Intercalary plate dynamics in Peridiniopsidaceae (Peridiniales) observed in the tiny dinophyte <i>Parvodinium</i> cf. <i>inconspicuum</i>

The epitheca of <i>Parvodinium</i> is predominantly comprised of four apical, two intercalary and seven precingular plates, but deviations from this general pattern are also known for particular species. <i>Parvodinium inconspicuum</i>, originally described from the Pacific, is a frequently encountered species also in Europe, although morphological traits that characterise it remain elusive at present. We collected plankton material in the Czech Republic, established monoclonal strains of these dinophytes and studied the morphology using bright field and fluorescence light microscopy and scanning electron microscopy. We further obtained rRNA gene sequence data to embed them in molecular phylogenetics. The strains under investigation are clearly elements of the Peridiniopsidaceae and showed little or no divergence among each other and to strains Buçaco and UTEX2255. Many cells of the new strains showed the regular epithecal tabulation of <i>Parvodinium</i>, but a considerable number of cells had a single intercalary plate in a symmetrical arrangement. We discuss the taxonomic identity of <i>P. inconspicuum</i>, to which the new accessions are assigned tentatively. The epithecal plate configuration of <i>Parvodinium</i> appears evolutionarily very dynamic, increasingly revealing the morphological heterogeneity of this taxon as research proceeds.