DISTRIBUTION PATTERN OF TREMATODES IN A LARGE SETTLEMENT OF THE PONTO-AZOV MOLLUSK LITHOGLYPHUS NATICOIDES (C. PFEIFFER 1828) (GASTROPODA, HYDROBIIDAE) FROM THE UPPER SECTION OF THE UGLICH RESERVOIR (UPPER VOLGA BASIN)

The Ponto-Azov gastropod mollusk Lithoglyphus naticoides (C. Pfeiffer, 1828) naturalized in the Uglich Reservoir relatively recently (in 2013-2015). The main objective of this study is a comparative analysis of the species diversity and structure of trematode communities in different biotopes, occupied by L. naticoides after the initial phase of its spread in the upper section of this water body (downstream the city of Dubna), characterized by a complex hydrological regime. Approximate coordinates of the geographical center of the large settlement of L. naticoides are 56°53ʹN, 37°25ʹE. It is found that by 2019-2021 the average density of L. naticoides in this settlement reached a consistently high value of about 450 ind./m2. Judging by the variability of shell height in adult mature L. naticoides individuals, even within the same settlement, optimum habitat conditions for this mollusk differ significantly in different biotopes. In a comparative aspect, data from two main hydrobiological sampling locations are considered: in the open part of the reservoir and at the mouth of a large tributary - the Dubna River. It is shown that at both locations the species composition of trematodes in L. naticoides is similar and includes, in particular, parthenitae of Apophallus muehlingi (Jägerskiöld, 1899), Apophallus (= Rossicotrema ) donicus (Skrjabin et Lindtrop, 1919), Parasymphylodora markewitschi Kulakowskaja, 1947, Nicolla skrjabini (Iwanitzky, 1928). However, Sanguinicola volgensis (Razin, 1929) parthenitae, dominant in L. naticoides in the open part of the reservoir, is not found in the sample of mollusks from the mouth of the Dubna River. As compared to the sampling site in the open part of the reservoir, at the mouth of the Dubna River the level of infection with parthenitae of other species is significantly lower: N. skrjabini and P. markewitschi - 7 times, Apophallus spp. - 8 times, Xiphidiocercaria sp. - 13 times. The ratio of males to females among mature individuals of L. naticoides turns out to be close to 1:1, which is usual for many freshwater mollusks. In general, the high occurrence (the infection prevalence - the proportion of infected individuals in the host sample P±SE, %) in the open part of the reservoir indicates the presence of prerequisites for serious outbreaks of helminthoses: Xiphidiocercaria sp. (9.16±2.52%), Apophallus spp. (11.45±2.78%), P. markewitschi (7.63±2.32%), N. skrjabini (7.63±2.32%), S. volgensis (63.36±4.21%). At the same time, according to the data obtained, further spread of L. naticoides in some tributaries of reservoirs in the Upper Volga basin may not have such serious parasitological consequences.