ZOOPLANKTON DIEL VERTICAL MIGRATION — A REVIEW OF PROXIMATE CONTROL

Diel vertical migration (DVM) is a characteristic behavioural pattern performed by zooplankton in which their vertical distribution changes over the 24-h day. Here the proximate control of zooplankton DVM is reviewed. Light has emerged as the major proximate cue controlling DVM behaviour and the understanding of zooplankton visual physiology and the light-mediated behaviour underlying DVM is expanding. Field and laboratory evidence exist to support each of the three major hypotheses for the exogenous role of light in DVM: (1) preferendum or isolume, (2) absolute intensity threshold, and (3) relative rate of change. Light may also play an endogenous role in DVM by entraining circadian rhythms in vertical movement or activity. This appreciation of the role of light has improved modelling efforts into the causes and consequences of DVM. The most important recent advance in the study of DVM is the recognition that this behaviour is a phenotypic response in many species and is most commonly activated by chemical cues (kairomones) from fish predators. High levels of kairomones signal high levels of predation pressure, and DVM-related photobehaviours, such as swimming responses on relative rates of irradiance change, are altered such that migration occurs and zooplankton achieve a refuge from visual predators.