Growth-rate dependent feeding rates in Daphnia pulicaria and Brachionus rubens: adaptation to intermediate time-scale variations in food abundance

The hypothesis was tested that zooplankton can adapt to fluctuationsof their food resources at intermediate time scales in orderto maximize their energy input. The cladoceran Daphnia pulicariaand the rotifer Brachionus rubens were cultured at a range offixed growth rates and then offered radioactively labelled algaeat limiting and non-limiting concentrations to determine theirfunctional response. Rotifers and cladocerans showed a remarkablysimilar response. Animals growing at low growth rates had highermaximum filtering rates, i.e. were better adapted to low foodconcentrations than those growing at high rates. The growthrate did not affect the maximum ingestion rate in any of thespecies, thus indicating that adaptation to the prevailing foodconditions was based on the process of food collection. Bothspecies were heavier when cultured at high growth rates. Plottingthe weight-specific maximum filtering rate versus the growthrate resulted in a pronounced negative slope. The slope forthe weight-specific maximum ingestion rate was also negative.Hence filtering and ingestion rates seem to be determined bythe size of the zooplankton, not by their weight.