Consumption rates of two rotifer species by zebra mussels Dreissena polymorpha

We demonstrated that zebra mussels Dreissena polymorpha collected from the Hudson River could consume two rotifer species that were common before the zebra mussel invasion. The clearance rates (volume of water filtered per hour) of zebra mussels differed when feeding on the two rotifer species but both decreased with an increase in rotifer density. The ingestion rates (biomass of rotifers per hour) for the two rotifer species also differed, but both increased with increasing rotifer density. This is the first experiment to measure zooplankton consumption by bivalve molluscs at different zooplankton densities. The feeding rates of mussels in this study were compared with those of phytoplankton and other zooplankton from previous studies. The diameter of the mussel inhalent siphon was correlated linearly with the shell length and tissue weight, and was usually an order of magnitude wider than rotifer sizes. It is concluded that bivalve suspension feeders not only act as food competitors but also as predators on zooplankton in the aquatic ecosystem.