Use of encapsulated live microalgae to investigate pre-ingestive selection in the oyster Crassostrea gigas

The involvement of algal chemical cues in the pre-ingestive selection of food particles in Crassostrea gigas was studied using a new approach. Live cells of two microalgal species, Nitzschia closterium and Tetraselmis suesica, were separately entrapped in small alginate microcapsules using an emulsification/internal gelation method. Microcapsule size was adjusted to be within the range of particles ingested by oysters. Using this technique, about 80% of microcapsules had a diameter ranging from 21 to 100 μm. The monitoring of entrapped algae showed that phytoplankton cells remained alive and maintained an active growth for at least 24 days. In particle selection bioassays, adult C. gigas were fed a mixture of microcapsules containing the above algae species as well as control empty alginate microcapsules. The comparison of the proportions of each microcapsule type in the diet and in pseudofeces revealed that those containing T. suesica were significantly ingested while those containing N. closterium were preferentially rejected. Since microcapsule material (alginate matrix) prevented physical contacts between algae cells and oyster feeding organs, this study clearly demonstrate that extracellular metabolites produced by microalgae play a crucial role in the pre-ingestive selection of particles in suspension-feeding bivalves.