Samenvatting

Blue mussels (Mytilus edulis) create niches in the Wadden Sea and therefore they are important for the Wadden Sea. Because there are less mussel beds in the Wadden Sea than would be expected and because of their important role, they are monitored since the mid-nineties. It was found that some beds were overgrown by the alien bivalve Pacific oyster (Crassostrea gigas). Probably co-existence of the two bivalves has advantages like protection from predation; mussels migrating down between oysters for refuge when predators were abundant. One of these predators, the Shore crab (Carcinus maenas), uses various size-related techniques to open mussels. For any size of crab, there is an optimal size of mussel to predate on. At the Royal Netherlands Institute for Sea Research (NIOZ), laboratory experiments have been conducted with mussel beds covered by oysters. The mussels may have settled themselves differently in the beds, than they would have done with predators around, like in the field. In the field they have anti-predatory responses caused by exposure to chemical cues from damaged conspecifics or from predators.