Development and interaction of ctenophores Beroe ovate Bruguière, 1789 and Mnemiopsis leidyi A. Agassiz, 1865 in the coastal zone of the Middle Caspian Sea

Aim. Analysis of the distribution and status of ctenophores В eroe ovata and Mnemiopsis leidyi , and assessment of the parameters of their interaction in the Middle Caspian Sea. Material and Methods. The material was obtained in the western and eastern parts of the Middle Caspian Sea in September–October 2022. Large ctenophores were collected using a big cone plankton net with a 0.5 m 2 opening and a 500 μm, mesh size. The number and size of large ctenophores were determined immediately after collection, larvae and eggs were counted in samples fixed with formalin to a final concentration of 4%. Zooplankton samples were collected by the Juday plankton net (0.1 m 2 opening, 180 μm mesh size). Results. In September‐October 2022, a developed population of В eroe ovata with active reproduction was recorded in the waters of the Middle Caspian Sea. The habitat area of B. ovata has significantly expanded to the north compared to 2021. Complete suppression of the Mnemiopsis population by the Beroe was observed on most of the western and eastern shelf, as well. The total number of Beroe reached 48 ind/m 2 , biomass – 12 g/m 2 . Single large individuals were found in the north of the Middle Caspian Sea at salinity 5.7 psu. The structure of mesozooplankton was still dominated by the small copepod Acartia tonsa . Conclusion. New data indicate the active development of the В eroe ovata in the Caspian Sea, which presents a fundamental challenge for the restoration of the Caspian ecosystem. The progressive adaptation of Beroe to the conditions of the Caspian Sea has led to its expansion to the north of the Middle Caspian Sea in areas with low salinity. A complete suppression of the M. Leidyi population by the new alien noted on most of the shelf indicates an increase in its pressure on the Mnemiopsis population compared to the corresponding period of 2021.