Refined morphological circumscription and epitypification of Pseudostaurosira polonica (Witak & Lange-Bertalot) E.Morales & Edlund (Staurosiraceae, Bacillariophyta), an overlooked freshwater diatom species in Europe

Pseudostaurosira polonica is a poorly known small araphid species, due to insufficient documentation and discussion of its morphological features. As the original type material seems to be no longer available and, moreover, based on fossil, eroded material, an epitype is designated based on an Italian population from the small mountain tarn Lago di Valagola (Brenta Dolomites, Italy). LM and SEM observations of this population increased our knowledge of the morphology of this rarely observed species. These observations and comparison with similarly shaped Pseudostaurosira species indicated that at least two different morphological groups can be separated within the genus Pseudostaurosira. The analysis of this species also highlights the importance of a better conservation of original material in larger, well-curated collections.