Peculiarities of the population of <Emphasis Type="Italic">Bythotrephes brevimanus</Emphasis> lilljeborg, 1901, in lake pleshcheevo (the Upper Volga Basin)

The morphological and biological features of Bythotrephes—a relatively rare crustacean in Lake Pleshcheevo—have been identified on the basis of vast material (several hundreds of individuals) collected in August 2010. The individuals were distinguished by a minimal body size, short caudal appendix, absence of a loop, and very small caudal claws and were identified as Bythotrephes brevimanus Lilljeborg, 1901. The morphology of the crustacean corresponds to the high water mineralization in the lake and the relative stability of the environment. Some peculiarities of molt and growth of the crustacean were distinguished. A chitinous integument of the caudal appendix formed during embryogenesis has a different structure than chitin formed during further molts. In August, the population of B. brevimanus in different areas of the lake was stable and consisted only of juveniles and parthenogenetic females (80%) (4–6 eggs). The largest number of crustaceans (150 ind./m³) was recorded in the periphery of the depression above depths of 6–7 m; 80 ind./m³ were recorded above depths of 11–12 m; and, above the depression, there were less than 30 ind./m³. The small density of crustaceans in the pelagial depends on the intensive grazing of this food object selected by fish in especially transparent water (color 15–20 and transparency 300–600 cm).