Two new freshwater species of the genus Achnanthidium (Bacillariophyta, Achnanthidiaceae) from Qingxi River, China

We describe two new Achnanthidium species, A. anhuense sp. nov. and A. qingxiense sp. nov., from Qingxi River, Anhui Province, China, based on light and scanning electron microscopy. Both species belong to the “A. pyrenaicum complex” of the Achnanthidium genus, based on their possession of transapically-elongated areolae and deflected external distal raphe fissures. A. anhuense sp. nov. has a slightly irregular linear-lanceolate valve with rounded or weakly protracted apices and a transapically rectangular or bow tie central area on the raphe valve. A. qingxiense sp. nov. has a linear-lanceolate valve with rounded apices and the axial area distinct expanded apices on the rapheless valve. Both species differ sufficiently from other similar species, based on valve outline, shape of the axial and central areas and striae density. These new species were all collected from stone substratum.