Quantitative effects of fish kairomones and successive light stimuli on downward swimming responses of Daphnia

Swimming in response to light change is considered the proximate mechanism underlying diel vertical migration. This behavioural mechanism is supposed to be tuned to the adaptive needs under natural conditions by the modifying influence of environmental variables, such as predator kairomones. We investigated to what extent fish kairomone level affects downward swimming in response to a continuous relative light intensity increase. At the higher kairomone levels Daphnia had significant higher displacement velocities. Thus, kairomone was perceived quantitatively and behaviour was altered correspondingly. Because the absolute difference in displacement velocity between treatments was small, we assumed that kairomone concentration could not explain much of the seasonal variability in diel vertical migration patterns in Lake Maarsseveen. Therefore, we hypothesised that besides swimming in response to relative changes in light intensity, other aspects of phototaxis were important also. The natural light intensity increase at early morning consists of a continuous increase in the rate of the relative light intensity change, which reach a maximum about 30–45 min before sunrise. After this maximum, the rates of the relative light intensity increase decrease again. Thus far, it was assumed that successive changes in light intensity acted independent on the swimming reaction of the daphnids. In this paper, we present results of experiments with Daphnia swimming in response to a combination of a continuous and an instantaneous increase in light intensity. A continuous relative light increase preceding an instantaneous increase in light intensity (step) enlarged the downward displacement velocity of Daphnia markedly. Hence, successive light stimuli did not act independently. With growing relative light increase rates preceding the step, the displacement velocity increased also. Moreover, the presence of fish kairomone further increased the velocity in response to the light change. Therefore, the rate of the relative change in light intensity is an incomplete description of the stimulus for diel vertical migration behaviour.