The Dynamics of Trematode Occurrence in Two Permanent Settlements of the Ponto-Azov Mollusk Lithoglyphus naticoides (C. Pfeiffer, 1828) (Gastropoda, Hydrobiidae) at the Northeastern Border of Its Range

The freshwater gastropod mollusk Lithoglyphus naticoides (C. Pfeiffer, 1828) is one of the most successful alien species in water bodies of the Volga River basin. One of the negative consequences of its spread is range expansion of trematode species, some of which are highly pathogenic to fish and fish-eating birds and mammals. The aim of this study is to analyze the features of formation of the trematode fauna in settlements of L. naticoides at the northeastern border of its range (Rybinsk Reservoir). It is found that the first permanent settlement of L. naticoides formed in 2005–2010 in coastal shallow waters in the lower part of the Volga reach of the reservoir. The mollusk did not spread rapidly throughout the entire water area of the reservoir outside the Volga reach zone with relatively high mineral water content. However, in 2011–2019, a new large settlement of L. naticoides was recorded in the upper fast-flow part of the Volga reach. A relatively low density of mature individuals in the settlements of L. naticoides (up to 50 ind./m 2 ) allows at least five trematode species to complete their life cycle: Apophallus muehlingi (Jägerskiöld, 1899), Apophallus (= Rossicotrema ) donicus (Skrjabin et Lindtrop, 1919), Parasymphylodora markewitschi Kulakowskaja, 1947, Nicolla skrjabini (Iwanitzky, 1928), and Sanguinicola volgensis (Razin, 1929). The prevalence of parthenitae of certain species depends on the reservoir water level regime, although the overall infestation of L. naticoides can exceed 90%. The slow-flowing lower part of the Volga reach is characterized by the dominance of P. markewitschi and trematodes of the genus Apophallus ; the upper section, which is close to the river type, is dominated by N. skrjabini and S. volgensis . In the future, in the Rybinsk Reservoir, we can expect naturalization of other trematode species associated with L. naticoides within its range.