In daphnids there is a complex change in life history as a function of conspecific density, which is induced by infochemicals released by the animals. The density dependent life history in Daphnia is a change in life history strategy, which enables daphnids to produce more surviving offspring at unfavourable environmental conditions, caused by high population density. Moreover, this density dependent life history change in daphnids can influence results in ecotoxicological testing in a strong way.

1. The change in life history data as a function of animal density was investigated in Daphnia magna, Daphnia pulex and Simocephalus vetulus. Experiments were carried out in flow through design at densities of 1 to 40 animals in 50 ml water volume. This ascertained that density effects were investigated independent of food effects, which affect life history in the same way as density. Increasing density increased the mortality of daphnids, caused longer juvenile development time, and distinctly reduced both growth and reproduction. However, mothers cultured in higher density produced bigger neonates (body length and dry mass). Moreover, neonate lipid content increased, which resulted in longer survival in starvation tests. In addition, the neonates of mothers cultured in higher density, when living single, grew faster and produced significantly more offspring, than neonates of single living mothers. These changes in life history parameters and the shift from quantity to quality in offspring can be seen as a change in reproduction strategy as a function of animal density. In the field, high animal densities are mostly followed by a lack of food due to overgrazing. Daphnids change their reproductive output from quantity to quality also as a direct response to food shortage. However, the reproductive tactics observed in the present study ascertains, that offspring starvation resistance is already increased before a lack of food starts. The animal density, thus, can be seen as proximate factor and the nutritional depletion as ultimate factor.

2. Infochemicals released by the daphnids themselves induce the change in reproduction strategy. These infochemicals are unspecific, and unstable but nevertheless effective. If daphnids live at very high densities (above 200 animals/L), the main factor that influences the animals is physical contact. Based on calculations, there are strong indications, that at very high densities a depression of filtration rate can be held responsible for the observed effects.

3. Studies revealed that in acute toxicity test neonates of mothers, cultured in higher density, were less sensitive to 3,4-dichloraniline than neonates of single living mothers. The bigger the neonates were and the more the lipid content increased the higher was the EC50 (until the three fold concentration). In contrast, there was no change in EC50 in tests with sodiumdichromate. Density effects have also to be taken in account in chronic toxicity tests, in which flow through designs are used (e.g. test of instable or volatile substances). Here 10 animals per vessel are allowed. With increasing concentration, several toxic substances increase the mortality of daphnids. Theoretical considerations and computations showed that in a flow through toxicity test with ten animals per vessel the EC50 will change as a consequence of mortality. At high toxicant concentrations often only few animals can survive. Consequently, depression of clutch size as a function of density is less intensive than in the control. Additionally, as compared to the control, the concentration of food will be higher at higher toxicant concentrations. Thus, two promoting effects counteract the inhibiting effect on reproduction of the tested substance: (1) A less intensive density effect and (2) a promotion by the increased concentration of food. In consequence, the EC50 can be up to 15 fold higher than in a test with single living animals.