FEEDING RATE OF DAPHNIA MAGNA STRAUS IN DIFFERENT FOODS LABELED WITH RADIOACTIVE PHOSPHORUS1

Abstract

The feeding rate of Daphnia magna was studied by measuring the radioactivity of animals fed on pure cultures of Escherichia coli, Saccharomyces cerevisiae, Chlorella vulgaris, and Tetrahymena pyriformis labeled with radioactive phosphorus. Below a certain concentration of each food, the feeding rate is proportional to concentration of food. Above that concentration, feeding rate is independent of concentration. Starved animals, when placed in a nonlimiting concentration of food, behave temporarily as if it were limiting and for a few minutes filter at the maximum rate. Although the maximum volume of the various foods eaten in unit time is not the same, it is probably determined more by digestibility than by size of the food cells. Filtering efficiency of Daphnia magna is independent of the size of food cells between 0.9 µ 3 and 1.8×10 4 µ 3 . Log‐phase Chlorella vulgaris was not observed to inhibit feeding, but senescent cells caused Daphnia magna to decrease the filtering rate and its maximum feeding rate.