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Abstract

Recent years have provided further rejections of the metabolic and demographic advantage hypotheses for ultimate causes of diel vertical migration (DVM) in zooplankton, but strong support for the mortality-avoidance hypotheses. "Normal" DVM seems to be a response to predation by visually orienting predators whereas reverse DVM patterns are a response to "normally" migrating invertebrate predators. Several predictions of this hypothesis have been confirmed by experimental tests and observations in the field. For example: The hypothesis explains the timing of DVM; more conspicuous zooplankton are the strongest migrators; migration amplitude depends on the optical properties of the water; fish abundance affects migration amplitude. The strongest support comes from the recent discovery that DVM is an inducible response triggered by a cue (mostly chemical) from the predator. DVM is modified by environmental factors such as food and oxygen and also the strength of the trade-off between maximum protection and energetic input. However, avoidance of light dependent mortality (presumably visual predation) seems to provide a unifying concept to explain the evolution of this widespread behavior.