141 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, Stichotrichia 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. INTRAMACRONUCLEATA: Class 1. SPIROTRICHEA – Ubiquitous and Morphologically Complex 142 7. Subphylum 2. INTRAMACRONUCLEATA: Class 1. SPIROTRICHEA 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, 1987)! 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