Over the past few decades, the jellyfish Craspedacusta sowerbii Lancester, 1880 spread around the world, including the main waterway of Ukraine – the Dnieper River. It is known that invasive species can lead to serious changes in the ecosystems in which they fall. Therefore, studies of the biology and ecology of freshwater jellyfish in new reservoirs are relevant. The aim of the work was to determine the features of the oogenesis of jellyfish C. sowerbii Lancester, 1880, predict the population size next year, and assess the potential threats they may pose to the ecology of the Dnieper reservoir. Objectives of the study: to take samples of jellyfish in the Dnieper reservoir; perform histological sections of gametes of female jellyfish; to study the course of gametogenesis of jellyfish oocytes; determine the percentage of eggs ready for ovulation in the first generation of the breeding season; to establish the values of the nuclear-cytoplasmic ratio in jellyfish oocytes; to determine the peculiarities of reproduction of jellyfish of the species C. sowerbii Lancester, 1880; to predict the number of jellyfish of the next generations and to assess the possible consequences of the rapid reproduction and spread of this jellyfish on the ecosystem and fishery use of the Dnieper reservoir. The article states that there is a mass reproduction of jellyfish C. sowerbii Lancester, 1880 in the Dnieper reservoir that can lead to losses of fish products in 5740 t/year. After analyzing the histological sections of jellyfish, it was found that their reproduction is portioned, the share of mature eggs in the first generation is about 13 %. It was found that mature eggs have an area of 839.75 μm2 and a diameter of 32 μm. It is estimated that next year we should expect an increase in the population of jellyfish by almost 50 %. On the basis of the conducted researches the course of oogenesis of freshwater jellyfish in the conditions of the Dnieper reservoir is determined.
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