Diel vertical migration

Diel vertical migration (DVM), also known as diurnal vertical migration, is a pattern of movement used by some organisms, such as copepods, living in the ocean and in lakes. The adjective "diel" (IPA: /ˈdaɪ.əl/, /ˈdiː.əl/) comes from Latin: diēs, lit. 'day', and refers to a 24-hour period. The migration occurs when organisms move up to the uppermost layer of the water at night and return to the bottom of the daylight zone of the oceans or to the dense, bottom layer of lakes during the day. DVM is important to the functioning of deep-sea food webs and the biologically-driven sequestration of carbon.

In terms of biomass, DVM is the largest synchronous migration in the world. It is not restricted to any one taxon, as examples are known from crustaceans (copepods), molluscs (squid), and ray-finned fishes (trout).

The phenomenon may be advantageous for a number of reasons, most typically to access food and to avoid predators. It is triggered by various stimuli, the most prominent being changes in light-intensity, though evidence suggests that biological clocks are an underlying stimulus as well. While this mass migration is generally nocturnal, with the animals ascending from the depths at nightfall and descending at sunrise, the timing can alter in response to the different cues and stimuli that trigger it. Some unusual events impact vertical migration: DVM can be absent during the midnight sun in Arctic regions and vertical migration can occur suddenly during a solar eclipse. The phenomenon also demonstrates cloud-driven variations.

The common swift is an exception among birds in that it ascends and descends into high altitudes at dusk and dawn, similar to the vertical migration of aquatic lifeforms.