PHENOTYPIC PLASTICITY OF THE FILTER SCREENS IN DAPHNIA - ADAPTATION TO A LOW-FOOD ENVIRONMENT

Daphnia can enlarge the area of the screens on its filtering limbs in response to low concentrations of food. Seven species of Daphnia had larger filter screens when grown at low levels of food compared to animals of the same size raised at high levels of food. The increase in screen area varied between 19 and 83% among species. Larger filter screens resulted in higher filtering and feeding rates. Threshold food concentrations for growth, calculated from the carbon budget for different Daphnia species, were biased when adaptation of the filter screens was ignored. Daphnids used for measuring assimilation rates are usually cultured at high levels of food and thus have small filter screens. If filter-screen enlargement is taken into consideration in estimating assimilation rates, thresholds come close to the realistic values measured in long-term growth experiments. Filter-screen adaptability is considered a response to varying nutritional conditions in the pelagic zone.