A new genus and species combination are proposed for Urocryptum tortum n. gen., n. comb., a scuticociliate with a polymorphic life cycle. This marine ciliate was isolated from a sample taken at Gokasho Bay in Mie Prefecture (Japan). Specimens from different phases of the growth cycle were examined in vivo and with two silver staining techniques. Three life-history stages were observed: an exponential growth phase stage (trophont), a stationary phase stage (tomite), and finally a resting stage (cyst). The exponential growth form is laterally flattened and ovoid; it has 20–24 somatic kineties (SK) and a typical complement of scuticociliate oral structures. Polykinetid 1 (Pk1) has two longitudinal files of 6 kinetosomes (Ks); sometimes one or two additional kinetosomes are located anteriorly. Polykinetid 2 (Pk2) has two files of 6 or 7 Ks; a third file of three widely spaced kinetosomes is located on its right side, as well as a small curved row of 5 Ks positioned on the right hand side of the posterior end of Pk2. Polykinetid 3 (Pk3) has 3 rows of 4, 5, and 7 Ks's perpendicular to the haplokinety (Hk) or paroral membrane. The stationary growth phase cell is spindle-shaped and has a similar number of SK that are much more closely spaced than in exponentially growing specimens. Oral infraciliary structures are reduced in size, having fewer kinetosomes and being positioned more anteriorly in the cell. Pk1 is composed of two files of 5–6 Ks, Pk2 has only two files of 6–7 Ks, and Pk3 has two to three rows of 3–4 Ks. The Hk is displaced anteriorly and becomes straight, losing the shape typical of exponential growth phase cells. Cysts are pyriform with a wide cyst wall; no infraciliary structures were visible.
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Vol. 48 • No. 3