Coupling of anti-predator defences in Daphnia: the importance of light

Here we test for the possible coupling of two kairomone-induced, anti-fish defences in Daphnia, life-history changes (LHC) and diel vertical migration (DVM) mediated by the environmental factor light. A gradient of five different light intensities that represents naturally occurring intensities in the lake water column was used in life-history experiments, and we show that LHC of a single Daphnia clone are inversely coupled to the ambient light intensity. Furthermore, we could show that the light intensity has to exceed a threshold to induce the LHC. We also observed an effect of the light intensity on the fish kairomone-mediated expression of a candidate gene (actin 3) in a way that the gene response differs between groups of low and high light intensities. We argue that the ambient light intensity an individual is exposed to and that is dependent on the position in the water column of a lake inversely determines the degree of LHC. These findings suggest a plastic coupling of these two anti-fish defences in Daphnia (LHC and DVM), which allows for an adjustment to fluctuating environments of standing freshwater ecosystems.