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 f low-through system. Daphnia were either kept at constant high or low f ood levels or subjected to alternating periods of high food and starva tion. Some animals were starved continuously after they had deposited their first clutch of eggs. Eggs were measured and weighed and their d ensity (dry mass per volume) was determined. The results support the m odel of Glazier (1992), which defines a region of 'reproductive constr aint' at very low food concentrations and a region of 'adaptive respon se' as food concentrations increase. Egg sizes were largest under cont inuously low food concentrations (0.1 mg C 1(-1)), which indicates tha t the maximum of Glazier's non-linear response curve is at very low fo od levels. Eggs produced during starvation were small, probably as a r esult of reproductive constraints. Egg density was about 0.37 mg dry w eight mm(-3) and did not differ between treatments.