Overall, currently known fauna of partenides and trematode larvae, present in the benthic gastropods, includes 29 species from 9 families of trematodes. (Chubrik, 1966; Podlipaev, 1979; Galaktionov and Marasaev, 1986; 1990, etc.) (Table 2.4.5). The general trend of infestation of benthic gastropods with trematoda species is the focal distribution of invasion, and it applies to marine and fresh water ecosystems alike (Ginetsinskaya, 1983). Distribution of trematode foci in gastropods of the Barents Sea is the following: In the northern and central parts of the sea trematode foci is absent; towards south, with the decrease of the sea depth, in the shallow water and on banks, there are some localized areas of infestation of gastropods with parasites that use fish as a final host.
In coastal Murman areas of, infections are found near littoral and sublittoral bays, gulfs and nearby islands. In such areas, there is a higher percent of infection in mollusks and a high diversity of trematode species. These are mainly parasites that use birds as their final host.
No infestation has been found in the littoral zone of the Pechora Sea as there is no macrobenthos present. Parasitic foci are located in the vast areas of the shallow water. Bird parasites are dominating there due to the ecological conditions of the region.
The shallow waters of the Pechora Sea aids formation of the foci of many species of helminthes. Infestation levels in gastropods is higher in this area than in the coastal Murman area. Invasion foci are formed due to the uneven spatial distribution of invertebrates and vertebrates, serving as intermediate and final hosts for parasites. Many different factors are required for a successful infestation, such as concentration of intermediate and final hosts and certain hydrodynamic, hydrological and hydrochemical regimes. Such a set of favourable conditions occurs locally.
The most pathogenic parasites in known foci are species from the Microphallus genus (Microphallidae family), which can be pooled into the group «pygmaeus». Infestation with these parasites leads to sterilization of the first host (mollusk). There is a very high level of infestation among older molluscs, which can reach up to 50-60 % (Galaktionov, 1982). Such a high level of invasion is observed in Littorina species living in small bays, especially inner areas and vast regions of the Pechora Sea. Secondly, infestation levels of young birds of some species (Somateria molissima) is almost 100%, and intensity of invasion is such, that it leads to high mortality, sometimes reaching 40% (Bianki, et al., 1979). Weak immune systems of young birds is likely a reason for this.