Distribution of the Chinese Mitten Crab Eriocheir sinensis H. Milne Edwards, 1853 in the Eastern Part of the Gulf of Finland

New data are presented on the distribution of the alien species the Chinese mitten crab Eriocheir sinensis in the basin of the eastern part of the Gulf of Finland, the Baltic Sea. The fact of widespread occurrence of E. sinensis in bottom communities has been confirmed. During the period from 2021 to 2023, 81 adult individuals of the mitten crab were recorded; 32 of them were studied in the laboratory. Of the studied individuals, males were dominant; the sex ratio (males : females) was 3.6 : 1; females with exterior egg sacs were not found. The carapace width of the studied crabs (n = 32) ranged from 46.6 to 85 mm, averaging to 63.2 ± 1.7 mm. Males were slightly larger than females; however, statistically significant differences in carapace width between males and females were not detected. The paper further discusses issues related to the origin of crabs inhabiting the eastern part of the Baltic Sea and the possibility of naturalization of this species in the basin of the eastern part of the Gulf of Finland, the Baltic Sea. The conclusion is drawn about the importance of systematic monitoring of E. sinensis in the Gulf of Finland basin.