THE DYNAMICS OF TREMATODE PREVALENCE IN TWO PERMANENT SETTLEMENTS OF THE PONTO-AZOV MOLLUSK <i>LITHOGLYPHUS NATICOIDES</i> (C. PFEIFFER, 1828) (GASTROPODA, HYDROBIIDAE) AT THE NORTH-EASTERN BORDER OF ITS RANGE

Abstract

The freshwater gastropod mollusk Lithoglyphus naticoides (C. Pfeiffer, 1828) is one of the most successful invasive species in waterbodies 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 for fish, fish-eating birds and mammals. The aim of this study is to analyze the formation features of the trematode fauna in settlements of L. naticoides at the north-eastern border of its range (the Rybinsk Reservoir). It is found that the formation of the first permanent settlement of L. naticoides took place in 2005-2010 in coastal shallow waters in the lower part of the Volga reach of the reservoir. There was no rapid spread of the mollusk throughout the entire water area of the reservoir outside the Volga reach zone with a relatively high mineral content of water. However, in 2011-2019, the formation of a new large settlement of L. naticoides was recorded in the upper part of the Volga reach characterized by faster flow. A relatively low density of mature individuals in the settlements of L. naticoides (up to 50 ind./m) makes it possible for 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), Sanguinicola volgensis (Razin, 1929). The prevalence of parthenitae of certain species depends on the reservoir water level regime but the overall infection 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 , but N. skrjabini and S. volgensis dominate in the upper section, which is close to the river type. In the future in the Rybinsk reservoir, we can expect naturalization of other trematode species associated with L. naticoides within its range.