Eschweiler, Nina (2011) Foes and friends : integration of invasive alien oysters into the epibenthic community of the Wadden Sea. (PhD/ Doctoral thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 120 pp.

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Abstract

Since the development of anthropogenic transport vectors, especially waterways, many exotic species have the opportunity to enter regions which they could not reach on their own before. If conditions are appropriate, establishment and further dispersal of alien species may rapidly proceed. Influences of the non-indigenous species on native species and the habitat are frequent and have become a thriving field of research. Effects can be detrimental or beneficial and may cause changes in living conditions as well as the ecological function of the respective species with further impact on the entire community structure. Deliberately introduced Pacific oysters Crassostrea gigas have successfully established feral populations in the Wadden Sea. This process is facilitated by rising temperatures and the absence of natural enemies. This study is a contribution to the question, how the invasive C. gigas fits into the recipient community, by exploring interactions with resident species. Oysters require hard substrate for settlement, which is rare on sandy or muddy tidal flats. Therefore, they preferentially settle on shells of mussels Mytilus edulis and periwinkles Littorina littorea, since both provide hard bottom surface. Results obtained by this study indicate, that oyster epibionts growing on snails almost stopped their mobility and reproductive output. A heavy spatfall may thus, strongly affect the periwinkle population with cascading effects on benthic community structure. Where oysters became dominant on mussel beds, experiments showed that mussels actively migrate downwards in between the larger oysters to seek shelter from predators. As a consequence of abandonment of the more suitable and favored positions on top of the reef, M. edulis exposed itself to higher interspecific competition for food and space. Mussel growth and condition index was reduced when placed at the bottom in between living oysters compared to top positions on the reef and in between empty oyster shells. The consequence is a trade-off between increased survival and reduced growth. Oyster growth, on the other hand, considerably decreased with increasing density of M. edulis. However, observations also indicate an increase of settling juvenile oyster epibionts on mussel shells with increasing mussel density. Presumably, this oyster overgrowth has similar negative effects on M. edulis as shown for the periwinkle L. littorea. In experiments, native juvenile starfish Asterias rubens preferred juvenile oysters over mussels as prey. This predation effect may explain why Pacific oysters have established on intertidal mussel beds but remained scarce subtidally where starfish abound. In conclusion, detrimental and profitable effects in interactions between resident species and alien oysters occurred in both directions. A new balance of advantages and disadvantages may arise, with a successful integration of C. gigas into the epibenthic community of the intertidal Wadden Sea.

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