BEHAVIORAL MECHANISMS CONTROLLING VERTICAL MIGRATION IN DAPHNIA

We used infrared video monitoring to study cladoceran vertical migrati on in laboratory tanks exposed to natural and simulated daylight cycle s. There are changes in the behavior patterns at different times of th e year, with both Daphnia magna and Daphnia longispina showing increas ed vertical migration and greater average depth later in the year. Thi s change in behavior pattern is due to temperature, not daylength, cha nges. Our animals show a complex endogenous migration pattern, with a strong downward excursion in the middle of the night, another before d awn, and then a morning rise, of which only the last depends on a visu al stimulus. We propose that the morning rise acts as a correcting mov ement bringing animals to a depth just safe from visual predators. The midnight sinking is unlikely to be due to a random spreading-out proc ess. Fish taste cues enhanced the dawn excursion, but not the midnight one. Responses to fish cues were most pronounced for D. magna at low temperatures.