INVERTEBRATES OF THE RYBINSK RESERVOIR AS SUPPLEMENTARY HOSTS OF TREMATODS SPHAEROSTOMUM-BRAMAE AND S-GLOBIPORUM (TREMATODA, OPECOELIDAE) -FIELD AND EXPERIMENTAL INVESTIGATIONS

An experimental study of Hirudinea (7 species), Oligochaeta (5), Turbe llaria (5), Gastropoda (18) and Bivalvia (2) receptivity to invasion o f cercaris Sphaerostomum bramae and Sph. globpoirum has been carried o ut. In Hirudinea only Erpobdella octoculata displays susceptibility to invasion. Most Oligochaeta (4 species) are highly susceptible to inva sion, the survival of their metacercaria being very low. In 21 days ol igochaets get rid of parasites and die. Turbellaria are not receptive to invasion, except Bothromesostoma essentii. Receptivity of Gastropod a and survival of their metacercaria are rather high; in large species it decreases with age. Small and medium individuals of mollusks in fa milies Bithyniidae, Valvatidae, Physidae, Planorbidae, Lymnaeidae are most invaded. Receptivity of Bivalvia and survival of their parasites larvae are lower. The leech E. octoculata and Gastropoda play a role o f basic additional hosts. The mollusks (Codiella, Physa, Lymnaea, Plan orbis, Anisus, Planorbarius), which inhabit the draining zone of the r eservoir, provide a source of fish infection during only three months in a year. The mollusks (Valvata, Bithynia, Contectiana) beyond the dr ained zone act in this way for over six months.