There is information obtained on species composition, parasitofauna, and structural characteristics of periwinkles in autumn period in three areas (Abram-Mys settlement, Mishukovo settlement, Retinskaya bay) of western Murman coast, and was made a comparison of data with periwinkles of Eastern Murman. The eastern Murman coast is characterized by 6 species of periwinkles, with 5 species noted on the western Murman coast in this study, because the molluscs Littorina compressa have not been found. The infection rate of males L. saxatilis and L. obtusata in senior size classes of East Murman exceeds the infection rate of females what leads to the difference in the sex structure ratio of molluscs. This fact is also characteristic for periwinkles of Western Murman. Periwinkles of Eastern Murman significantly exceed periwinkles of Western Murman in shell height, shell width, and densities. The difference in maximum age is a one year for L. saxatilis molluscs of Western and Eastern Murman; there is no difference for L. obtusata. Maximum age for L. littorea is 13+, for Eastern Murman is 28+. The dominance of trematodes with a life cycle without a free-standing larval stage and a second intermediate host is typical for periwinkles of Western and Eastern Murman.

The study was carried out in 2018 in the littoral zone of Kola Bay during the autumn period in the areas of Abram Mys Settlement, Mishukovo Settlement and Retinskaya Bay. Molluscs of the genus Littorina were collected from various substrates: stones, algae, metal construction and soil. In addition, samples were taken not from a particular substrate, but along the borders of the littoral zone to study species composition of molluscs and common level of infection rate in study areas. Our data showed that L. saxatilis infested by mature trematode parthenitae prefer being on the surface of algae, metal structure, stones at low tide instead of hiding under the stones, in the thickness of algae and in other possible shelters. The study of age distribution among infested Littorina molluscs living on different substrates showed that the ratio of age groups is different and varies from 1+ to 13+. Distribution data are established and considered for infested molluscs by trematode parthenitae of the group ”pygmaeus”: Microphallus pygmaeus, M. piriformes, which are characterized by the absence of free-living cercariae. Trematode parthenitae of other species which have a stage of free-living cercaria in their life cycle are absent in molluscs during the autumn period.