Do zebra mussels grow faster on live unionids than on inanimate substrate? A study with field enclosures

The zebra mussel Dreissena polymorpha has invaded numerous freshwaters in Europe and North America and can foul many types of solid substrates, including unionid bivalves. In field experiments we compared growth rates of dreissenids on live specimens of the freshwater bivalve Anodonta cygnea to growth rates of dreissenids on stones. Dreissena density in the study lake was about 1000 m(-2) in most places, Anodonta density approximately 1 m(-2) and about 50% of the Anodonta were infested with 10-30 Dreissena. In summer/autumn small dreissenids generally grew faster on live Anodonta than on stones. Similar trends were observed for spring, but differences of growth increments between dreissenids on live Anodonta and stones were usually not significant. Dreissenids settled down or moved towards the ingestion/egestion siphons of Anodonta and ingestion siphons of dreissenids were directed towards siphons of Anodonta. These results suggest that dreissenids can use the food provided by the filter current of the large Anodonta.