New cavum-bearing Planothidium (Achnanthidiaceae, Bacillariophyceae) species from the Kamchatka Peninsula, Russia

Two new species of the genus Planothidium isolated from a river and hot spring in the Kamchatka Peninsula, Russia, are described on the basis of morphological and molecular data. These are P. tyushovii sp. nov. and P. dyakonovii sp. nov. The new species are characterised by the presence of a covered depression in the centre of the rapheless valve (i.e., a cavum). Comparison of the new species with closely related taxa is given and the phylogenetic relationships of the Planothidium frequentissimum species group are discussed.