Mollusken op waterlelies, 3. Slakken op waterlelies in Oostzaan

On two localities in Oostzaan, province Noord-Holland, the Netherlands, the leaves of two populations of an unknown species of Waterlily Nymphaea (one population with rose coloured flowers) were checked for the presence of aquatic molluscs. Only freshwatersnails were encountered: eight different species at one locality, six at the other. The introduced sessile snail Ferrissia clessiniana abounded at the locality with the lowest diversity in species. Maybe Ferrissia, with a mean density of 29 specimens per leaf, outcompeted most of the local species. A follow up research has to show whether a natural drop in the density of Ferrissia will lead to an increase of the snail diversity on the Nymphaea-leaves.