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Abstract The Class SPIROTRICHEA is an extremely 
diverse assemblage that, except for protocruziids and 
phacodiniids, is characterized by having replication 
bands pass through the macronucleus during the 
DNA synthesis phase. The name of the group refers 
to the prominent adoral zone of paramembranelles 
(AZM) that spirals out over the anterior end, some-
times completely enclosing it. The class is divided 
into seven subclasses. These ciliates are found in al-
most every microhabitat that ciliates can be encoun-
tered, and there are even symbiotic forms. Spiro-
trichs feed on a diversity of prey, from bacteria to 
other ciliates. They locomote typically using somatic 
polykinetids, called cirri, although several included 
groups have simpler somatic kinetids. Division mor-
phogenesis is as diverse as the included subclasses, 
ranging from mixokinetal to hypoapokinetal. Spiro-
trichs typically have multipolar mating systems, and 
hold the phylum record for the number of mating 
types - over 100! Molecular biological research on 
the developing macronuclei of spirotrichs demon-
strated the presence of polytene chromosomes and 
the ultimate differentiation of macronuclear DNA 
characterized by gene-sized pieces. 
Keywords Protocruziidia, Phacodiniidia, Licno-
phoria, Hypotrichia, Oligotrichia, Choreotrichia, 
 The SPIROTRICHEA is a diverse assemblage of 
ciliates that are cosmopolitan, being found in almost 
any habitat where ciliates are encountered even 
from hydrothermal vent sites at over 2,000 m deep 
(Small & Gross, 1985). The class name arises from 
the characteristically spiralling nature of the adoral 
zone of membranelles or adoral zone of oral poly-
kinetids , which emerge from the oral cavity and 
spiral in a counter-clockwise direction as they wrap 
around the anterior body surface. The vast majority 
of species have sparse somatic ciliation, which may 
take the form of compound ciliary structures like 
 cirri and bristles . Those forms with a holotrichous 
ciliation and somatic dikinetids (e.g., Protocruzia ) 
are considered closer to the ancestral archetype, and 
recent molecular phylogenies bear this out (i.e., 
Bernhard et al., 2001; Hammerschmidt et al., 1996; 
Shin et al., 2000). Spirotrichs are typically medium-
sized ciliates (i.e., 100-200 µm in length), although 
exceptionally small oligotrichs can be 5 µm in length 
while exceptionally large, cannibalistic stichotrichs , 
up to 900 µm long, may differentiate in some species 
(e.g., Onychodromus , Wicklow, 1988). Spirotrichs are 
behaviorally either planktonic (i.e., oligotrichs , 
 choreotrichs ) or substrate-oriented and benthic (i.e., 
 hypotrichs , stichotrichs ). Spirotrichs are predominantly 
free-swimming although the most distinctive group 
in the class, the tintinnid choreotrichs , might be con-
sidered sessile as tintinnids secrete a lorica in which 
they reside attached while using their oral cilia 
for locomotion. Some tintinnids can attach to the 
substrate with the lorica (e.g., Foissner & Wilbert, 
1979). The loricae of more than 30 genera have been 
 fossilized , in most cases genera with no known con-
temporary species. Fossils date from the Ordovician 
period of the Paleozoic Era , 400-500 million years 
ago, up to the Pleistocene Period about 1 million 
years ago, but the greatest diversity of tintinnid 
 fossils occurred in the Mesozoic Era (Loeblich 
& Tappan, 1968; Tappan & Loeblich, 1968, 1973). 
 Chapter 7 
 Subphylum 2. 
SPIROTRICHEA – Ubiquitous 
and Morphologically Complex 
 Since Corliss’s observation (1979) that the lit-
erature on this group was select, it has exploded, 
driven by interests at two ends of the biological 
hierarchy – the molecular and the ecological. 
Stimulated both by the description using molecular 
techniques of DNA processing in the macronu-
cleus of the stichotrich Stylonychia mytilus by 
Ammermann, Steinbrück, von Berger, and Hennig 
(1974) and by mass culturing methods in both sti-
chotrichs (Laughlin, Henry, Phares, Long, & Olins, 
1983) and hypotrichs (Roth, Lin, & Prescott, 1985; 
Schmidt, 1982), the developmental genetics of 
these ciliates, particularly Oxytricha , Stylonychia , 
and Euplotes , has become a fertile area of research 
(e.g., for reviews see Gall, 1986; Klobutcher & 
Herrick, 1997; Prescott, 1994, 1998, 2000). At the 
other extreme, Kofoid and Campbell (1929, 1939) 
had demonstrated the abundance and diversity of 
tintinnids in the oceanic plankton , highlighting 
their possible ecological significance. Pomeroy’s 
(1974) vision of the importance of microbial organ-
isms in ocean food webs coupled with conception 
of the “ microbial loop ” by Azam et al. (1983) led 
to an explosion of interest in the ecology of plank-
tonic ciliates, which are dominated particularly 
by oligotrichs and choreotrichs among which the 
 tintinnids are placed. Our understanding of the 
autecology of these latter two groups, which often 
represent the bulk of abundance and biomass of 
ciliates in the plankton (e.g., for reviews see Lynn 
& Montagnes, 1991; Pierce & Turner, 1993; Sherr 
& Sherr, 1988), has also benefited from the devel-
opment of laboratory culture methods (Gifford, 
1985; Gold, 1973). 
 Finally, stichotrichs and hypotrichs have been 
of particular interest to biologists interested in the 
development of pattern in cells. Stichotrichs , like 
 heterotrichs , have considerable regenerative pow-
ers, which have also made them useful models for 
developmental biologists. The literature is rich and 
has demonstrated among other things that pattern 
is controlled at both a global or cellular level to 
position organelles and at the organellar-organellar 
complex levels to construct individual “pattern 
units” (Frankel, 1989; Grimes, 1982; Grimes, 
McKenna, Goldsmith-Spoegler, & Knaupp, 1980). 
Gates (1978b, 1988) has shown by quantitative 
analysis of ventral cirral and dorsal kinetid patterns 
that there is an underlying morphometric theme 
within the genus Euplotes . Genetic variations of 
pattern have been demonstrated in both the ventral 
(Génermont, Demar, Fryd-Versavel, Tuffrau, & 
Tuffrau, 1992) and dorsal (Heckmann & Frankel, 
1968) ciliature of Euplotes species. In the sticho-
trich Paraurostyla weissei , Jerka-Dziadosz and 
Dubielecka (1985) have demonstrated the genetic 
basis for a pattern mutant that develops multi-left 
marginal cirral files. Moreover, Jerka-Dziadosz 
(1976) has shown that numbers of oral polykinetids 
and marginal cirri are proportional to cell size in 
P. weissei . This research on pattern formation and 
variation in spirotrichs should give taxonomists 
cause to reflect on the significance of the charac-
ters chosen to distinguish species. Dramatic dif-
ferences in the number of cirral files may be due 
only to a single gene mutation (Jerka-Dziadosz & 
Dubielecka, 1985), while differences in the abso-
lute number of elements may merely be a function 
of cell size (Jerka-Dziadosz, 1976). Ultimately, the 
strong test for a new species will be demonstration 
of mating segregation. Demonstration of mating 
incompatibility involves considerable work, espe-
cially for the geneticists of stichotrichs and hypot-
richs , whose species typically have multipolar 
mating-type systems (Ammermann, 1982; Caprette 
& Gates, 1994; Dini & Luporini, 1979; Heckmann, 
1963), possibly including over 100 mating types 
in Stylonychia mytilus (Ammermann, 1982) and 
hiding cryptic species (Dini, Bracchi, & Gianní, 
 Corliss (1979) included the heterotrichs (see 
Chapter 6 ) and armophorids – clevelandellids 
– odontostomatids (see Chapter 8 ) in the Subclass 
 Spirotricha of the Class POLYHYMENOPHOREA . 
These groups are now excluded from the Class 
 SPIROTRICHEA (see below Taxonomic 
Structure ), a position supported by ultrastructural