The ciliates represent a ubiquitous group of protists with representatives inhabiting most marine, freshwater, and terrestrial habitats (Corliss 1979). Their small size, rapid reproductive rate, and ability to form desiccation-resistant resting stages ensure easy dispersal of species and colonization of suitable habitats (Fenchel and Finlay 2004). Ciliates occur from the poles to the tropics and from alpine regions to the deep sea. They survive in extreme environments, including hot springs, hypersaline lakes, and desert settings, with many species adapted to anaerobic conditions. Free-living species can be found swimming in the water column, living within interstices of flocculent sediment or tidal sands, attached to hard or soft substrates, and creeping along soil particles or epiphytic mosses. Symbiotic and parasitic species live in association with a wide variety of hosts, including other protists, planktonic and benthic invertebrates, reptiles, fish, and mammals. Most ciliates feed on bacteria, microalgae, or other protists; however, some are photosynthetic, and others consume host tissues. Ciliates are generally viewed as playing pivotal roles in microbial food webs, as they regenerate nutrients through excretion (Caron and Goldman 1990) and transform bacterial and microalgal biomass into larger particles that are easily exploited by metazoan grazers (Azam et al. 1983, Stoecker and Capuzzo 1990, Gifford 1991). The number of ciliate species inhabiting the biosphere is uncertain, but estimates range as high as 30,000 (Foissner 1999), with about 7,200 species being formally described (Corliss 1979). Of these, a relatively small percentage has been reported from the Gulf of Mexico.
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