The earth’s greatest mass migration, (apparently) referred to as the diel vertical migration and also known as the diurnal vertical migration, takes place every single day in the ocean. In temperate and tropical regions, zooplankton migrates to the ocean surface at night and then moves down again during the hours of daylight. In a single day, this vertical movement may range from 400 to 1,000 meters, depending on the size and type of animal involved. In polar regions, where darkness lasts for several months, zooplankton migrates up and down on a seasonal basis, being at the surface during summer and at depth in winter.
It is thought that zooplankton rises to feed on the phytoplankton that lives in the surface waters, but then retreats to depth for safety, or possibly because it expends less energy in cooler water.