Morphology, ontogenesis and molecular characterization of Atractos contortus Vörösváry, 1950 and Stichotricha aculeata Wrzesniowskiego, 1866 (Ciliophora, Stichotrichida) with consideration of their systematic positions

Atractos contortus, a rare highly spiralized hypotrich ciliate, has not been studied by modern methods. Atractos contortus and Stichotricha aculeata are currently assigned to the Spirofildae. A Canadian population of A. contortus and an Idaho population of Stichotricha aculeata, were characterized using live observation, silver impregnation, scanning electron microscopy, and 18S rDNA sequencing. Atractos contortus has two frontal and one buccal cirrus, two marginal and two ventral rows arranged in left-handed helices, and four dorsal kineties. Left and right marginal rows make two and a half and three turns; left and right ventral rows are shortened. Frontoterminal, postoral, transverse and caudal cirri are absent. During morphogenesis the parental adoral zone is completely retained. Cirral rows replicate by intrakinetal and de novo processes. New right marginal and right ventral rows arise from the parental right ventral row. The parental right marginal row is morphogenetically inactive. Dorsal kineties replicate apokinetally. The parental dorsomarginal kinety and some parental marginal cirri persist for some time after cytokinesis. Phylogenetically, A. contortus groups with Uroleptus species and not spirofilids. The Idaho S. aculeata matches populations from previous reports. Monophyly of the Spirofilidae is rejected. Placement of genus Atractos in the family, Atractosidae nov. nom. is proposed.