Functional response and particle size selection of Halteria cf. grandinella, a common freshwater oligotrichous ciliate

In laboratory experiments, we studied the growth and feeding characteristics of Halteria cf. grandinella, a common and widespread oligotrichous ciliate in freshwater plankton. Particle-size-dependent feeding rates were measured with fluorescent latex micropheres (0.22 to 4.23 mu m diameter) and natural food organisms (bacteria, Synechococcus sp., Chlorella minutissima) in short-term feeding experiments. N. cf. grandinella ingested all but the smallest (0.22 mu m) particles offered and demonstrated a concentration dependent type-2 functional response. Maximum clearance rates were obtained with 2.76 mu m latex beads (0.6 mu l h(-1)). Clearance rates declined for smaller and larger particle sizes but even with the suboptimal size classes, 0.47 and 1 mu m, high maximal ingestion rates were measured (19 and 13 beads ciliate(-1) min(-1), respectively). The maximum clearance rates for bacteria and algae were close to those of similar sized latex beads, and no discrimination of particles based on properties other than size could be detected. The physiological state of the ciliates did not seem to have an impact on the relative pattern of the particle-size-dependent functional response curves, but the absolute feeding rates decreased nearly 10-fold after prolonged starvation. The experiments demonstrated that this small filter-feeding ciliate is an omnivorous species which is able to efficiently exploit the planktonic prey size spectrum from 0.5 to 5 mu m, covering heterotrophic and autotrophic pico- and nanoplankton in the diet.