(Table 1) Mean sizes of Hydrobia ulvae juveniles in the littoral zone of the Chernaya Inlet (Kandalaksha Bay, White Sea) in July-September 1995

Based on observations and experiments carried out within the White Sea silty-sandy littoral zone in 1994-1997 data on biology of development and behavior of Hydrobia ulvae juveniles over water column and in sediments were obtained. Hydrobiid juveniles 0.125-0.150 mm in size appear in plankton during the second half of June and in two to three weeks they precipitate on sediments reaching 0.300-0.350 mm in size. Specific biological features of the White Sea hydrobiids are a short reproductive period and a short period of juvenile growth related to long under-ice time and decelerated warming of shallow waters. Distribution of juvenile individuals of H. ulvae is primarily determined by hydrodynamics and microtopography of the littoral zone. Redistribution of the juveniles permanently takes place, since all size groups of the juveniles are equally subjected to migration. During the first few weeks after settling mortality of juvenile mudsnails is 85%.