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left 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
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 , 
(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,