Particle selection in the ribbed mussel Geukensia demissa and the Eastern oyster Crassostrea virginica: Effect of microalgae growth stage

We studied particle selection in the ribbed mussel Geukensia demissa, an important suspension-feeding inhabitant of estuaries and intertidal zones of salt marshes along the Atlantic coast of North America. Adult mussels were fed on several mixtures of microalgal cultures (1) in exponential or (2) in stationary phase of growth, and the proportional occurrence of algal species in pseudofeces was examined by flow cytometry. The Eastern oyster, Crassostrea virginica, was chosen as a reference. Results showed that both mussels and oysters were able to selectively ingest or reject our experimental microalgae. Moreover, the pre-ingestive particle selection was affected by microalgal growth phase, particularly in mussels. For instance, the sorting efficiency index increased significantly in mussels fed with a blend made of Nitzschia closterium, Isochrysis sp. and Tetraselmis suesica harvested in stationary growth phase, as compared to the same blend made with microalgae in exponential growth phase. Isochrysis sp. and T. suesica were preferentially ingested by both bivalves whereas N. closterium, was preferentially rejected in pseudofeces. These results demonstrate particle selection in ribbed mussel and underline the effect of algae growth phase on the sorting mechanisms.