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The effect of relative increases in light intensity on photobehaviour was studied in the hybrid  Daphnia galeata  ×  hyalina . We first carried out a series of experiments to study the influence of fish kairomone on several response variables of light-induced swimming. With fish kairomone present, an increase in the percentage of reacting daphnids to 100% was found at almost all ecologically occurring relative light change rates that were above threshold. The relationship between the relative increase in light intensity (stimulus) and the time expiring between the onset of the stimulus and the start of the downward swimming response was not influenced by fish kairomone, nor did kairomone alter the functional relationship between stimulus strength and downward displacement velocity, although velocity increased. During the previous experiments, various light change rates were applied, but per test run these rates were constant. The natural relative light increase in the early morning consists of continuously increasing relative light change rates, turning into decreasing rates after the maximum is reached ∼30–45 min before sunrise. In a second series of experiments, one step increase in the rate of the relative light change was applied. We found that displacement velocity was higher than that expected if the second rate of increase had been given alone. Taking into account the newly found stimulus-acceleration effect, light-induced swimming might explain the amplitude of diet vertical migration found in lakes.

The effect of accelerations in light increase on the phototactic downward swimming of Daphnia and the relevance to diel vertical migration