Features of Spatial Distribution of Alien Mollusks of the Genus Dreissena in a Large Tributary of the Reservoir of the Middle Volga (by the Example of the Sviyaga River)

Invasions of alien species of mollusks are of great interest as a result of their serious environmental and economic consequences. We present new information on the distribution and quantitative characteristics of mollusks from the genus Dreissena in one of the tributaries of the Kuybyshev Reservoir. The fact of the penetration of these species up to the upper reaches of the Sviyaga River has been revealed. The quantitative values of the Dreissena vary unevenly, gradually increasing toward the mouth of the river. Native species of mollusks dominate in the upper and middle reaches of the river, and alien ones in the lower reaches. The comparison of morphometric characteristics of shells in the river revealed significant differences between two related dreissenid species, with the exception of the river mouth. The important factors in the distribution and development of two species of Dreissena in the Sviyaga River are flow rate, depth and width of the river section, and the absence of a solid substrate in the water.