INFLUENCE OF FOOD QUANTITY AND LEAD-EXPOSURE ON MATURATION IN DAPHNIA-MAGNA - EVIDENCE FOR A TRADE-OFF MECHANISM

1. The start of reproduction is an important life-history trait in both ecology and ecotoxicology. Effects of food availability and toxic stress on maturation in female Daphnia magna are presented. 2. Five instars to maturation were observed under conditions of moderate to high food levels. Age and size at subsequent stages of the maturation process, i.e. ovary provisioning and egg deposition, depended on the rate of body growth. 3. At low food, a trade-off between reproduction and growth occurred. Within the same environment, some females reproduced at early age and small size, whereas others gave priority to growth over reproduction. In the latter resorption of yolk from the ovaries was observed. 4. Except for the production of malformed young, effects of lead on maturation resembled increased food stress. With abundant food, toxic effects were different to effects at low food. The results of standardized laboratory tests may therefore underestimate toxic effects at reduced food availability. 5. The phenomena observed in our studies can only partly be explained by a food-independent size threshold for maturation, which initiates resource allocation to reproduction; a minimum food uptake is required to enable the production of eggs.