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clearly confirmed plagiopylids and trimyemids as 
sister taxa (Baumgartner et al., 2002; Embley & 
Finlay, 1994; Lynn & Strüder-Kypke, 2002). To 
these, we can now add the odontostomatids , based 
on the SSUrRNA gene sequence of the odontosto-
matid Epalxella (Stoeck et al., 2007). We currently 
recognize two orders: the Order Plagiopylida and 
the Order Odontostomatida . 
 The Order Plagiopylida is characterized by 
the typical sandwich-like arrangement of the 
 hydrogenosome-methanogen assemblages. This 
order includes the families, Plagiopylidae , SONDERI-
IDAE , and TRIMYEMIDAE . Genera in the former 
two families typically have a striated band structure 
(see Somatic Structure ), which trimyemids lack. 
 The Order Odontostomatida was established as 
a group by Lauterborn (1908), and remains one 
of the smallest ordinal groups outside those that 
are monotypic (e.g., Licnophorida , Phacodiniida , 
 Protocruziida ). It includes three families: 
the Epalxellidae , the Mylestomatidae , and the 
 Discomorphellidae . These are typically small cili-
ates with a prominent dorsal keel and often elon-
gate, spine-like processes. The somatic ciliature 
is reduced to what are considered vestiges of 
the perizonal kineties of armophorid ciliates, to 
which odontostomatids were originally related 
(Jankowski, 1964b; Tuffrau, 1992; Tuffrau & de 
Puytorac, 1994). 
 14.2 Life History and Ecology 
 Plagiopylids and odontostomatids are key indicators 
of the ciliate sulfureta community, which also 
includes ciliates from the Classes HETERO-
(Dyer, 1989; Fenchel, 1987). They are consistently 
represented in surveys of these habitats, which 
are characterized primarily as being anoxic or 
at most with very low concentrations of oxygen. 
 Plagiopyleans have been found in freshwater habi-
tats in Europe (Finlay & Maberly, 2000; Madoni & 
Sartore, 2003; Sola, Guinea, Longás, & Fernández-
Galiano, 1988), Africa (Dragesco, 1972), and North 
America (Bamforth, 1963; Beaver & Crisman, 
1989b), and in chloride lakes (Madoni, 1990). 
They are typically restricted to the sediment layers, 
often in microhabitats with high concentrations of 
dissolved sulphide (Esteban, Finlay, & Embley, 
1993) and of mesotrophic to hypereutrophic sta-
tus (Beaver & Crisman). Plagiopyleans have 
also been observed in the coastal sediments and 
sands of marine and estuarine habitats in Eurasia 
(Agamaliev, 1974; Dragesco, 1962; Fauré-Fremiet, 
1973; Fauré-Fremiet & Tuffrau, 1955; Fenchel 
et al., 1977), North America (Borror, 1963; Dyer, 
1989; Nerad, Schaffer, Small, & Mangold, 1995), 
the Gulf of Arabia (Al-Rasheid, 1999b), and the 
 Sea of Japan (Ozaki & Yagiu, 1941). Plagiopyla
may extend its distribution into the water column 
of marine habitats when the oxycline changes 
its vertical placement as the seasons progress 
(Fenchel, Kristensen, & Rasmussen, 1990). Where 
abundances have been recorded in the water col-
umn, Plagiopyla rarely exceeds 1 ml −1 (Fenchel et 
al., 1990; Massana & Pedrós-Alió, 1994), while 
 odontostomatids can increase their relative abun-
dance in sediments during periods of anoxia, reaching 
more than 50 ml −1 of sediment (Fenchel, 1993). 
 Plagiopyleans have never been recorded from soils 
(Foissner, 1998a). 
 Plagiopyleans have also been conspicuous endo-
symbionts from hosts as diverse as sea urchins 
(Grolière, de Puytorac, & Grain, 1980b; Lynch, 
1930; Poljansky & Golikova, 1959) and the hip-
popotamus (Thurston & Grain, 1971). 
 Our understanding of feeding and growth in 
 plagiopyleans is primarily derived from research 
on Trimyema and Plagiopyla . Strains of Trimyema 
compressum have been fed over 50 strains of bacte-
ria , including both Gram-positive and Gram-negative 
species, as well as a variety of methanogens . 
This ciliate indiscriminantly ingested all bacteria 
(Schulz, Wagener, & Pfennig, 1990). However, 
its growth was limited to a smaller subset of the 
strains, although both Gram-positive and Gram-
negative strains and strains of Archaea supported 
some growth (Baumgartner et al., 2002; Schulz et al.,
1990; Yamada, Kamagata, Nakamura, Inamori, & 
Nakamura, 1994). Bacterial carbohydrates are the 
most important energy source for these anaerobic 
ciliates (Holler, Gälle, & Pfennig, 1994). Yields 
of over 9,000 ciliates ml −1 were recorded when 
T. compressum ingested a strain of Desulfovibrio 
vulgaris (Yamada et al., 1994). Threshold concen-
trations of bacteria to support growth were in the 
range of 10 7 ml −1 (Schulz et al., 1990). Plagiopyla 
nasuta has been grown on a mixed assemblage of 
natural sediment-derived bacteria and its feeding 
and growth dynamics studied by uptake of fluores-
cently labeled bacteria (FLB) (Massana, Stumm, 
& Pedrós-Alió, 1994). Plagiopyla could consume 
over 4,000 bacteria ciliate −1 h −1 but had growth 
rates that were very low compared to aerobic 
ciliates of similar size feeding at these uptake rates 
(Massana et al., 1994). This confirms the general 
view that gross growth efficiency of these anaerobic 
14.2 Life History and Ecology 271
ciliates is about 25% that of aerobes, although 
it can be increased by the presence of symbiotic 
 methanogens (Fenchel & Finlay, 1990b, 1991a, 
1991b). In the field, the realized growth rates 
of Plagiopyla are much lower. Thus, the natural 
abundances of these ciliates are unlikely to exert 
control on natural bacterial populations (Massana 
& Pedrós-Alió, 1994). 
 Plagiopyleans have conspicuous and abundant 
symbiotic bacteria associated both as ectosymbi-
onts and as endosymbionts (Fenchel et al., 1977; 
Berger & Lynn, 1984). It is now clear that these 
endosymbiotic associations have been estab-
lished repeatedly, and therefore have also been 
lost repeatedly. Wagener, Bardele, and Pfennig 
(1990) demonstrated that Methanobacterium 
formicicum could be functionally integrated into 
Trimyema cells that were endosymbiont-free. In 
natural populations, repeated losses and functional 
integration are demonstrated by the sister spe-
cies P. nasuta and Plagiopyla frontata that have 
endosymbionts related to the different methanogen 
genera Methanocorpusculum and Methanolobus , 
respectively (Embley & Finlay, 1994). As noted 
above, this symbiotic association increases the 
 growth efficiency of the ciliate. Moreover, it is 
mutualistic as it also provides a refuge for the 
 methanogens , which avoid competition with sul-
phate reducing bacteria , avoid the toxic effects of 
environmental oxygen, and have a ready supply of 
 hydrogen (Fenchel & Finlay, 1992; Müller, 1993). 
Plagiopyla frontata may have over 3,000 methano-
gens per cell. The cell division of these bacteria, 
which is synchronous with their host ciliate’s cell 
division, may be controlled somehow by the ciliate. 
Excess bacterial production is transferred to the 
ciliate host in a fashion similar to the endosymbio-
sis with Chlorella species found in other ciliates 
(Fenchel & Finlay, 1991c). 
 14.3 Somatic Structure 
 Plagiopylids are ovoid to elongate ovoid ciliates that 
may show some dorsoventral flattening (Fig. 14.1). 
 Odontostomatids are laterally-compressed ciliates 
with a rigid and often ribbed, armor-like pellicle. 
Spiny processes are often present, both posteriorly 
and anteriorly (Fig. 14.2). Trimyemids and odonto-
stomatids can be about 15 µm in length while some 
 plagiopylids can exceed 200 µm in length (Nerad 
et al., 1995; Sola, Guinea, Longás, & Fernández–
Galiano, 1989b). Trimyemids have an apparently 
helicoidally disposed and sparse somatic ciliature 
(Fig. 14.1). The helicoidal disposition is due to the 
patterning of kinetids in the up to 60 somatic kine-
ties in some species. Trimyemids may also have a 
 caudal cilium complex, which Baumgartner et al. 
(2002) interpreted