 The life cycle of a typical prostome Holophrya
(formerly Prorodon ) has been characterized by 
Hiller and Bardele (1988) (Fig. 13.1). As noted 
above, there is a considerable recent literature 
on the distribution and abundance of prostome 
ciliates, especially those found in the plankton of 
a variety of aquatic environments. Prostomes have 
extremely broad feeding preferences, primarily due 
to the range in size of taxa in this class. Smaller 
 prostome species are microphagous bacterivores 
while larger species can consume filamentous 
algae. Common genera, such as Balanion (now 
includes Pseudobalanion ), Bursellopsis , Coleps , 
Holophrya (formerly Prorodon ), and Urotricha , 
have been recorded from habitats around the world, 
as diverse as small temporary ponds (Foissner, 
1984b; Madoni & Sartore, 2003) and soils in 
 Europe (Berger, Foissner, & Adam, 1984) and glo-
bally (Foissner, 1998a). They have been found in 
 saline lakes in Europe (Esteban, Finlay, & Embley, 
13.2 Life History and Ecology 259
1993), Australia (Post, Borowitzka, Borowitzka, 
Mackay, & Moulton, 1983), and Chad (Pourriot, 
Iltis, & Leveque-Duwat, 1967). Prostomes have also 
been recorded in the interstitia of marine coastal 
habitats of Black and Caspian Seas (Agamaliev, 
1971; Kovaleva & Golemansky, 1979) and in leaf 
litter associated with coastal mangroves (Dorothy, 
Satyanarayana, Kalavati, Raman, & Dehairs, 2003). 
They are conspicuous in the marine plankton off 
 North America (Dolan, 1991; Martin & Montagnes, 
1993; Montagnes, Lynn, Roff, & Taylor, 1988) and 
 Europe (Dale & Dahl, 1987b; Edwards & Burkill, 
1995; Leakey, Burkill, & Sleigh, 1993) where they 
show extreme tolerances to fluctuations in salinity 
(Ax & Ax, 1960). Prostomes have been recorded 
from rivers (Foissner, 1997b) and lakes worldwide, 
in South America (Barbieri & Orlandi, 1989), 
 North America (Beaver & Crisman, 1989a; Lynn 
& Munawar, 1999), Europe (Carrias, Amblard, & 
Bourdier, 1994; Müller, 1991; Pfister, Auer, & Arndt,
2002; Stensdotter-Blomberg, 1998; Stenson, 1984; 
Zingel, Huitu, Makela, & Arvola, 2002), Israel 
(Hadas & Berman, 1998), Australia (Esteban, 
Finlay, Olmo, & Tyler, 2000; James, Burns, & 
Forsyth, 1995), and in Lake Baikal (Obolkina, 
1995, 2006). In deeper lakes, prostomes are com-
mon in the epilimnion and metalimnion (Zingel 
& Ott, 2000), and even in the hypolimnion (Guhl, 
Finlay, & Schink, 1996). Other freshwater habitats 
include rice fields (Madoni, 1986, 1996) and small 
ponds (Finlay et al., 1988). 
 Since prostomes are found in soils , these ciliates 
must encyst . Cysts can be extremely cryptic, even 
in species found in freshwater (Hiller & Bardele, 
1988; Leipe, 1989; de Puytorac, & Savoie, 1968). 
The cyst wall of the tomonts of Cryptocaryon is 
composed of several layers of fibrous material, 
presumably derived from the many cortical muco-
cysts (Matthews, Matthews, & Burgess, 1993). 
Tannreuther (1926) noted that the resting cysts of 
Holophrya (formerly Prorodon ) have thicker cyst 
walls than the temporary cysts (Fig. 13.1). 
 Abundances vary due to a number of expected 
factors, such as food abundance (Finlay et al., 
1988), but may also be influenced by ultraviolet 
irradiance (Barcelo & Calkins, 1980), pH (Weisse 
& Stadler, 2006), and temperature (Montagnes & 
Weisse, 2000; Weisse, 2006). Prostomes often tend 
to numerically dominate the plankton , especially 
in the early summer months in temperate lakes 
(Müller, 1989). As noted above, prostomes can 
reach exceedingly high densities. Nevertheless, 
typical peak abundances for Urotricha