Effect of Varying Oxygen Concentrations on the Filtering Rate of Daphnia Pulex

Knowledge of factors affecting feeding efficiency is of vital importance to understanding any organism's relationship to its environment. The present study reports on the effect of one factor–variation in O_2 concentration–on the filtering rate of a common freshwater zooplankton species, Daphnia pulex. Measurements of D. pulex filtering rate were made under exposure to a wide range of O_2 concentrations and over periods of time ranging from < 1 h to 15 days. Oxygen was found to have a significant effect on the filtering rate in that below concentrations of 3 mg O_2/l the rate decreased sharply; however, following prolonged exposure (> 8—12 h) to low O_2 conditions the filtering rate gradually increased and eventually D. pulex were able to filter much more rapidly than under normal O_2 concentrations. These increases in filtering rates under conditions of low O_2 are directly correlated with the animals' increased production of hemoglobin when little O_2 is present.