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Based on the results of cruises and long-term monitoring at the outer shelf of Sevastopol Bay, the interannual dynamics of the state of the population of the alien ctenophore Beroe ovata in the Black Sea for 2000-2021 was analyzed. It was established that in recent years the ctenophore was found in plankton earlier than usual, the period of its active pelagic life increased to 7-8 months, and the body length in juveniles decreased from 40 to 20-25 mm. Three seasonal periods are distinguished, which, due to their temperature conditions, were especially significant for the species: January - February, April - May and July - August. The high temperature of sea water in July - August usually contributes to an increase in abundance and biomass of the ctenophore, as well as a decrease in the average size. The period of active life of B. ovata depends on the warming of the sea in May, as well as on the biomass of Pleurobrachia pileus . B. ovata early appearance leads to faster supression of Mnemiopsis leidyi , preventing it from reaching a high abundance and biomass in summer.

The research both of the quantity and size structure of the invader Mnemiopsis leidyi (A. Agassiz 1865), as well as the mesozooplankton abundance, species diversity, the food spectrum, trophic relations, and predatory impact by M. leidyi on prey populations in the western and eastern sectors of the open Black Sea was carried out in July - August 2017. M. leidyi biomass varied from 92 to 250 g m during the period of maximum seasonal development. The food spectrum of M. leidyi was dominated by copepods Acartia spp. and Calanus euxinus (Hulsemann 1984). A clear spatial difference in the quantity of M. leidyi and zooplankton, the intensity of food consumption and predatory impact was observed. In the western offshore part of the sea, the high food ingestion rate by M. leidyi (the mean specific daily ration was 45.50±8.31% С) and intensive predatory impact on zooplankton (up to ~50% of Acartia sp. biomass) resulted in the strong decrease in mesozooplankton biomass. In contrast, in the eastern sector M. leidyi predation had no effect on zooplankton community maintaining daily grazing of ~6% Acartia spp. biomass.

Abundances, biomasses, size structures, and reproduction rates of the comb jellies Mnemiopsis leidyi and Beroe ovata were studied in the shelf area of the Black Sea near Sevastopol in 2013–2014. Longterm data on the population development were analyzed. The peculiar feature of M. leidyi development in recent years is a shift of its development to an earlier time of the season and a significant decrease in its reproduction rate: in the summer months in 2014, the average reproduction rate of adult ctenophores (30–70 mm) constituted only 20% of that in 2004. Such shift was related to the increase in temperature in the upper homogeneous layer, which, in turn, resulted in a decrease in its feeding rate.

The feeding rates of ctenophore Mnemiopsis leidyi were estimated using the data of the abundance and food composition in the inshore waters of the northwestern Black Sea in 2009–2010. The clearance rate varied in regard to the different foods. The maximum rates were observed when M. leidyi consumed Bivalvia veligers (400 L ind.−1 day−1); the minimum rates were observed when it fed on Copepoda (35 L ind.−1 day−1). The feeding rate increased in accordance with the water temperature in the range from 13°C to 27°C, but dropped when the temperature was higher than 27–28°C. These data were used to calculate the predatory impact of the M. leidyi population on the different groups of its prey and on the zooplankton community in total. The predatory pressure on each prey group and on the zooplankton community was significantly lower for the period of 2009–2010 compared to the previous years. This indicated the decrease in the predatory pressure of the M. leidyi population on the zooplankton community.