Trade-off between increased survival and reduced growth for blue mussels living on Pacific oyster reefs

Pacific oysters Crassostrea gigas (Thunberg 1793) have been introduced into the Wadden Sea (North Sea, Germany) in the mid of the 1980s and have invaded native blue mussel Mytilus edulis (L.) beds. The latter turned into oyster reefs where mussels seem to be relegated to the bottom in between the much larger oysters. By combining field and laboratory experiments, we reveal how mussels react to cohabitation with the invasive oysters. Mussels subjected to direct contact with crabs Carcinus maenas migrate from top to bottom positions between oysters in both field and laboratory experiments within 22 days. Shell growth was significantly reduced for mussels placed on the bottom compared to mussels at the top of an oyster reef. Condition index was lower for mussels on the bottom of the reef irrespective of whether placed between dead or living oysters. We conclude that mussels experience a trade-off between survival and food supply and prefer to take refuge from predation even when this decreases growth and condition. This mechanism may have facilitated the take-over of C. gigas on M. edulis beds in the European Wadden Sea.

Research highlights
► Blue mussels respond to direct contact with predatory crab Carcinus maenas.
► C. maenas provokes a downward migration of Mytilus edulis inside an oyster reef.
► Growth and condition index of M. edulis on bottom of a reef are reduced.
► Mussels experience a trade-off between survival and food supply.