Abstract 18 How to cite the abstract: Foissner W., Berger H. (1999): Morphological characterization of two nomen nudum hypotrichs (Protozoa, Ciliophora): Oxytricha nova and Oxytricha trifallax. - The Journal of Eukaryotic Microbiology, Supplement 46: 11A, Abstract 86. Morphological Characterization of Two nomen nudum Hypotrichs (Protozoa, Ciliophora): Oxytricha nova and Oxytricha trifallax. WILHELM FOISSNER and HELMUT BERGER, Universitaet Salzburg, Institut für Zoologie, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria. Oxytricha nova and O. trifallax were named and established as viablegenetic system (via frozen resting cysts) by molecular biologists, but never determined or described in a classical morphological sense. Thus, their identity is unknown and both are nomen nudum species according to the International Code of Zoological Nomenclature. In the present paper, this bewildering situation is rectified by investigating offsprings of the original populations. It is shown, by a detailed literature review and morphological analysis, using life observation, silver impregnation and scanning electron microscopy, that both populations belong to a single morphotype, viz. Sterkiella histriomuscorum, a cosmopolitan species very frequent in limnetic and terrestrial habitats. However, on the molecular level, O. nova and O. trifallax are very distinct, suggesting that they are different species. Thus, S. histriomuscorum is a complex of sibling species. For the sake of nomenclatural continuity and priority, we suggest to identify O. trifallax as S. histriomuscorum and to establish O. nova as a new species. Both species are diagnosed by a combination of morphological and molecularbiological characters. Field populations of S. histriomuscorum should be designated as "Sterkiella histriomuscorum complex", if no molecular data are available to decide whether they belong to O. nova, S. histriomuscorum, or to another not yet described species of the complex. Supported by the Austrian FWF, Project P-12367-Bio. Links Homepage www.protozoology.com Publication list of Helmut Berger Publisher Helmut Berger Main publication to this topic Keywords: Ciliophora, Hypotrichia, Sterkiella, Oxytricha, nomenclature, Histrio, Histriculus, morphogenesis, cyst, biometry, Hypotrichida, Hypotricha, Spirotrichea, Spirotricha, Ciliates, Ciliata, cell division, Zellteilung, Protist, Top Dr Helmut BERGER Consulting Engineering Office for Ecology - Technisches Büro für Ökologie Radetzkystrasse 10, 5020 Salzburg, Austria, Europe Phone +43-(0)662-432538; Fax +43-(0)662-443139; email office@protozoology.com; http://www.protozoology.com