Four small-celled Planothidium species from Central Europe proposed as new to science

Four small-celled taxa are presented and described – Planothidium werumianum, P. pumilum, P. rhombiculum and  P. rostratoholarcticum. Planothidium werumianum, P. pumilum and  P. rhombiculum are proposed as new to science and  P. rostratoholarcticum as a new name for Achnanthes lanceolata var. rostrata Hustedt 1911. The latter, coming from Germany, is transferred to Planothidium in the species rank. The new name – P. rostratoholarcticum − is necessary to avoid a junior homonym, i.e., Planothidium rostratum (Østrup) Lange-Bertalot 1999, a species described from tropical Thailand. The new taxa were compared to several other, more or less similar small-celled Planothidium spp., such as P. rostratum (Østrup) Lange-Bertalot, P. minutissimum (Krasske) Lange-Bertalot, P. granum (Hohn & Hellerman) Lange-Bertalot, P. daui (Foged) Lange-Bertalot, P. frequentissimum (Lange-Bertalot) Lange-Bertalot and P. engelbrechtii (Cholnoky) Round & Bukhtiyarova. Significant differences could be found through light- and electron microscopic fine structure analysis. Planothidium pumilum and P. rostratoholarcticum occur mainly in eutrophic alkaline rivers and lakes with medium to high conductivity, likewise in estuaries, lagoons, and backwater of the North Sea and the Baltic Sea. Planothidium werumianum and  P. rhombiculum were found in small carbonate-rich rivers originating from medium altitude mountains.