Effects of an invasive mussel, Arcuatula senhousia, on local benthic consumers: a laboratory C-13-labeling study.

Filter-feeding bivalves play an important ecological role in aquatic systems through benthic-pelagic coupling. However, the mechanisms by which invasive mussels influence the food uptake by local consumers are little known. We tested the interactive effects of the invasive mussel Arcuatula senhousia and the native clam Ruditapes decussatus on the uptake of 13C-enriched microalgae, Isochrysis sp., by two polychaetes, the surface deposit feeder Cirriformia tentaculata and the subsurface deposit feeder Phylo foetida. Polychaetes were exposed to two different densities of A. senhousia and R. decussatus used alone or together. In each treatment, the 13C-concentration throughout the sediment horizon (0-0.5, 0.5-2.5 and 2.5-5 cm) was also quantified. Results showed that, irrespective of bivalve densities, the 13C-uptake by C tentaculata was lower in the presence of the two bivalves together than in the presence of one bivalve species (either A. senhousia or R. decussatus). This was consistent with decreased 13C-concentrations in the 0-0.5 cm sediment layer in the treatment with the two bivalves together. Contrary to C. tentaculata, no changes were observed among treatments in the 13C-uptake by P. foetida. This possibly reflected a limited 13C-enrichment of the lower sediment layers. Our main findings are: (1) A. senhousia reduces the 13C-uptake by R. decussatus, and (2) A. senhousia reduces the availability of phytoplankton-derived C for deposit feeders by impairing the feeding, incorporation and biodeposition of R. decussatus. We conclude that the interaction of A. senhousia with native bivalves needs to be taken into consideration in future food web studies.