Are introduced oysters (Crassostrea gigas) hampering the recruitment of indigenous bivalve filter feeders?

No abstracts are to be cited without prior reference to the author.

Since their first introduction in 1964, Japanese oysters (Crassostrea gigas) are spreading rapidly throughout Dutch estuaries. They may affect Dutch ecosystems profoundly. One way in which they might affect indigenous filter-feeding bivalves is by filtering their pelagic larvae, thereby hampering their recruitment. We studied the ability of adult oysters to filter bivalve veliger larvae of different species. Inhalant feeding current velocities of Japanese oysters and blue mussels (Mytilus edulis) were assessed using Digital Particle Image Velocimetry. These velocities were compared with average swimming speeds of bivalve veligers from literature. Inhalant feeding current velocities of both oysters and mussels were higher than swimming speeds of bivalve veligers, indicating that both species might be able to filter large amounts of veliger larvae. However, fluid deformation rate profiles revealed inhalant oyster feeding currents to be more diffuse than inhalant mussel feeding currents. This may cause veligers to be less able to detect feeding currents of oysters in time, increasing the chances of being filtered. Additionally, in the Oosterschelde estuary larval numbers in the water column above an oyster bed were compared with larval numbers above a reference site. Significantly less mussel larvae were found above the oyster bed than on the reference site. This supports the theory that adult oysters filter veliger larvae. Larval numbers of oysters however showed completely the opposite. More oyster larvae were present above the oyster bed than above the reference site.