Two new Planothidium (Achnanthidiaceae, Bacillariophyceae) species with a sinus from the Kamchatka Peninsula, Russia

Two new species from the genus Planothidium are described from the Kamchatka Peninsula, Russia, based on morphological and molecular analysis: Planothidium piipii and P. novograblenovii. The new species are characterised by a horseshoe-shaped structure in the form of a sinus on the rapheless valve. Comparison with closely related species is provided; the new species are distinguished by various traits, including valve outline and dimensions, striation patterns, and sinus shape. According to molecular data, strains of the new species form a subclade within a clade that includes species of the Planothidium lanceolatum complex.