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 har dness. The first method used gut analysis to estimate the selectivitie s of daphnids feeding in a broad size range of a single particle type, including polystyrene beads (4-60 mu m diameter) in the laboratory an d Eudorina colonies (10-90 mu m) in the field. In the second method, D aphnia of different sizes fed in a mixture of 6.5 mu m Chlamydomonas a nd one of eight test particles. Smaller daphnids were less effective i n 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 resu lts of both methods show that prey size and hardness are both very imp ortant in determining daphnid feeding selectivity. For a given particl e size, 'soft' algae(naked and gelatinous flagellates) are more readil y ingested than 'hard' algae (diatoms and dinoflagellates), and 'hard' algae are more readily ingested than polystyrene beads. Daphnia can f eed effectively on algae that are 2-5 times larger than the largest in gestible bead.