Patterns of diatom treatment in two coexisting species of filter-feeding freshwater gastropods

To assess trophic partitioning among sympatric gastropod species in ancient lakes, we quantified diatoms in the guts of two coexistent Baikal gastropod species and tested for differences in species, size, and fracturing of large and small diatoms by taenioglossan radulae. In May 2010, the diatom Synedra acus dominated the littoral phytoplankton and gut contents of Baicalia turriformis and Teratobaikalia ciliata (Baicaliidae), both inhabiting the rocky Baikal littoral. In laboratory experiments, both ctenidial filter-feeding gastropods were fed with two diets of cultivated Synedra acus of different cell sizes: > 150 mu m and < 100 mu m. Field and laboratory studies revealed intact diatom cells (often with green chromatophores) and fragmented frustules of diatoms < 60 mu m in the guts of both species. The two baicaliids varied in the number of ingested microalgae. In addition, they exhibited significantly different efficiencies for breaking large diatoms; B. turriformis broke large diatoms into more fragments than T. ciliata. The differences in the utilization of large and small diatoms by gastropods are discussed in terms of the relationships among coexisting species. Small diatom survival is considered from the view of interactions between producers and their consumers in the freshwater food web.