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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. The first published records of ciliates collected in the Gulf of Mexico are 3 papers on species from the coast of Louisiana by J. C. Smith (1898, 1900, 1904). Two papers by Jacobs (1912, 1914) on endosymbionts of sea urchins in the Dry Tortugas appeared a decade later. More than a decade and a half elapsed before the next papers on the ciliate fauna appeared. The 1930s and 1940s were a period of relatively vigorous investigation of the ciliates of the Gulf, almost all of it originating from the Tortugas Laboratory of the Carnegie Institution at the western tip of the Florida Keys. The Tortugas papers comprise 2 distinct groups: those on free-living ciliates (Bullington 1931–1940, Pearse 1932a, b) and those on endosymbionts of sea urchins, annelids, and corals (Powers 1933, 1935, Wichterman 1939, 1940, 1942a, b). A notable exception was Noland’s classic paper of 1937 on free-living ciliates 57 4 Ciliated Protists (Ciliophora) of the Gulf of Mexico D. Wayne Coats and John C. Clamp  Ciliophora. After Pratt 1916. 58 ~ Ciliated Protists (Ciliophora) 1974, Welch 1977, Overstreet 1978, Turner, Postek, and Collard 1979, Clamp 1989, Landers, Zimlich, and Coate 1999). Three of these papers (Phillips 1973, Welch 1977, Landers, Zimlich, and Coate 1999) stand out by virtue of the fact that they deal with apostome ciliates, specialized ectocommensals of crustaceans that are, in general, poorly known. Studies of the strictly planktonic ciliates that inhabit open waters of the Gulf of Mexico are largely limited to taxonomic listings of loricate tintinnid species retained by fine-mesh plankton nets. Among these, the works of Balech (1967a,b, 1968), Cosper (1972), Calderón-Aragón and López-Ochoterena (1973), and Aladro-Lubel (1974) are the most comprehensive, providing extensive coverage of coastal areas, particularly the northeastern Gulf, and reasonable detail of oceanic communities. In addition , Balech (1972) reviewed published data for several oceanographic regions, including the Gulf of Mexico, to examine vertical zonation of tintinnids and their potential use as indicators of upwelling. Mention of planktonic ciliates from the Gulf also can be found in reports on red tides (e.g., Lackey and Hynes 1955) and diets of larval fishes and zooplankton (Govoni, Hoss, and Chester 1983, Stoecker and Govoni 1984, Govoni and Chester 1990, Larson 1991). The systematics of many taxa in the phylum Ciliophora are currently in a state of flux. This is mainly the result of a flood of new information relative to taxonomic placement , especially from modern techniques such as electron microscopy and gene sequencing. This is reflected in the necessity of designating several of the species listed in the following checklist as incertae sedis. The most up-to-date and accepted classification of ciliates is the one presented in Lynn and Small (2000), and we have used it to structure our checklist. There is also a useful website (http://www .uoguelph.ca/~ciliates/) that includes some problematic taxa not treated in Lynn and Small (2000). We used it to establish the...

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