The freshwater jellyfish Craspedocusta sowerbii, one of Russia's 100 most dangerous invasive species, was found in a reservoir in Astrakhan thanks to the vigilance of local residents.

Live jellyfish were delivered to the Caspian Branch of the P.P. Shirshov Institute of Oceanology of the Russian Academy of Sciences from the waters of the Volga Backwater for further analysis.

Having obtained live specimens, junior researcher Maria Biryukova, after examining the specimens and analyzing literary sources, discovered that the jellyfish Craspedocusta sowerbii had taken up residence in the city's reservoir. The specimens were preserved in alcohol and will be transferred to the Institute of Oceanology's specialized laboratory for confirmation of the discovery and its identification through genetic analysis.

Based on the current situation regarding the spread of this jellyfish throughout the Russian Federation, the region closest to the Astrakhan region where this jellyfish was encountered is the Volgograd region at the Volga-Don Canal in the summer of 2023. It is assumed that this jellyfish could have entered the region in various ways: with water flows from the Volga-Don Canal downstream, in the ballast waters of ships (including in the kingston boxes of bottom fittings) or by migrating avifauna.

The newly discovered Craspedocusta sowerbii is a freshwater tropical jellyfish native to the waters of the Amazon River. This hydrozoan has two main developmental stages: the hydroid polyp and the medusa. The polyps are very small, translucent, and up to 1 mm long. They attach to substrates such as rocks, mollusk shells, stems, and floating leaves of higher aquatic plants. Hydrozoans can withstand water temperatures as low as 4°C, but at this temperature, polyp growth and asexual reproduction cease.

Jellyfish are adult, sexually mature specimens (both sexes) up to 2-2.5 cm in diameter, translucent with a white, or sometimes light cream, tint in the gastrovascular region and around the tentacles. Jellyfish emerge when water temperatures warm to 23-30°C. Jellyfish of both sexes release their reproductive products into the water, where fertilization occurs. Their eggs fall to the bottom of the reservoir, transforming into microhydras, which can "walk" and move along the substrate. Microhydras, in the form of tentacle-less polyps, continue to reproduce, but asexually (by budding).

Craspedocusta is a predator and actively feeds on zooplankton, consuming prey items and potentially compromising the food supply for planktivorous fish. This jellyfish prefers slow-moving or stagnant, well-warmed waters. It can withstand drying waters by transforming into a podocyst, entering a dormant state. The maximum survival time of a podocyst of this jellyfish, according to literary sources, is 40 years. Craspedocusta sowerbii exhibits characteristics of a cosmopolitan species, but due to temperature factors, it does not inhabit the cold regions of our country adjacent to the Arctic zone.

In Russia, these jellyfish are found in the Moscow, Tula, Tver, and Nizhny Novgorod regions, the freshwater part of the Gulf of Finland near St. Petersburg, the reservoirs of Novorossiysk, the Don River, the Volga region (Bashkortostan), and in the artificial lake of the village of Smolenshchina near Irkutsk (Eastern Siberia).

Work and observations on this invader will continue. The upcoming work will assess the status of the hydromedusa population and its spread across urban water bodies, followed by a forecast of its development and dispersal throughout the Volga Delta.