LAUSR

The main biological features of two species of the genus Eurytemora have been studied: the relict E. lacustris (Poppe, 1887) and the alien E. velox (Lilljeborg, 1853), which can be found in natural environments on the territory of the Republic of Belarus. The relict species E. lacustris has been found only in two mesotrophic dimictic lakes belonging to the Baltic Sea basin: Volchin (maximum depth 32.9 m) and Vecheliye (35.9 m). E. velox has now colonized the main rivers of the south of Belarus and their accessory reservoirs belonging to the basins of the Baltic and Black seas. E. lacustris is characterized by an uneven distribution in the water column and location in deep water layers with temperatures below 13°C. Seasonal and daily vertical migrations have been established for E. lacustris. With age, the amplitude of migrations increases, reaching values of more than 10 m of depth for adults. E. velox is recorded in the coastal biotopes of watercourses and shallow water bodies. In the recent two years, the average density of E. lacustris in Lake Vecheliye has been established as about 2000 ind./m3, while their percentage in the zooplankton was 6%. In Lake Volchin, its density has been 2-5 times lower in the same period, and there this species’ percentage in the plankton did not exceed 2%. The density of E. velox varies from 20 to 12 000 ind./m3, their average density is 1631 ind./m3, and their percentual share in the zooplankton does not exceed 1%. For E. lacustris, the body length at all stages of development has been determined. It has been established that males have a greater body length in comparison with females, due to the relatively longer male abdomen, which is not typical for other species of freshwater planktonic copepods. In Belarus, E. lacustris is a bicyclic species, whereas E. velox is polycyclic.