Daphnia grazing on natural bacteria1

Ten to sixteen Daphnia were placed in small vials filled with natural water from a lake and incubated for 0.5 to 2.0 h. The small (ca. 0.10 µ 3 ) bacteria were counted on Nuclepore filters by an Acridine Orange direct count method with epifluorescence. Filtering rates were calculated from counts of bacteria before and after the incubations. For D. pulex , D. middendorffiana , and D. longiremis the filtering rates ranged from 0.2 to 1.5 · ml animal −1 · h −1 depending on the size of the animal and temperature. Simultaneous comparisons with rates of filtering on larger yeast cells indicated that these bacteria were filtered at about 30% the rate for yeast.