Diel Vertical Migration of Aquatic Crustaceans—Adaptive Role, Underlying Mechanisms, and Ecosystem Consequences

The phenomenon of diel vertical migration (DVM) of planktonic crustaceans, recognized by biologists for at least 2 centuries, is a special case of habitat selection behavior by pelagic animals, with their depth preference changing over a diel cycle in a way that prevents encounters with visually oriented predators (mostly fish). Migrating populations usually move toward cold, dark deep-water strata deprived of algal food when there is sufficient ambient light and move back to food-rich and warm surface waters after dusk. DVM has been recognized in pelagic representatives of all aquatic phyla of the animal kingdom and is considered the most massive diel biomass displacement on Earth. DVM can be observed in nearly all lentic freshwater and marine environments. As zooplankton occupy the central position in pelagic food webs, their massive migrations dramatically affect ecological functioning of offshore biota, particularly the efficiency of primary production utilization, energy flow, and biogeochemical pathways of essential nutrients such as carbon fluxes. The phenomenon of DVM is perhaps the most suitable for quantitative description and the major environmental factors underlying the fitness consequences of DVM, including vertical gradients of light intensity (predation risk), temperature related metabolic rates, food concentration (growth and fecundity), and others, are easy to monitor track in the field and to manipulate in laboratory systems. DVM, as inducible behavior, can be experimentally manipulated, both in the field and in the laboratory, which, in turn, makes it possible to design experiments convenient for testing specific hypotheses on various proximate and ultimate factors underlying this behavior. These characteristics make DVM suitable for investigating the evolution of animal behavior, its adaptive value, and ecosystem consequences. In the fondest memory of our friend Konrad Ciechomski with whom we made, years ago, our first steps into the world of plankton migrations.