FILTRATION OF PARTICLES FROM SUSPENSION BY THE AMERICAN OYSTER CRASSOSTREA VIRGINICA

1. Particle filtration by the oyster Crassostrea virginica was studied in the 1.0 to 12.0 µ size range in relation to naturally occurring particles and to kaolinite suspensions in filtered river water. 2. Oysters were held in troughs of flowing water under conditions similar to their natural environment. Particle number and volumes entering and leaving the troughs were enumerated using a Coulter electronic particle counter. Particle diameter was expressed as that of a sphere having a volume equal to the particle. 3. Results of the study were expressed as per cent of total particles removed in various size increments or in volumes removed over the same size range. 4. Oysters filtered naturally occurring particles in the 1.0 to 3.0 µ range with about one-third the efficiency as larger particles. Above 3.0 µ there was no change in efficiency with increasing particle size. For kaolinite particles, oysters removed particles in the 1.0 to 2.0 µ range with about half the efficiency as larger particles. Above this size there was no change. 5. When results are expressed in terms of per cent removal, the importance of the small sized particles is minimized. In terms of volume, particles in the 1.0 to 3.0 µ range constitute the largest single size fraction over the 1.0 to 12.0 µ range. 6. The consistent presence of an inflection point at a particle size around 2.0- 3.0 µ in the curve for per cent removed vs. particle size is an indication that the distance between adjacent latero-frontal cilia is the factor that determines the smallest particle size that can be completely retained by the oyster gill. 7. Presence of the inflection point is also interpreted as negating the existence of a mucous sheet over the gills, as suggested by MacGinitie (1941). 8. Particles in the 1.0 to 3.0 µ range may play an important role in the nutrition of oysters and other lamellibranchs. Their removal by these organisms may also be an important factor in sedimentary processes.