Rigidosticha italiensis n. gen., n. sp. (Ciliophora, Spirotricha), a novel large hypotrich ciliate from the soil of Lombardia, Italy

The morphology of Rigidosticha italiensis n. gen., n. sp., which was found in a soil sample collected from an uncultivated field in Lombardia, Italy, was investigated using live observation and protargol staining. Rigidosticha n. gen. is characterised by a rigid body, undulating membranes resembling a Steinia pattern, oxytrichid frontal ciliature, distinct mid-ventral cirral pairs, transverse cirri, one right and one left row of marginal cirri, absence of dorsal kinety 3 fragmentation, more than two dorsomarginal rows, and caudal cirri. The new species shows the following features: size in vivo 230–330 × 100–170 μm, on average 230 × 115 μm in protargol preparations; two ellipsoidal macronuclear nodules; 51 adoral membranelles; one buccal cirrus; one parabuccal cirrus; two frontoterminal cirri; 16 mid-ventral cirral pairs; and three transverse cirri. Rigidosticha mainly differs from Rigidothrix, Afrophrya, Uroleptus, and Territricha, in having the undulating membranes in Rigidosticha (vs. oxytrichid and cyrtohymenid) pattern. The oxytrichid frontal ciliature and midventral pattern in the present species further support the CEUU (Convergent Evolution of Urostylids and Uroleptids) hypothesis.