Until recently, about 40 modern freshwater representatives of the genus Cyclotella (Kützing) Brébisson were known in Russia. Most of them have been recently transferred to the genera Discostella Houk et Klee, Handmannia M.Peragallo, Pantocsekiella K.T.Kiss et Ács, and to date 11 species and varieties of the genus: C. ambigua Grunow, С. atomus Hustedt var. atomus, C . atomus var. gracilis Genkal et Kiss, C . baicalensis et Meyer, C. caspia Grunow, C. choctawhatcheeana Prasad, C . cryptica Reimann, Lewin & Guillard, C . distinguenda Hustedt, C. meduanae Germain, C. meneghiniana Kützing and C . minuta (Skvortzow) Antipova are known from water bodies and watercourses in Russia. Cyclotella cryptica and C. distinguenda belong to rare species of Russian flora. The study of the species morphology was undertaken from the river mouth of the Khara River, a tributary of hyperhaline Lake Elton (Volgograd Region, Russia). Scanning electron microscopy has made it possible to estimate the variability ranges of the main diagnostic quantitative features (valve diameter, number of striae in 10 µm, number of central fultoportulae and location of marginal fultoportulae). The literature data on morphological variability of C. cryptica is limited and our studies address the shortcomings. Findings of the species in fresh water bodies of Russia and published illustrations cast doubts on the accuracy of identification of C. cryptica, and this finding in the Khara River is suggested to be the first for Russia. For C. distinguenda, the maximum diameter of the valve is specified, ranges of variability of the number of striae in 10 µm and the size of initial cells are disclosed.

Keywords: Bacillariophyta, Cyclotella cryptica, C. distinguenda, morphology, scanning electron microscopy, Khara River, Volgograd Region, Russia

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