There is a theory that jellyfish entered the Azov Sea from the Black Sea through the Kerch Strait when the water level in the Azov Sea dropped significantly.
Fish and Other Inhabitants of the Azov Sea

Diversity of the Azov Sea Fauna
The Azov Sea is home to many species of fish, shrimps, shellfish and other creatures. In addition, hundreds of species of various animals and birds live on the coast of the Azov Sea, its sandy spits and islands. In many parts of the Azov Sea, national parks and reserves have been created in which rare animals and birds are protected. An example of such a national park on the Azov Sea is Biryuchy Island, which is home to more than 250 species of birds and animals.
Azov Fish
Dozens of fish species inhabit the Azov Sea, differing in origin, size and diet. A small sprat and a huge beluga live in Azov, herbivorous fish species and predatory sharks live, some fish feed on plankton, and some eat other types of fish - in a word, the entire spectrum of marine fauna is observed in the Azov Sea. Some types of fish have always lived in the Azov Sea - for example, Azov gobies, and some were brought and acclimatized, such an example is the pilengas, whose homeland is the Far East. And because of the change in the salinity of the water, unusual species of fish began to swim in the Azov Sea - dolphins and sharks.
Other Inhabitants of the Azov Sea
In addition to fish, other species of living creatures also live in the Azov Sea. For example, several species of shrimp live in the waters of the Azov Sea, and jellyfish feel great in its waters, which at one time got into the Azov Sea from the Black Sea through the Kerch Strait. And although jellyfish are not the indigenous inhabitants of the Azov Sea, in recent decades they have fully acclimatized in it, and are not going to disappear anywhere.
Two species of jellyfish live in the Azov Sea, which got into it from the Black Sea through the Kerch Strait - Cornerot and Aurelia.
There are indeed sharks in the Azov Sea, one of the varieties of sharks lives in this sea - the Black Sea katran, common in the Black Sea.
Azov shrimp is a valuable marine product and an excellent delicacy, and also serves as food for various types of fish in the Sea of Azov.
On the coast of the Azov Sea, a mass death of the Azov goby is observed annually, while dozens of tons of fish are thrown ashore.
The Azov Sea is one of the richest areas of the World Ocean with fish, and the Azov Sea ranks first in terms of fish catch per unit area.