THE INFLUENCE OF PREY HARDNESS ON DAPHNIAS SELECTIVITY FOR LARGE PREY

Two comparable methods were used to study the feeding of four species of Daphnia on large spherical particles which differed in size and hardness. The first method used gut analysis to estimate the selectivities of daphnids feeding in a broad size range of a single particle type, including polystyrene beads (4-60 mu m diameter) in the laboratory and Eudorina colonies (10-90 mu m) in the field. In the second method, Daphnia of different sizes fed in a mixture of 6.5 mu m Chlamydomonas and one of eight test particles. Smaller daphnids were less effective in feeding on large test particles. Nonlinear regression was therefore used to estimate the Daphnia body size at which the clearance rate on a test particle was reduced to 50% of that for Chlamydomonas. The results of both methods show that prey size and hardness are both very important in determining daphnid feeding selectivity. For a given particle size, 'soft' algae(naked and gelatinous flagellates) are more readily ingested than 'hard' algae (diatoms and dinoflagellates), and 'hard' algae are more readily ingested than polystyrene beads. Daphnia can feed effectively on algae that are 2-5 times larger than the largest ingestible bead.