Ciliophora
Introduction

This chapter concerns the aloricate or "naked" ciliates (for the purpose of this chapter, non-tintinnid ciliates), rather fragile organisms occurring virtually everywhere: in salt and fresh water (pelagic, benthic, interstitial), soil, tree bark, snow, ice brine, sewage treatment plants, anoxic environments, etc. Several species are ecto- or endocommensals but only a few are true parasites. Some 8000 extinct and extant species are currently known, but many more still await discovery (Lynn and Corliss, 1991).

The Ciliophora, although a highly differentiated and diverse assemblage, are considered a monophyletic taxon. With the exception of the small primitive groups primociliatids (without nuclear dualism) and karyorelictids (with a diploid, nondividing macronucleus), ciliates are characterized by having two kinds of nuclei: a micro- and a macronucleus. They also have few to many cilia or compound ciliary organelles (at least in some stages of their life cycle), a complex infraciliature composed of single, paired, or multiple basal bodies with closely associated microtubules and microfibrils. And they conjugate, which is a sexual phenomenon. Some ciliates are mouthless (e.g., astomes and Myrionecta rubra) or polystomic (suctorians), but the vast majority have a single feeding apparatus. Most ciliates possess pellicular alveoli (unit membrane-bound sacs beneath the plasma membrane) and tubular mitochondrial cristae (Lynn and Corliss, 1991).

Most ciliates are naked, but several groups and species form a more or less robust lorica (e.g., Tintinnoinea). Aloricate ciliates comprise several size classes ranging from about 10 µm to 4.5 mm, but stalked forms and colonies may become distinctly larger (Corliss, 1979). They are trophically diverse, most being heterotrophs (bacterivores, herbivores, carnivores, detritivores) consuming a wide variety of prey (e.g., bacteria, cyanobacteria, coccolithophorids, algae, fungi, flagellates, ciliates, small metazoans, organic particles, and dissolved organic matter; Corliss, 1979). Some, especially planktonic oligotrichs, are mixotrophs, retaining functional chloroplasts from their prey and deriving nutritive benefits from them (Laval-Peuto and Rassoulzadegan, 1988; Stoecker et al., 1989a; Sanders, 1991). At least one species, Myrionecta rubra, is apparently exclusively autotrophic depending on a cryptophycean endosymbiont (Taylor et al., 1971; Lindholm, 1985; Crawford, 1989).

Reproduction of ciliates is typically by asexual binary fission. Many ciliates divide homeotropically (homothetogenically), i.e. the fission plane is transverse to the main body axis, but oligotrichs divide enantiotropically, i.e. both daughters (proter, opisthe) are connected at their posterior ends (Fauré-Fremiet, 1953; Petz and Foissner, 1992). Sessile and sedentary species (e.g., chonotrichs, suctorians, peritrichs, folliculinids) often reproduce by budding, a kind of unequal binary fission. The smaller bud (larva) looks very different from the parental cell and is usually a dispersal form (Corliss, 1979; Small and Lynn, 1985). Some species (e.g., colpodids, the freshwater fish parasite Ichthyophthirius) divide into more than two offspring (palintomy, polytomy), which usually takes place in a reproductive cyst. Multiple budding occurs in some suctorians and chonotrichs (Jankowski, 1973; Lynn and Corliss, 1991). During conjugation two specimens of presumably differing mating types fuse partially, their nuclei also merging. After separation of the organisms, both exconjugants divide.

Ciliates have been investigated ever since Antoni van Leeuwenhoek first discovered free-living forms with his microscope (actually a hand lens) in 1674. Many new species were described about 100 years later by O. F. Müller, and in the early 19th century e.g., by M. Bory de St. Vincent, C. G. Ehrenberg and F. Dujardin. In the late second half of that century, O. Bütschli (1887-1889) established the first comprehensive ciliate classification. In the early 20th century, A. Kahl produced a comprehensive guide to genera and species of almost every major ciliate group, a work still highly valued today. Nowadays, interest in the often neglected ciliates is increasing again since it is becoming clear that they play an important role in the ecosystem. They can be valuable bioindicators used, for instance, in assessing the quality of running waters. Ciliates are also used as ideal model organisms in cell biology and are popular study objects in molecular biology and biochemistry.

(Ci. table 1)

Because of their fragility and lack of a permanent skeleton, naked ciliates are under-represented in marine plankton samples and were thus largely neglected in the past. However, they make up the bulk of the ciliate community in the pelagic realm and can be up to 50 times more abundant than the tintinnids (e.g., Smetacek, 1981; Tumantseva and Kopylov, 1985; Dale, 1987; Pierce and Turner, 1992) reaching densities as high as 1.36 x 106 ind./l (Dale and Dahl, 1987a; Table 1). Recent investigations show that aloricate ciliates occur in high numbers and consume significant quantities of autotrophic and heterotrophic microbial production. They are eaten in turn by other protozoans, many metazoans (especially copepods), and fish larvae. Thus, they are an important link between lower trophic levels and the larger zooplankton consumers (Fig. Ci 1 ; e.g., Pomeroy, 1974; Taylor, 1982; Azam et al., 1983; Porter et al., 1985; Jonsson, 1986; Sherr et al., 1986; Albright et al., 1987; Fenchel, 1987, 1988; Sherr and Sherr, 1987, 1988; Rassoulzadegan et al., 1988; Stoecker et al., 1989b; Bernard and Rassoulzadegan, 1990; Lynn and Montagnes, 1991; Pierce and Turner, 1992; Lonsdale et al., 1996). But as species identification is sometimes difficult and time-consuming (e.g., because of silver impregnation), ecological investigations were often restricted to size-classes or even communities. Species composition and distribution are still not well-researched.