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175
Abstract The Class ARMOPHOREA represents 
a new assemblage of ciliates, and one of the two 
“ riboclasses” as their establishment is completely 
dependent upon small subunit rRNA gene sequences 
that showed affi nities of the two included orders 
– Armophorida and Clevelandellida. The ciliates in 
this class occupy anoxic habitats. Armophorids are 
typical of sapropelic habitats, but can be benthic or 
planktonic, while clevelandellids are endosymbi-
onts in the digestive systems of a wide variety of 
invertebrates, particularly insects, and some verte-
brates, particularly amphibians. While their somatic
dikinetids are quite different, armophoreans are all 
characterized by having their mitochondria trans-
formed to hydrogenosomes, organelles that provide 
hydrogen to the methanogenic bacterial symbionts 
of these ciliates. The oral structures of the two orders
are also divergent: membranelle-like in armophorids
and heteromembranelles in clevelandellids. Stomato-
genesis is pleurotelokinetal. The macronucleus is 
of simple form, but polytene chromosomes develop 
after conjugation and the macronuclear DNA ulti-
mately differentiates into gene-sized pieces. Armo-
phorids, because of their habitat preferences, are 
particularly good bioindicators of anoxic aquatic 
environments. 
Keywords Endosymbiont, cathetodesmal fi bril, 
 sulfureta, secant system 
 The ciliates included in this class are typically small 
to medium-sized cells. Armophoreans are free-
swimming and typically holotrichously ciliated. 
However, their body ciliation can vary from many, 
densely ciliated kineties in some clevelandellids 
to only anterior and posterior cirrus-like tufts 
in some armophorids. All species have multiple 
adoral polykinetids, ranging from around a dozen 
in some armophorids to several dozens in some 
 clevelandellids . These ciliates are very restricted 
in their distribution. Although world-wide, they 
are confined to sediments, both aquatic (Fenchel, 
1993) and terrestrial (Foissner, 1987), and the 
water column (Fenchel et al., 1995), where oxygen 
tensions are extremely reduced to absent. They are 
also found as endocommensal symbionts in the 
digestive tracts of a variety of metazoans, rang-
ing from selected invertebrates (Albaret, 1970b; 
Hackstein & Stumm, 1994) through to amphibians 
(Affa’a, Ndongo, & Granosik, 1995). Interest has 
increased in the group recently because they harbor 
endosymbiotic methanogenic bacteria , which can 
themselves produce the greenhouse gas, methane . 
There can be thousands of methanogenic bacteria 
per ciliate (van Bruggen, Stumm, & Vogels, 1983), 
producing significant quantities of methane , which 
is then liberated into the environment (Fenchel & 
Finlay, 1992; Hackstein & Stumm, 1994). 
 The name of the class, ARMOPHOREA , is 
derived from the subordinal name originally pro-
posed by Jankowski (1964a, 1964b) to include 
only the caenomorphid heterotrichs, which he 
argued derived from a Metopus -like ancestor. It 
derives from the Latin arma , meaning weapons (or 
it derives from the Latin armus meaning shoulder), 
and refers to the fact that caenomorphids have the 
appearance of military helmets (or the caenomor-
phid body is twisted to give the appearance of a 
shoulder). Although not highly similar, a number 
 Chapter 8 
 Subphylum 2. 
INTRAMACRONUCLEATA: Class 2. 
ARMOPHOREA – Sapropelibionts that 
Once Were Heterotrichs 
176 8. Subphylum 2. INTRAMACRONUCLEATA: Class 2. ARMOPHOREA
of clevelandellids have conspicuous polysaccha-
ride “skeletal” elements in their cortex, an “armor” 
of a different sort (see Albaret, 1970a). 
 Like the Class SPIROTRICHEA , there is no 
conspicuous synapomorphy for members of this 
class. They are united by the following three fea-
tures. First, they are restricted to anaerobic habitats 
and are typically dependent upon methanogenic 
symbionts . Although this is not a unique feature 
for the Class ARMOPHOREA (see particularly 
Chapter 12. Class PLAGIOPYLEA ), we predict 
that the metabolic dependence on hydrogenases 
in this class will be shown to have a common 
phylogenetic origin. Second, clevelandellids and 
 armophorids share pleurotelokinetal stomatogen-
esis of the adoral polykinetids , a feature shown 
by members of other classes (Foissner & Agatha, 
1999). Finally, they show strong similarities in the 
sequences of their small subunit rRNA (SSUrRNA) 
genes (Embley et al., 1995; Hackstein, Van Hoek, 
Leunissen, & Huynen, 2001; van Hoek, van Alen, 
Sprakel, Hackstein, & Vogels, 1998). This class 
could be called the first “ riboclass ” of ciliates, 
since its monophyly is predicted by sequence 
analyses of the SSUrRNA genes. However, we do 
not yet have a signature sequence that would char-
acterize the class. 
 8.1 Taxonomic Structure 
 The two major groups – the clevelandellids and 
 armophorids – included in this class have long 
been considered heterotrichs because of their pos-
session of multiple adoral polykinetids (Fig. 8.1). 
Corliss (1979) considered them to be suborders 
within the Order Heterotrichida . However, early 
ultrastructural analysis demonstrated clear differ-
ences between the somatic and oral structures of 
 clevelandellids and their presumed “ heterotrich ” 
relatives. The somatic dikinetids do not give rise to 
 postciliodesmata , their kinetodesmal fibril is differ-
ently shaped, and there is a prominent left-directed 
striated cathetodesmal fibril arising adjacent to the 
anterior kinetosome (Paulin, 1967; de Puytorac & 
Grain, 1969, 1976). Although there is still no pub-
lished account devoted solely to the ultrastructure 
of armophorids , Schrenk and Bardele (1991) have 
indicated differences between the somatic kinetid 
of the armophorid Metopus and those of cleve-
landellids . It does appear that Metopus may have 
 cathetodesmal-like fibrils , which do not appear 
striated. Little research has been done on members 
of this class, outside the recent interest in their 
symbiotic methanogens (see below Life History 
and Ecology ). 
 We place armophorids and clevelandellids in the 
Class ARMOPHOREA primarily based on their 
strong association derived from sequence simi-
larities of the SSUrRNA gene: the clevelandellids 
Nyctotherus and Nyctotheroides strongly group 
with the armophorids Metopus and Caenomorpha
(Embley et al., 1995; van Hoek et al., 1998). Both 
Jankowski (1968b) and Albaret (1975) have sug-
gested that clevelandellids may have derived from 
metopids through transformation of the cortical 
patterning, following a suggestion by Villeneuve-
Brachon (1940). Therefore, we place these two 
groups together and elevate them to ordinal status, 
as others have done (Lynn & Small, 1997, 2002; 
de Puytorac, 1994a; Small & Lynn, 1985). 
Following Jankowski (1964a, 1964b, 1968b) and 
Albaret (1975), we assume that the free-living 
armophorids represent the descendants of the 
ancestral group from which the endosymbiotic cleve-
landellids evolved. 
 The Order Armophorida includes two fami-
lies: the Family Metopidae and the Family 
 Caenomorphidae (Fig. 8.1). In most forms, there 
is a slight twist left to the anterior end of the 
body, which is covered by up to five perizonal or 
epistomial kineties (e.g., Fernández-Galiano & 
Fernández-Leborans, 1980; Jankowski, 1968b). 
This twist becomes pronounced in derived forms 
and in all caenomorphids (Fig. 8.1). Caenomorphids 
are not typically holotrichous, but rather may have 
the somatic ciliation restricted to anterior and pos-
terior cirrus-like tufts. 
 The Order Clevelandellida has not changed in 
composition since Corliss (1979). It contains five 
families: the Family Nyctotheridae , the Family 
 Sicuophoridae , the Family Clevelandellidae , 
the Family Inferostomatidae , and the Family 
 Nathellidae . The latter two families are mono-
typic. Clevelandellids are densely ciliated, often 
laterally compressed ciliates with manyleft serial
oral polykinetids that are hidden in a groove-like 
peristome and deep oral cavity or infundibulum 
(Fig. 8.1). These obligate endosymbionts are com-
mensal in a wide range of hosts: Nyctotherus is 
Fig. 8.1. Stylized drawings of representative genera from the two orders in the Class ARMOPHOREA . Order 
 Armophorida : the metopids Bothrostoma and Metopus , and the caenomorphid Caenomorpha . Order Clevelandellida : 
Nyctotherus and Clevelandella
8.1 Taxonomic Structure 177
178 8. Subphylum 2. INTRAMACRONUCLEATA: Class 2. ARMOPHOREA
found in oligochaetes , insects , and myriapods ; 
Nyctotheroides is found in frogs and toads ; and 
Clevelandella is found in wood roaches and ter-
mites (see Life History and Ecology ). 
 Systematic research on members of this group 
has been done by literally a handful of investigators,
following monographic work on the armophorids 
and caenomorphids by Jankowski (1964a, 1964b) 
and on the clevelandellids by Albaret and coworkers 
(Albaret, 1975; Albaret & Njiné, 1976). Exploration 
of the biodiversity of clevelandellid symbionts of 
 African anurans has been expanded considerably 
by Affa’a (1980, 1983, 1988b) while Affa’a (1989) 
and Grim (1992) have described new genera sym-
biotic in fishes (see also Earl, 1991). 
 Esteban, Fenchel, and Finlay (1995) have taken 
a conservative approach in their revision of 
Metopus , reducing 76 nominal species to 22 mor-
phospecies. It will be up to molecular systematists 
to determine if these morphospecies are really as 
phenotypically variable as presumed by Esteban 
et al. (1995). 
 8.2 Life History and Ecology 
 Armophoreans, like most ciliates, are globally dis-
tributed. A novel technical approach to their study 
used electromigration to extract these often sedi-
ment-dwelling ciliates from their habitats (Wagener, 
Stumm, & Vogels, 1986). Free-living armo phorids 
have been found in freshwater and marine habitats in 
 Eurasia (e.g., Agamaliev, 1974; Finlay & Maberly, 
2000; Grolière & Njiné, 1973; Guhl, Finlay, & 
Schink, 1996; Madoni & Sartore, 2003) and North 
America (Bamforth, 1963; Borror, 1963), and chloride 
lakes (Madoni, 1990). In these habitats, they are part 
of the sulfureta community, which may also include 
ciliates from the Classes HETEROTRICHEA , 
 PLAGIOPYLEA , and OLIGO HYMENOPHOREA 
(Dyer, 1989; Fenchel, 1987). Foissner (1987, 1995b) 
recorded metopids from temperate and tropical soils 
in which they survive by encystment. Encystment 
is crucial to the transmission between hosts of the 
clevelandellids, all of which are endosymbionts 
in both terrestrial and aquatic metazoans. These 
ciliates have been recorded from diverse hosts: 
 insects (Hackstein & Stumm, 1994; Lalpotu, 1980a, 
1980b; Zeliff, 1933), millipedes (Albaret, 1970b; 
Hackstein & Stumm; Lalpotu, 1980c), molluscs (Laval 
& Tuffrau, 1973), sea urchins (Biggar & Wenrich, 
1932; Grolière, de Puytorac, & Grain, 1980b), fishes 
(Grim, 1998; Grim, Clements, & Byfield, 2002; Grim, 
Reed, & Fishelson, 1995/1996; Jankowski, 1974a), 
 amphibians (Albaret, 1975; Affa’a et al., 1995; 
Wilbert & Schmeier, 1982), and reptiles (Geiman & 
Wichterman, 1937; Takahashi & Imai, 1989). 
 Free-living armophorids are restricted to anoxic 
or microaerobic habitats, such as the anoxic hypo-
limnion in lakes and bays or the anoxic layers in
sediments. The armophorids Caenomorpha and 
Metopus can reach abundances of more than 
5,000 l −1 in the water column, but are typically 
much less abundant than this (Fenchel & Finlay, 
1991a; Fenchel, Kristensen, & Rasmussen, 1990; 
Guhl & Finlay, 1993; Guhl et al., 1996). Armophorids
increase their relative abundance in sediments during 
periods of anoxia, reaching more than 50 ml −1 of 
sediment (Fenchel, 1993; Finlay, 1982). These
ciliates survive best at low oxygen concentrations.
They exhibit a chemosensory response to oxygen 
concentration: they increase their swimming speed 
at higher oxygen concentrations and show ciliary 
reversals when leaving anoxic conditions and enter-
ing an oxygen zone (Fenchel & Finlay, 1990a). The 
abundances of symbiotic clevelandellids depend 
partly on the host. Wilbert and Schmeier (1982) 
recorded hundreds of Nyctotheroides in some frog 
hosts while Gijzen and Barugahare (1992) recorded 
over 10 4 ml −1 Nyctotherus in the hindgut of the 
 American cockroach Periplaneta americana . 
 Armophoreans typically feed on heterotrophic 
and phototrophic purple bacteria , and typically 
grow more slowly than comparably-sized aerobic 
ciliates with generation times in the order of days 
(Fenchel & Finlay, 1990b). Metopus requires bacte-
rial abundances of more than 10 7 ml −1 for maximum 
growth (Massana, Stumm, & Pedrós-Alió, 1994). 
The abundance of Caenomorpha is correlated with 
the abundance of its photosynthetic bacterial prey, 
Thiopedia , suggesting that there is chemosensory 
tracking of prey by this ciliate predator (Guhl & 
Finlay, 1993). While Guhl and Finlay (1993) con-
cluded that Thiopedia production is controlled by 
Caenomorpha , Massana and Pedrós-Alió (1994) 
concluded in another habitat that anaerobic cili-
ates do not likely control bacterial production. The 
 growth efficiencies of anaerobic ciliates are quite 
low, less than 10%. Although these ciliates are not 
dependent upon their intracellular endosymbiotic 
methanogenic bacteria, their growth rates can, in 
some cases, be reduced if deprived of their bacteria.
Although there is yet no direct evidence, the metha-
nogens in these cases may be supplying the host
ciliate with organic excretions to enhance the growth
rate (Fenchel & Finlay, 1991b). 
 One of the first surveys of symbiotic bacteria 
was that of Fenchel, Perry, and Thane (1977) who 
reported both ectosymbiotic and endosymbiotic 
bacteria in the armophoreans Caenomorpha and 
Metopus . Endosymbiotic methanogenic bacteria 
have been reported in members of both orders 
of armophoreans (e.g., Fenchel & Finlay, 1991a; 
Gijzen & Barugahare, 1992). Many of these bacteria 
have been confirmed to be methanogens , which 
can number from hundreds to over 8,000 per ciliate
(Fenchel, 1993). They can take various shapes 
from elongate rods, up to 7 µm in length, to coccoid 
forms, about 0.5 µm in diameter. Methanogens were 
identified first on the basis of a characteristic, fluo-
rescent, deazaflavin coenzyme F 420 (van Bruggen 
et al., 1983). Van Bruggen, Zwart, van Assema, 
Stumm, and Vogels (1984) and Van Bruggen 
et al. (1986) were first to isolate and characterize 
the methanogens to the genera Methanobacterium
and Methanoplanus . Use of the polymerase chain 
reaction has increased the diversity of methano-
gens to include potentially other genera, such as 
Methanolobus and Methanocorpusculum (Embley 
& Finlay, 1994). In both free-living and symbiotic 
 armophoreans , unrelated ciliates may contain the 
same methanogen species while the same ciliate 
species may at different times or in different hosts 
carry different methanogen species. This demon-
strates that losses and acquisitions of methanogens 
are continually occurring and some may be quite 
recent acquisitions (Embley & Finlay, 1993; van 
Hoek et al., 2000b). We do not yet know how the 
association is established since the bacteria lie in 
the cytoplasm not surrounded by a cell membrane. 
 Methanogen symbiosis has attracted recent inter-
est because methane is a greenhouse gas. Thus, 
ciliates could potentially contribute indirectly to 
 greenhouse gases by “growing their own meth-
ane producers.” Indeed, significant amounts of 
 methane production have been attributed to these 
ciliate endosymbionts. Up to 95% of the methane 
production in certain marine habitats has been 
attributed to the ciliates (Fenchel, 1993), but in 
other habitats methanogenesis derived from ciliate 
endosymbionts is a transient and minor contribu-
tion (Schwarz & Frenzel,2005). In contrast, over 
80% of the methane produced by the American 
cockroach can be attributed to ciliates (Gijzen & 
Barugahare, 1992). In other anaerobic habitats, 
stimulation of bacterial production by ciliate graz-
ing can enhance methane production , here not 
by endosymbiotic bacteria , but by the free-living 
 methanogens . Organic acids, such as acetate and 
propionate, excreted by the ciliates may stimulate 
bacterial growth (Biagini, Finlay, & Lloyd, 1998). 
 Research on the endosymbiotic armophoreans , 
the clevelandellids , has primarily focussed on the 
symbionts of frogs and insects . The amphibians
of Cameroon have provided a rich resource to 
probe the biology of the clevelandellids . Frog’s 
eggs are not infected and frog’s with a direct life 
cycle were never found to carry ciliates. The small 
 frog Phrynodon sandersoni provides a “natural 
experiment” to confirm these facts. Its tadpoles
develop without a digestive tract; of course, the 
 tadpoles are uninfected and so are the adults 
(Amiet & Affa’a, 1985). Affa’a and coworkers 
(Affa’a, 1988a; Affa’a & Amiet, 1985, 1994; 
Amiet & Affa’a, 1985) have concluded that there 
are three general life histories of infaunation. First, 
the ciliates may be found only in the juvenile or 
 tadpole stages of the host: this applies to such species 
as Nyctotheroides brachystomus , Neonyctotherus
reticulatus , and Parasicuophora aberrans . Second, 
other species, such as Nyctotheroides heteros-
tomus and Prosicuophora basoglui , infaunate only 
the adult stage. Finally, both tadpole and adult 
stages are infaunated by other species, such as 
Nyctotheroides teochii . 
 We do not know what factors control the dis-
appearance of ciliates from the tadpole or the 
appearance of ciliates in the adults. Affa’a (1986b) 
has shown that gonadotropins induce encystment 
in Prosicuophora and Nyctotheroides . It may be 
that the changes at metamorphosis of the tadpoles 
induce encystment in those forms that occur only in 
the tadpole and induce excystment in those forms 
that occur only in the adult. Ingestion of cysts is 
probably the main mode of transmission, although 
infection by live ciliates may occur since the feces 
of adult frogs have an abundance of ciliates (Amiet 
& Affa’a, 1985). The prevalence of a ciliate species 
in a frog host varies from one locality to another, 
although it is not yet clear what factors determine 
8.2 Life History and Ecology 179
180 8. Subphylum 2. INTRAMACRONUCLEATA: Class 2. ARMOPHOREA
this variability (Affa’a, 1986a, 1988a). Geographic 
variation has also been reported for Nyctotherus
species that infect cockroaches : similar ciliate 
genotypes can occur in different insect genera 
at the same or distant localities (van Hoek et al., 
1998). The ciliates apparently have no effect on 
the amphibian hosts. However, those resident in 
 cockroaches may significantly increase the growth 
rate and body weight of their hosts (Gijzen & 
Barugahare, 1992). 
 Reid and John (1983) characterized the cysts of 
the clevelandellids as flask-shaped, noting similarities
to those of the heterotrichs (see also Esteban et al., 
1995; Takahashi & Imai, 1989). Cysts are crucial 
to the maintenance of the life histories of the endo-
symbiotic clevelandellids and must certainly be 
important for those armophorids , such as Metopus , 
which are found in soils. How widely cyst-forming 
is distributed in other members of the class remains 
to be determined. 
 8.3 Somatic Structures 
 Armophorean ciliates are quite variable in shape 
and size. Clevelandellids are intermediate in size 
at around 100 µm; armophorids can range up to 
300 µm in length. Shapes are also quite variable. 
 Armophorids , especially caenomorphids , have a 
rigid, armor-like pellicle with processes and spines , 
but larger metopids can be quite flexible. The 
 armophorid body is developed into an anterior lobe 
that can become quite twisted, and along which 
travel the perizonal or frontal kineties (Fig. 8.1). 
Smaller forms may have somatic ciliature reduced 
to anterior and posterior cirrus-like tufts. 
 On the other hand, clevelandellids are very 
densely ciliated with closely packed somatic kine-
ties. These somatic kineties converge on each other 
forming what are called sutures or secant systems 
(Fig. 8.1). In clevelandellids , these are typically 
preoral, apical, caudal, and postoral; the length and 
precise positions of these secant systems is used in 
distinguishing genera (e.g., Affa’a, 1983; Albaret 
& Njiné, 1976; Earl, 1991; Grim, 1998). 
 The cell membrane is underlain by an alveolar 
layer that may be conspicuous in some caenomor-
phids (Fenchel et al., 1977), but it is apparently very 
compressed, or perhaps even absent, in metopids 
(Fenchel & Finlay, 1991a) and clevelandellids 
(de Puytorac & Grain, 1969). 
 Somatic kinetids are dikinetids throughout the 
class. However, as with the Class SPIROTRICHEA , 
there is considerable diversity in kinetid structure 
within the Class ARMOPHOREA . Unfortunately, 
much of this research remains to be published, 
appearing only in abstract form or as schematic 
drawings without micrographic support (Tuffrau 
& de Puytorac, 1994). We will rely on these but 
caution that detailed descriptions need to be pub-
lished to corroborate the drawings (Fig. 8.2). The 
armophorid somatic dikinetid is characterized as 
follows: a ciliated anterior kinetosome with a tan-
gential transverse ribbon at triplets 3, 4, 5 and a cili-
ated posterior kinetosome with a well-developed 
 divergent postciliary ribbon and a laterally-directed 
 kinetodesmal fibril at triplets 5, 6, 7 that may not be 
striated (Schrenk & Bardele, 1991). Other micro-
tubules have been reported to accompany the ante-
rior transverse ribbon near triplets 5 or 6 while a 
pair of presumably transverse microtubules is situ-
ated between the two kinetosomes opposite triplet 
4 of the posterior kinetosome (Da Silva Neto in de 
Puytorac & Tuffrau, 1994; Esteban et al., 1995) 
(Fig. 8.2). Foissner and Agatha (1999) observed 
by silver-staining what might be well-developed 
 cathetodesmal fibrils in several Metopus species. 
The postciliary microtubular ribbons extend along-
side each other in the cortical ridges (Fig. 8.3). 
 Paulin (1967) and de Puytorac and Grain (1969) 
provided the first evidence of the clevelandellid 
 somatic dikinetid of Nyctotherus and Sicuophora , 
respectively. Grim (1998) has provided some 
information on the dikinetid of the clevelandellid 
Paracichlidotherus . The clevelandellid dikinetid 
can now be characterized as follows: a ciliated 
anterior kinetosome that bears a tangential trans-
verse ribbon at triplets 4, 5 and a striated cathe-
todesmal fibril extending to the lateral left from 
an origin near triplet 2; and a ciliated posterior 
kinetosome with a divergent postciliary ribbon and 
a kinetodesmal fibril homologue at triplets 5, 6 
(Fig. 8.2). Grim reported two transverse microtu-
bules associated with the posterior kinetosome of 
Paracichlidotherus . The striated cathetodesmal fibrils 
of clevelandellids may be bifurcated (Fernández-
Galiano, 1986; de Puytorac & Grain; de Puytorac
& Oktem, 1967). De Puytorac and Grain (1969) 
illustrated the cathetodesmal fibril of Sicuophora
as having two origins, one as indicated above on 
the anterior kinetosome and the other on the poste-
rior kinetosome near the base of the kinetodesmal 
Fig. 8.2. Schematics of the somatic kinetids of representatives of the Class ARMOPHOREA . ( a ) Dikinetid of 
Metopus . ( b ) Dikinetid of Paracichlidotherus . ( c ) Dikinetid of Nyctotherus . ( d ) Dikinetid of Sicuophora (from Lynn, 
1981, 1991)
8.4 Oral Structures 181
fibril homologue. No micrographic evidence is 
presented for this interpretation so we have revised 
our drawing accordingly (Fig. 8.2). 
 We need to havesome detailed reinvestigations 
of armophoreans before any generalizations can be 
made about their somatic dikinetids . A further intrigu-
ing physiological observation is that Nyctotherus
ovalis switches swimming direction in response 
to voltage changes rather than showing a ciliary 
reversal. Moreover, this behavior appears to be 
influenced by host-dependent factors (van Hoek 
et al., 1999). 
 Contractile vacuoles are present in armopho-
reans . The cytoproct is often conspicuous, and in 
 clevelandellids may open to the outside by a cilia-
lined channel. 
 Mucocysts appear to be present in the cortex 
of clevelandellids (Paulin, 1967; de Puytorac & 
Grain, 1969) and armophorids (Esteban et al., 
1995). 
 Finally, mention must be made of the apparent 
absence of mitochondria with tubular cristae in all 
 armophoreans . The mitochondria in these ciliates 
have evolved into hydrogenosomes (van Hoek, 
Akhmanova, Huynen, & Hackstein, 2000a; Boxma 
et al., 2005). These hydrogenosomes have a hydro-
genase that uses electrons derived from pyruvate 
oxidation to reduce protons and generate hydrogen 
(Fenchel & Finlay, 1991a; Müller, 1993; Voncken 
et al., 2002). The hydrogen is typically used in 
 armophoreans by endosymbiotic methanogens (see 
Life History and Ecology ). 
 8.4 Oral Structures 
 The armophoreans were placed until recently 
with the heterotrichs because of their holotrichous 
somatic ciliation and the presence of multiple oral 
polykinetids forming an adoral zone . The two or 
182 8. Subphylum 2. INTRAMACRONUCLEATA: Class 2. ARMOPHOREA
three rows of kinetosomes of the oral polykinetids 
are hexagonally packed. In armophorids , a third or 
fourth row of kinetosomes is added continuing the 
hexagonal packing (Esteban et al., 1995; Foissner & 
Agatha, 1999). Armophorid oral polykinetids have 
been called paramembranelles . Clevelandellids 
typically have three rows of kinetosomes hexago-
nally packed, but a fourth, shorter row lies directly 
opposite to, rather than hexagonally packed with, 
the kinetosomes of the third row, leading to their 
designation as heteromembranelles because of 
the different packing of these kinetosomes of the 
fourth row (de Puytorac & Grain, 1976). This dif-
ferent packing leads to a different orientation and 
beating of the cilia that was nicely revealed in some 
published micrographs (Paulin, 1967; Takahashi & 
Imai, 1989). 
 The adoral zones of armophorids and clevelan-
dellids may be quite extensive, spiralling around 
the body one or more times in some armophorids 
Fig. 8.3. Somatic cortex of Metopus whose postciliary ribbons extend alongside each other into the cortical ridges. 
This schema was constructed based on the brief descriptions provided in reports by Schrenk and Bardele (1991) and 
Esteban et al. (1995)
(Fig. 8.1). The clevelandellids have a deeper 
oral cavity called an infundibulum where the 
 heteromembranelles typically occur (Tuffrau & 
de Puytorac, 1994). Postciliary ribbons are associ-
ated with the kinetosomes of the posterior row in 
both armophorids and clevelandellids (Tuffrau & 
de Puytorac). 
 Paroral structures are quite variable in the class. 
 Armophorids appear to have a single file of cilia, 
which may be derived from linearly arranged 
oral dikinetids (Esteban et al., 1995; Foissner & 
Agatha, 1999; Sola, Serrano, Guinea, & Longás, 
1992). Clevelandellids have a paroral with two 
sets of cilia deriving from two files of kinetosomes 
separated by a ridge (Grim, 1998; Paulin, 1967; 
de Puytorac & Grain, 1969; Takahashi & Imai, 
1989), termed a diplostichomonad by de Puytorac 
and Grain (1976). The oral structures of armopho-
reans are underlain by complex fibrillar structures 
and microtubules. The filamentous components 
are implicated in the movement of vesicles to the 
food vacuole forming region (Eichenlaub-Ritter & 
Ruthmann, 1983). 
 8.5 Division and Morphogenesis 
 There have been only a few papers on cell divi-
sion and division morphogenesis of armophoreans 
since Wichterman (1936) described division in 
Nyctotheroides (= Nyctotherus ). He observed the 
oral primordium to develop subequatorially. Since 
silver-staining was not used, kinetosomal replica-
tion was not detailed. As far as we know, armo-
phoreans divide while swimming freely. Foissner 
(1996b) has characterized stomatogenesis as pleu-
rotelokinetal (i.e., occurring within or at the end of 
several somatic kineties). 
 Two studies on the armophorids , Metopus and 
Caenomorpha , demonstrated pleurotelokinetal 
stomatogenesis . Martín-González, Serrano, and 
Fernández-Galiano (1987) showed that the oral 
primordium in Caenomorpha develops by prolif-
eration from the posterior ends of many perizonal 
somatic kineties . The primordial field splits later 
in development with an anterior portion devel-
oping into the paroral and the posterior portion 
developing into the oral polykinetids . In Metopus , 
a number of posterior dorsolateral somatic kineties
begin to proliferate kinetosomes (Foissner & 
Agatha, 1999). These differentiate as the oral 
polykinetids (Fig. 8.4). The paroral differenti-
ates later. Foissner and Agatha (1999) interpreted 
it to develop from kinetosomes derived from 
 perizonal kineties . However, it is just as pos-
sible from the evidence presented that paroral 
dikinetids could derive from “anterior” or “right-
side” kinetosomes in a fashion very similar 
to that reported for Caenomorpha . If this were 
the case, there would be strong similarities in 
Fig. 8.4. Division morphogenesis of Metopus , a representative of the Class ARMOPHOREA . ( a ) Kinetosomal 
replication begins at the “equatorial ends” of a number of somatic kineties. ( b ) Oral polykinetids assemble through 
side-by-side alignment of dikinetids units. ( c ) The posterior ends of several somatic kineties adjacent to the develop-
ing oral region disassemble, and it may be that the paroral ( d, e ) is assembled from these as division proceeds. (from 
Foissner & Agatha, 1999.)
8.5 Division and Morphogenesis 183
184 8. Subphylum 2. INTRAMACRONUCLEATA: Class 2. ARMOPHOREA
 stomato genesis between these two genera. 
Caenomorpha undergoes a complicated post-sto-
matogenesis morphogenesis, reminiscent of the 
 enantiotropic division of some oligotrichous spiro-
trichs (Martín-González et al., 1987). Considering 
the current evidence, we are not convinced that the 
differences between metopids and caenomorphids 
are sufficient to justify ordinal status for these 
two groups, as suggested by Foissner and Agatha 
(1999).
 Santos, Guinea, and Fernández-Galiano (1986) 
have provided a preliminary account of stoma-
togenesis in the clevelandellid Nyctotherus . Breaks 
occur in somatic kineties posterior to the oral region 
and kinetosomal proliferation occurs at the anterior 
ends of these breaks. A lateral groove develops as 
proliferation proceeds and primordium elements 
on the posterior wall of the groove differentiate as 
oral polykinetids while those on the anterior wall 
develop as paroral dikinetids, eventually forming 
the two files of the diplostichomonad (Santos et al., 
1986). This is clearly a pleurotelokinetal stomato-
genesis , showing significant similarities to that of 
the armophorids. 
 8.6 Nuclei, Sexuality 
and Life Cycle 
 Armophoreans have the typical complement of 
 macronucleus and one or more micronuclei . The 
macronuclei can also be variable in number in 
 caenomorphids , sometimes numbering more than 
four (Fig. 8.1). In smaller forms, the macronucleus 
is typically globular to ellipsoid, but in larger 
 clevelandellids it can become elongated and quite 
irregular in shape. The macronucleus of some 
 clevelandellids is “suspended” from the cortex by 
microfibrillar strands that collectively are called 
the karyophore (Fig. 8.1). 
 Eichenlaub-Ritter and collaborators have under-
taken some detailedultrastructural studies on 
micronuclear and macronuclear division in the 
 clevelandellid Nyctotherus cordiformis . Macronuclei 
divide by intramacronuclear microtubules that are 
primarily responsible for the elongation of the 
macronucleus, which is also accompanied on its out-
side by scattered extramacronuclear microtubules 
(Eichenlaub-Ritter & Tucker, 1984; Hamelmann, 
Eichenlaub-Ritter, & Ruthmann, 1986). Micronuclear 
mitosis is an endomitosis, typical of ciliates (Raikov, 
1982). There may be three “classes” of micro tubules, 
identified by their differing responses to drugs and 
temperature, which function to accomplish micronu-
clear mitosis : (1) manchette microtubules underlying 
the nuclear envelope; (2) interpolar and kinetochore 
microtubules, which function during anaphase; and 
(3) stembody microtubules, which function during 
telophase to separate the putative micronuclei to 
each progeny cell (Eichenlaub-Ritter & Ruthmann, 
1982a, 1982b). Microtubules in the dividing nuclei 
may have more than the canonical 13-protofila-
ments (Eichenlaub-Ritter, 1985; Eichenlaub-Ritter 
& Tucker). 
 Conjugation has been studied in only a few 
examples of armophoreans since the description of 
it in Nyctotheroides (= Nyctotherus ) by Wichterman 
(1936). It is not established what factors stimulate 
conjugation in free-living forms. Wichterman (1936) 
observed it occurring only in transforming tadpoles 
of the frog Hyla versicolor . This lead to specula-
tion that gonadotropins or some other physiological 
signal derived from the host may cue these ciliates 
to begin conjugation. However, Sandon (1941a) 
observed conjugation in Paranyctotherus isolated 
from the adult clawed frog Xenopus laevis , sug-
gesting that other factors are involved. Affa’a and 
Amiet (1994) have confirmed that conjugation can 
occur in all stages of the frog life cycle – tadpoles , 
transforming individuals, and adults. Gonadotropin 
injections induced conjugation in Prosicuophora , 
even when immature stages were treated (Affa’a, 
1986b). Thus, it is unlikely that one single factor 
stimulates conjugation . 
 Fusion of the conjugants occurs in the anterior 
region, and in some Metopus species total conjuga-
tion may occur (Noland, 1927). The micronuclei 
of each partner typically undergo three maturation 
divisions – two meiotic divisions followed by a 
mitosis of one of the four haploid products (Raikov, 
1972; Martín-González et al., 1987). In the total 
 conjugation of Metopus , the cytoplasm of one con-
jugant flows into the partner carrying the gametic 
nucleus or nuclei with it. However, the old macro-
nucleus is left in the cortical shell of the disgarded 
partner (Noland, 1927). Following fusion of the 
gametic nuclei to form the synkaryon , armopho-
reans typically have one post-synkaryon division 
with one nucleus becoming the new micronucleus 
and the other becoming the new macronucleus. In 
species with more than one macronucleus, there 
may be additional post-synkaryon divisions (see 
Martín-González et al., 1987). 
 Development of the macronuclear anlage in 
 armophoreans is an extremely long process: 
Golikova (1965) recorded it taking up to 2 weeks in 
Nyctotheroides (= Nyctotherus ) while Noland (1927) 
observed a mininimum of 1 week in Metopus . In 
both these genera, it appears that polytene chromo-
somes are formed at one stage during anlage devel-
opment. Golikova (1965) concluded that one giant 
 polytene chromosome may form in Nyctotheroides
by the end-to-end joining of the individual chromo-
somes. This giant chromosome later fragments both 
transversely and longitudinally to yield the macro-
nuclear chromosomes (Vinnikova & Golikova, 
1978). Ultimately, the macronuclear chromosomes 
fragment into gene-sized pieces as happens in the 
Class SPIROTRICHEA (see Chapter 7 ), a fact 
that Riley and Katz (2001) have confirmed by 
molecular analyses of the macronuclear DNA of 
both armophorids and clevelandellids. 
 8.7 Other Features 
 The free-living armophorids have been recog-
nized for some time as strong indicators of anoxic 
aquatic environments (e.g., Bick, 1972; Foissner, 
1988a; Sládecˇek, 1973). They are commonly found 
in soils (Foissner, 1987) and have been recorded 
from a variety of municipal landfill sites in the 
United Kingdom, where they undergo an encyst-
ment - excystment cycle in response to starvation 
and water loss (Finlay & Fenchel, 1991). 
8.7 Other Features 185