Preferential ingestion of organic material by the American oyster Crassostrea virginica

Using an artificial diet, composed of silt < 32 p in diameter and the alga Tetraselmis suecica, it was demonstrated that the oyster Crassostrea virginica could significantly reduce the concentration of algae voided in the pseudofaeces (measured as extracted chlorophyll pigment) by over 50 %, compared to levels in the food.More importantly, it was also shown that for C. virginica fed natural seston at concentrations between 4 to 20 mg I-', the proportion of energy, carbon and nitrogen voided in the pseudofaeces could also be reduced significantly compared to that in the food.As the organic material in natural seston is from a wide range of sources -e.g.phytoplankton of different sizes, bacteria, detritus particles, etc.these results indicate that C. virginica has a well developed ability to ingest preferentially various types of organic material and to reject other particles as pseudofaeces.This discriminatory mechanism must be able to operate on individual particles despite the fact that they are bound in viscous mucus.We hypothesise (based on literature information for the properties of molluscan mucus) that the viscosity of the mucus in which the food particles are entrapped may be significantly reduced by the ciliary action on the ridged surfaces of the opposed labial palps.This reduced viscosity mucus is possibly moved to the free edge of the palp where, with a cessation of the mechanical stimulation, it regains its original viscosity.The individual particles may then be subject to chemical testing by chemoreceptors which determine whether a particle is moved over the palp ridge to the mouth or is admitted to the deep rejection tracts.These rejected particles move to the free edge of the palps where they are re-incorporated in the mucus and rejected as pseudofaeces.