A method for determining food selection by zooplankton1,2

Abstract

A method of testing the preference of filter feeders for food organisms in a mixture is described. Daphnia pulex (De Geer) was offered a mixture 3H‐labeled bacteria and 11C‐labeled algae and the radioactivity of double‐labeled samples was measured in a liquid scintillation counter. Only the smallest Daphnia (< 1.5 mm) filtered bacteria preferentially from a mixture of Micrococcus (1.5 µm) and the pennate diatom Nitzschia actinastroides (length 28 µm). The larger animals ingest bacteria and algae without differentiation.