Three new subaerial Achnanthidium (Bacillariophyta) species from a karst landform in the Guizhou Province, China

Three new subaerial species of Achnanthidium are examined and described from a karst landform of central-south China based on light and scanning electron microscopy observations, and are compared with similar taxa worldwide. A. mediolanceolatum sp. nov. is elliptical-lanceolate, not linear-lanceolate as is more typical in this genus, as well as having a bow-tie shaped central area on the raphe valve and a large lanceolate axial area on the rapheless valve, features which together make it easy to distinguish this species from others. A. parvulum sp. nov. and A. guizhouensis sp. nov. are linear-elliptical in shape, which is more common for Achnanthidium and they differ from other species mainly in size, raphe, density of striae and shape of the central area. We compared these three new species with morphologically-similar taxa, and present their ecological settings and distributions as well. The relationship between Achnanthidium and Psammothidium is discussed, and the data from these new species from Guizhou presented herein suggest the two genera are difficult to separate.