Exploitation of natural food sources by two sympatric, invasive suspension-feeders: Crassostrea gigas and Crepidula fornicata

The natural diets of the introduced suspension-feeders Crassostrea gigas (Thunberg) and Crepidula fornicata (L.) were determined at a mid-latitudinal oyster-farming site within their European range (Bourgneuf Bay, France). Carbon and nitrogen stable isotope deviations of Pacific oysters and slipper limpets were compared with potential food sources on 3 sampling dates (March, July and November 2003). Four end-members were assimilated by the 2 species: C sub(3) angiosperm detritus, macroalgae-C sub(4) plant detritus, marine phytoplankton and benthic diatoms. Given the lack of source digestibility data for suspension- feeders, and these 2 species in particular, extreme feasible combinations of relative end-member contributions were calculated according to 2 assimilation scenarios, using either IsoSource software or a concentration-dependent model. For both Crassostrea gigas and Crepidula fornicata, benthic and planktonic microalgae dominated diets on the 3 sampling dates. Planktonic microalgae were ingested in greater proportions than benthic species in July and November; however, benthic diatoms also formed a constant and significant part of diets in these months, and were consumed in greater proportions than planktonic species in March. Plant (especially macroalgal) detritus played a major role in the diets of the 2 suspension-feeders, notably in March 2003 when it became the principal ingested source. The substantial contribution of plant detritus to the natural diets of these species has not previously been reported. Although Crassostrea gigas and Crepidula fornicata showed significantly different isotopic deviations in March and July 2003, trophic niches of Crassostrea gigas and Crepidula fornicata overlapped on all 3 sampling dates, with a greater ingestion of identical sources in November. These 2 invasive species could therefore be trophic competitors in the context of end- member supply limitation. Contrary to previous analyses conducted on these 2 species in Europe, this study reports significant dietary overlap. Ecosystem- specific diet studies of invasive species are thus necessary in order to understand trophic overlap/ competition as a function of the diversity and availability of local food sources.