Selective grazing on periphyton: a determinant of freshwater gastropod microdistributions

SUMMARY. 1. In a cuirophic pond in southern England. the snail Lymnaca peregra (Mull.) is associated with submersed macrophyies, mainly Elodea canadensis Michx. In contrast, the snail Planorbis vortex (Linn.) is associated with the emergent macrophyte Glyceria maxima (Hartm.) Holmberg.
2. L. peregra grazed selectively on filamentous green algae found only on E. camtdensis. P. vortex selected diatoms. Detritus, which was 5 times more abundant on G. maxima than on E . canadensis. comprised about 60% of the diet of P. vortex.
3. Results of experiments giving both snail species a choice between periphyton-detritus removed from the two macrophytes were consistent with the field observations. L. peregra c hose pcriphyton-detritus from E. canadensis whereas P. vortex chose that from G. maxima.
4. Although other factors may have influenced the distributions of L. peregru and P mriex in Radley Pond, food choice was probably the most important proximate factor.