Analysis of the type of Fragilaria construens var. subsalina (Bacillariophyceae) and description of two morphologically related taxa from Europe and the United States

Fragilaria construens var. subsalina is a common diatom found in a wide variety of aquatic systems worldwide. It was described by Hustedt (1925) from brackish waters in Oldesloe. Germany; a lectotype specimen was established by Simonsen (1987). We analysed the ultrastructure of specimens from the lectotype material by means of light (LM) and scanning electron microscopy (SEM) and show that this taxon should be placed at the species level within the genus Pseudostaurosira (as Pseudostaurosira subsalina). A similar but distinct taxon from the United States, found in waters with low electrolyte content, is proposed as a new species of Pseudostaurosira, namely Pseudostaurosira americana sp. nov. We also describe a second new species from Spain (Pseudostaurosira alvareziae sp. nov.), found in waters with moderate conductivity, which was misidentified as P. subsalina in the past. A morphometric analysis of the valve shape supports the morphological separation between the three species. The present study clarifies the taxonomy, ecology and systematic position of these three taxa and provides a comparison with other, similar taxa.