Relation between particle size selection and clearance in suspension‐feeding ciliates

Abstract

Suspension‐feeding, ciliated protozoans retain particles by sieving the feeding currents through ciliary organelles and show well‐defined size spectra with respect to the particles retained and ingested. When different species of ciliates are compared, clearance is correlated with the particle size most efficiently retained. Thus, species specialized on particles of bacterial dimensions (0.2–1 µm) have a strongly reduced clearance, due to a decreased rate of water propulsion through the filter, from that of species specialized on larger food particles. It is suggested that this is related to the increased resistance to water flow in filters with a decreasing porosity, since cilia can only generate a small hydrostatic pressure. Ciliary suspension feeders specialized on small particles have relatively high requirements with respect to the minimum food particle concentration; they could not maintain populations at bacterial concentrations typical of offshore waters.