Trophic interactions between two introduced suspension-feeders, Crepidula fornicata and Crassostrea gigas, are influenced by seasonal effects and qualitative selection capacity

The effects of season and qualitative selection capacity on trophic relationships between two sympatric invasive suspension-feeders, Crepidula fornicata and Crassostrea gigas, were investigated in Bourgneuf Bay (France) from January 2003 to June 2004. Carbon and nitrogen stable isotopic deviations, δ13C and δ15N, of common Atlantic slippersnails and Pacific oysters were analysed relative to isotopic composition and availability of end-members. Slippersnail deviations were less variable over the sampling period compared with those of oysters. Significant differences between δ13C and δ15N of C. fornicata and C. gigas were found from winter to early summer, and linked to major isotopic changes in oysters. We identified three distinct seasonal periods: January to March when oysters were 15N-enriched compared to slippersnails and to themselves at other times of the year, April to June–July when oysters showed a 15N-depletion and a more marked 13C-depletion compared to slippersnails and to themselves at other times of the year, and July–August to December when both species presented similar carbon and nitrogen deviations. Species-specific differences in qualitative selection capability may explain these seasonal differences in isotopic deviations. Whereas the isotopic composition of the indiscriminate suspension-feeding slippersnails reflects the composition of the seston throughout the year, the oyster is capable of qualitative selection. The oyster isotopic compositions are consistent with a facultative activation of selection mechanisms under conditions of qualitative and quantitative food limitation, notably the preferential ingestion and assimilation of the dominant organic source in the suspended pool. We conclude that C. fornicata and C. gigas are trophic competitors only in winter and spring at this site, where detrital end-members are major POM components. These results underscore (1) the importance of long-term (annual) studies in the evaluation of potential trophic competition, and (2) the necessity to include the qualitative selection capacities of suspension-feeders in future interpretations of trophic relationships in marine coastal ecosystems.