Life history and demographic responses to different food regimes were investigated in three clones of Daphnia magna. Green alga Scenedesmus obliquus served as food source and was given in five amounts, which covered the range from near threshold to excess food concentrations, from 0.05 through 0.15, 0.5, 1.5 to 4.5 mg C l -1 . The animals were kept under given food regime from first instar until their natural death. Food regime had a significant effect on Daphnia survival, which was shortest under the highest food regime. However, the response in expected reproduction of an individual differed between clones, with one benefiting more from higher food supply than the others. Life history trade-offs were seen as shortened lifespan versus higher investments in early reproduction.

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.

No abstract

It has previously been shown that the level of Daphnia proteins detected by commonly used anti-HSP antibodies, with molecular weights corresponding to those of heat shock proteins, can be modified by various kinds of stress, including the threat of predation. In this study, we demonstrate for the first time that these proteins change their expression in response to thermal stress. Thus, it is concluded that Daphnia heat shock proteins, as components of the cellular stress defense, are involved in both, abiotic and biotic stress.