Mollusk of the family Bithyniidae (Gastropoda, Littorinimorpha) of the Lower Volga plains river

Background. Preservation of stable and sustainable natural conditions in aquatic ecosystems of various territories is the most urgent problem of nature management in many regions of the world. Occurring changes together with the anthropogenic factor intensify the processes of destabilization of watercourse biocenoses. The resulting transformations in the environment have already led to irreversible changes in the biosphere and a reduction in the number of species. The main causes of biodiversity reduction are habitat destruction, degradation and fragmentation. The most important component of aquatic ecosystems is the representatives of benthic fauna - benthos. One of the elements in the composition of macrozoobenthic are mollusks, as one of the most numerous and widespread groups of organisms, clearly responding to the changes in habitat conditions. Therefore, the purpose of our work is to study the distribution patterns and quantitative indicators of Bithynia tentaculata (Linnaeus, 1758) in the plains river sections with varying degrees of exposure to environmental factors. Materials and methods. In July 2017, hydrobiological samples were collected from the source to the mouth of the Samara River were examined 17 stations. Processing of the collected material was carried out in laboratory conditions. The number and biomass of individuals were counted and necessary morphometric measurements were made. The analysis of the obtained data by means of modern statistical programs was performed. Results. As a result of the study of the Samara River, individuals of the mollusk B. tentaculata were registered in 30% of the sample. The species was found in biotopes characterized by various abiotic and biotic conditions of the environment. In the river it prefers shallow (up to 3 m) unpolluted areas rich with vegetation. Quantitative values in the Samara River were low and significantly inferior to values in one of its tributaries by a factor of 4. The maximum values were noted at the station near the village Borskoe. The comparative analysis of morphological parameters of mollusk shells (height and width) at the river sites did not allow to reveal clear distinctions connected with ecological features on them. It was only noted that the presence of all size groups of the species in the middle reaches may indicate the most satisfactory conditions for B. tentaculata. A canonical analysis of the dependence of the species relative to the vectors of the gradient of environmental factors showed significant (p≤0,05) results for macrophyte overgrowth area, nitrate nitrogen content, dissolved oxygen and calcium concentrations in water. In order to understand the minimum number of factors affecting clam individuals, the rocky sediment criterion was applied, with results indicating 4 environmental variables explaining more than half (54%) of the variance. Conclusions. The obtained results allowed us to supplement the available information on the distribution of the Lower Volga region malacofauna. The species preferences in a complex of ecological conditions characteristic for biotopes of the Samara River have been revealed. All these new data will allow to carry out more effectively measures on preservation of biodiversity of aquatic ecosystems.