Description and ultrastructure of araphid diatom species (Bacillariophyceae) morphologically similar to Pseudostaurosira elliptica (Schumann) Edlund et al.

P>Several populations identified and reported from North America and Mongolia under the names Fragilaria elliptica Schumann or Staurosira elliptica (Schumann) Williams et Round were analyzed in detail using light microscopy (LM) and scanning electron microscopy (SEM). The analysis yielded three variants that can not be ascribed to 'elliptica' of the type. Furthermore, two of the variants could not be related to published taxa and we describe them as new species, Staurosira ambigua sp. nov. and Staurosira dimorpha sp. nov. A third variant was identified as Pseudostaurosira trainorii Morales, a taxon originally reported from ponds and a river in the north-eastern USA. The combined LM and SEM analysis used here allowed for a more thorough circumscription of taxonomic boundaries among morphologically similar taxa and for better delimitation of their geographic distributions. A discussion of the taxonomy and ecology of these taxa based on literature and recent ecological data is presented.