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

The effect of relative increases in light intensity on photobehaviourwas studied in the hybrid Daphnia galeata x hyalina. We firstcarried out a series of experiments to study the influence offish kairomone on several response variables of light-inducedswimming. With fish kairomone present, an increase in the percentageof reacting daphnids to 100% was found at almost all ecologicallyoccurring 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 stimulusand the start of the downward swimming response was not influencedby fish kairomone, nor did kairomone alter the functional relationshipbetween stimulus strength and downward displacement velocity,although velocity increased. During the previous experiments,various light change rates were applied, but per test run theserates were constant. The natural relative light increase inthe early morning consists of continuously increasing relativelight change rates, turning into decreasing rates after themaximum is reached