Zooplankton exhibit a variety of daily cycles including vertical and horizontal migrations, changes in feeding behavior and alternating reproductive states. The most popular hypothesis to explain the adaptive advantage of diel vertical migration is predator avoidance, i.e., nocturnal
vertical migrations afford protection from visually feeding predators, whereas reverse (diurnal) migrations result from avoidance of nocturnally migrating, nonvisual predators. Proposed metabolic advantages of vertical migration have received much attention, but relatively little experimental
support. Nocturnal migrations may also represent an adaptive behavior for avoidance of damaging solar radiation. One is struck with the variations in diel behavior patterns and the apparent plasticity of zooplankton in adapting diel behaviors to fit specific environments. Recent studies considering
multiple causes of vertical migration show much promise. Problems and improvements in studies of diel zooplankton behavior are discussed.
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