EGG SIZE AND EGG MASS OF DAPHNIA-MAGNA - RESPONSE TO FOOD AVAILABILITY

The influence of different food availability on egg size and egg mass in Daphnia magna Straus was studied in long-term experiments using a flow-through system. Daphnia were either kept at constant high or low food levels or subjected to alternating periods of high food and starvation. Some animals were starved continuously after they had deposited their first clutch of eggs. Eggs were measured and weighed and their density (dry mass per volume) was determined. The results support the model of Glazier (1992), which defines a region of 'reproductive constraint' at very low food concentrations and a region of 'adaptive response' as food concentrations increase. Egg sizes were largest under continuously low food concentrations (0.1 mg C 1(-1)), which indicates that the maximum of Glazier's non-linear response curve is at very low food levels. Eggs produced during starvation were small, probably as a result of reproductive constraints. Egg density was about 0.37 mg dry weight mm(-3) and did not differ between treatments.