Environmental stress and local adaptation in Daphnia magna

The effects of fish kairomones, crowding chemicals, and day length on the l ife-history traits of a set of 16 Daphnia magna clones, derived from four p opulations that differ in fish-predation pressure, were studied. Significan t among-population differences were observed, the differences being in conc ordance with the hypothesis of local adaptation. The among-population genet ic differences were not mediated through a change in response to fish kairo mones, but through an overall smaller body size, smaller eggs, and a higher number of eggs in clones derived from habitats in which fish are present. Using a model, we show that the observed changes in life-history characteri stics may lead to differences in fitness under different predation regimes, such that populations from habitats with fish have highest fitness under f ish-predation regimes and populations without a fish background have higher fitness values under invertebrate predation regimes.