Rising temperatures associated with global warming present a challenge to the fate of many aquatic organisms. Although rapid evolutionary response to temperature-mediated selection may allow local persistence of populations under global warming, and therefore is a key aspect of evolutionary biology, solid proof of its occurrence is rare. In this study, we tested for genetic adaptation to an increase in temperature in the water flea Daphnia magna, a keystone species in freshwater systems, by performing a thermal selection experiment under laboratory conditions followed by the quantification of microevolutionary responses to temperature for both life-history traits as well as for intraspecific competitive strength. After three months of selection, we found a microevolutionary response to temperature in performance, but only in one of two culling regimes, highlighting the importance of population dynamics in driving microevolutionary change within populations. Furthermore, there was an evolutionary increase in thermal plasticity in performance. The results of the competition experiment were in agreement with predictions based on performance as quantified in the life table experiment and illustrate that microevolution within a short time frame has the ability to influence the outcome of intraspecific competition.

Cyanobacteria may reduce Daphnia fitness through their toxicity, low nutritional value or poor manageability. In this study, the relative importance of the low nutritional value and poor manageability of cyanobacteria on Daphnia life history were determined. To separate these two aspects of cyanobacteria impact, short or long filaments of Cylindrospermopsis raciborskii were given as food to five clones of Daphnia magna either alone or together with the green alga Scenedesmus obliquus. Feeding with short cyanobacterial filaments caused reductions in growth rate (,45%) and fecundity (,60%) in some clones. The deleterious effects caused by cyanobacterial poor nutritional quality could be completely overcome by supplementation with highquality food. Long cyanobacterial filaments caused greater reductions in growth rate (,65%) and fecundity (,80%) in the most affected clones, and the addition of high-quality food only partially diminished the effects of poor filament manageability. In Daphnia fed with long filaments, there was no correlation between body size at maturity and fecundity, and the maximum fecundity was reached by individuals of intermediate size. Decreased body size may, thus, represent an adaptive mechanism against interference with food collection caused by long filaments. The pattern of responses to different food regimes was similar in all Daphnia clones, but they differed in the absolute values of the life history parameters affected. Differences in the fecundity of Daphnia caused by the presence of cyanobacteria may affect clonal composition of cladoceran assemblages, thus, cyanobacteria can shape microevolutionary changes in Daphnia populations.

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 many eutrophic lakes, ability to cope with filamentous cyanobacteria is crucial to Daphnia fitness, as the filaments can reduce food intake by interference with food-particle retention. Two basic mechanisms were proposed to explain food collection by Daphnia: mechanical sieving, in which filtering appendages act as sieves, and direct interception, in which appendages act as paddles and water does not pass through. As was recently suggested, both mechanisms may be active, and their relative importance is determined by the Reynolds number on filtering appendages. Mechanical sieving seems particularly sensitive to the interference from filamentous cyanobacteria, which can clog the meshes while passing through the filtering chamber. We therefore hypothesized that in the presence of filaments, Daphnia minimizes the interference with filtration by decreasing the Reynolds number on the filters and by thus reducing the relative importance of sieving. To test this hypothesis, we examined the responses of nine clones of the Daphnia longispina group to the presence of nontoxic, filamentous cyanobacteria. The presence of cyanobacteria triggered in Daphnia responses of both behavioral (descending to deep, cold waters) and morphological nature (decreased intersetal and intersetular distances in the filtering apparatus). Both responses led to a decrease in the Reynolds number on the filters. Moreover, the two responses were inversely correlated: individuals with larger meshes descended to colder strata than did those with smaller meshes. These modifications of phenotype are regarded as adaptive, since they allow for minimized filamentinduced interference with the filtration process.

In many species, males are shorter-lived than females, and, mostly anecdotally, shorter lifespan was also attributed to Daphnia males. This does not necessarily stay in accordance with the biological roles of the sexes in Daphnia. Daphnia females maximize their fitness by maximizing the number of produced offspring, which incurs costs associated with quick attainment of large body size: metabolic costs of fast growth and increased risk of predation. In contrast, Daphnia males maximize fitness by maximizing the number of fertilized females, and seem to follow the strategy that enables them to maximize the lifetime female encounter rate, which should increase with lengthening lifespan. As arguments exist both in favour and against males living longer than females, we tested for differences in physiological lifespan of Daphnia magna males and females. Although maximum observed lifespan was always equal or longer in males than in females, no statistically significant differences were found. The results indicate that Daphnia males should not be considered short-lived anymore.