Deep water 'dwarfs' and 'giants' among endemic Baikal gastropods

It is suggested that existence of "dwarfs" and "giants" among Baikal gastropods depends on several factors. "Dwarfs" occurs in habitats 1) where sedimentation rate is low and thick silt layer is absent (the small shell without long periostracal nodules does not allow the animal to stay on the soft sediment surface; 2) where there is hard substratum suitable for laying egg capsules; 3) where there is intensive water exchange assuring supply of food (detritus and phytoplankton). Distribution of "giants" capable to lay egg capsules on their own shells is not confined by availability of hard substratum. They dwell on soft sediments because they have large light shell with high whorl allowing them to migrate on silt surface. "Giants" live in places with high sedimentation rates, constant supply of food (including died fish), and higher degree of water mineralization. "Giants" and "dwarfs" inhabit only oxidized substrates. Neither "giants", nor "dwarfs" inhabit the flat bottom of Baikal. "Giants" occur on near-slope parts of the lake, while the "dwarfs" prefer underwater elevations.