Re-discovery and novel contributions to morphology and multigene phylogeny of Protospirella mazurica (Raabe, 1968) Aescht, 2001 (Ciliophora: Pleuronematida), an obligate symbiont of the river nerite Theodoxus fluviatilis Linnaeus, 1758 (Mollusca: Gastropoda).

Although the river nerite Theodoxus fluviatilis (Gastropoda: Neritimorpha) has an exceptionally broad geographical and ecological distribution, it carries only four ciliate species: Hypocomella quatuor, Protospirella mazurica, Scyphidia sp., and Trichodina baltica. After more than a half-century gap, we re-discovered P. mazurica in a Danubian population of T. fluviatilis (haplotypes F31 and F34) and characterized it using an integrative morpho-molecular approach. Protospirella mazurica is distinguished by (i) a small, elongate-ellipsoidal to ovoidal body, (ii) a broadly ellipsoidal macronucleus accompanied by a single globular micronucleus, (iii) a subterminal contractile vacuole, (iv) about 24 somatic kineties, (v) thigmotactic ciliature composed of about 10 kineties shortened posteriorly to form a parenthetical system, and (vi) a long inverted J-shaped paroral membrane associated with three unequally long membranelles. According to the present phylogenetic analyses of two mitochondrial and three nuclear markers, P. mazurica robustly clusters within the order Pleuronematida (Oligohymenophorea: Scuticociliatia) along with other symbiotic members of the families Hemispeiridae and Thigmophryidae as well as free-living representatives of the paraphyletic family Cyclidiidae. In light of the present phylogenetic analyses, we consider the family Ancistridae to be a junior synonym of the family Hemispeiridae, which collates 14 genera in our classification framework.