The effect of chronic exposure of freshwater cladoceran Daphnia magna to low, environmentally relevant concentrations i.e 4 μgL^-1of ibuprofen (a nonsteroidal anti-inflammatory drug) in a laboratory experiment was studied. We observed the key life history traits of first and fifth generation individuals: age and size at first reproduction, number of first clutch eggs and individual growth rate. Moreover, chosen molecular/subcellular markers of experimental animals stress response such as triglyceride content, heat shock proteins (HSP) expression and DNA:RNA ratio were collected. Overall, chronic exposure to ibuprofen had no significant effect on the molecular markers nor on the life history parameters of the Daphnia. It did, however, caused lethal morphological deformities in embryos and juvenile daphnids. Depending on the clonal affiliation, exposure to a low dosage of ibuprofen over five generations resulted in the deformation of ∼3%-∼10% of the first clutch of offspring. Also, up to 90% of females carried at least one deformed embryo. This is the first time that research has revealed such an effect of ibuprofen on D. magna.

In two independent experiments, we compared: (1) water depth selection (and accompanying temperature selection) by male and female Daphnia magna under different kinds of environmental stress, including the presence of filamentous cyanobacteria, the risk of predation from fish, and the presence of toxic compounds; and (2) sex-dependent production of heat shock proteins (HSP60, 70, and 90) in response to a sudden change in temperature. Male D. magna selected deep water strata, which offer a relatively stable environment, and thereby avoided the threat of predation and the presence of toxic compounds in surface waters. Correlated with this behavior, males reduce their molecular defenses against stress, such as the production of heat shock proteins (HSPs), and do not maintain the physiological machinery that triggers an increase in HSP levels in response to stress. In contrast, female D. magna actively select habitats that offer optimal conditions for growth and production of offspring. Consequently, females are exposed to variable environmental conditions that may be associated with increased stress. To permit survival in these different habitats, D. magna females require molecular mechanisms to protect their cells from rapid changes in stress levels. Thus, they maintain high constitutive levels of the heat shock proteins from HSP 60, 70, and 90 families, and they have the potential to further enhance the production of the majority of these proteins under stress conditions. The results of this study indicate that the separate habitats selected by male and female D. magna result in different patterns of HSP production, leading us to hypothesize that that male and female Daphnia magna adopt different strategies to maximize the fitness of the species.