Morphological Characteristics of Kellicottia bostoniensis (Rousselet, 1908) (Rotifera: Brachionidae) from the Water Bodies of the Middle Volga Basin

—The morphological characteristics of the North American invasive species Kellicottia bostoniensis Rousselet, 1908 from 29 different types of water bodies and watercourses of the Middle Volga basin, have been studied. The water bodies serving as habitats for K. bostoniensis are in several areas that significantly differ in a set of characteristics, such as geological structure, relief, climate, degree of anthropogenic impact, and hydrochemical and hydrophysical parameters. The size of K. bostoniensis individuals from most of the studied water bodies and watercourses have been measured and analyzed for the first time. A high positive correlation has been revealed between the total body length and posterior spine length ( R 2 = 0.79, p < 0.01), as well as between the total body length and anterior spine length ( R 2 = 0.75, p < 0.01). A relationship has been revealed between the lengths of the anterior and posterior spines ( R 2 = 0.58, p < 0.01). K. bostoniensis individuals from the studied water bodies and watercourses are characterized by a large size (the median values of the total body length are 236–440 μm). This size is similar to that of individuals from the native habitats of this species in North America and some biotopes of Europe and European Russia. In all the water bodies in Russia and other counties (Belarus, Sweden, Finland, Brazil, and Argentina) where the morphological variability of K. bostoniensis was previously studied, the largest body size was recorded for the invasive species in the Sura River. K. bostoniensis individuals recorded in deep mesotrophic water bodies with a high transparency are larger than those in shallow eutrophic water bodies with a lower transparency in the Middle Volga basin. Our data on the morphological characteristics of the invasive species in the water bodies of the Middle Volga basin significantly expand the existing data on the morphological variability of K. bostoniensis individuals and their dependence on environmental factors.