Identification and ontogenesis of the nomen nudum Hypotrichs (Protozoa : Ciliophora) Oxytricha nova (= Sterkiella nova sp n.) and O. trifallax (= S. histriomuscorum)

Oxytricha nova and O. trifallax were named and established as viable geneti c systems (via frozen resting cysts) by molecular biologists, but never det ermined or described in a scientific way. Thus, their identity is unknown a nd both are nomen nudum species according to the International Code of Zool ogical Nomenclature. In the present paper, this bewildering situation is re ctified by investigating offspring of the original populations. It is shown , by a detailed literature review and morphological and ontogenetical analy sis, using live observation, silver impregnation and scanning electron micr oscopy, that both populations belong to a single morphotype, viz. Sterkiell a histriomuscorum (Foissner, Blatterer, Berger and Kohmann, 1991), a cosmop olitan species very frequent in limnetic and terrestrial habitats. However, on the molecular level, O. nova and O. trifallax: are very distinct, sugge sting that they are different species. Thus, S. histriomuscorum is a comple x of sibling species. For the sake of nomenclatural continuity and priority , we suggest identifying O. trifallax as S. histriomuscorum and establishin g O. nova as a new species, Sterkiella nova sp. n. Both species are diagnos ed by a combination of morphological, ontogenetical and gene sequence chara cters. Field populations of S. histriomuscorum should be designated as gene sequence "Sterkiella histriomuscorum complex" if no molecular data are ava ilable to decide whether they belong to S. nova, S. histriomuscorum, or to another not yet described species of the complex.