Ultimate causes of diel vertical migration of zooplankton: New evidence for the predator-avoidance hypothesis

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