Migration in the Ocean Twilight Zone

Do animals in the ocean twilight zone migrate?

In short, yes. As the sun sets and darkness descends on the ocean’s surface, many animals living in the ocean twilight zone—like fish, mollusks, crustaceans, and others—rise up to the surface en masse to feed on plankton and other microorganisms. When the sun reappears at dawn, they descend back to the murky depths, where they’ll stay until night comes again. This phenomenon is called the Diel Vertical Migration, and is the largest migration of animal life on earth. The migration’s name comes from the latin dies, or day, which describes the regularity with which it occurs. The process plays a major role in the health of the marine ecosystem, and may even help pull CO2 out of the atmosphere—yet scientists are only starting to understand its importance. 

Why does vertical migration from the ocean twilight zone occur?

There are a number of possible reasons for vertical migration. One explanation is that it helps animals living in the twilight zone avoid predators, which can hunt by seeing the faint outline of their prey above them. Traveling into shallower water to feed during daytime might be dangerous for many species, by moving at night, they may be able to avoid being spotted. Another possible reason for this mass vertical migration is that it may help animals conserve energy by feeding in the warm surface waters at night and residing in the cooler deep waters during the day. Alternately, organisms feeding on the bottom in cold water during the day may migrate to surface waters at night in order to digest their meal at warmer temperatures.

What dangers do migrating animals face?

Each time an animal migrates up or down in the water, it runs a dangerous gauntlet. Along the way, it may face passive predators like jellyfish, which lie in wait to ensnare, entangle, or engulf the vertical commuters. It may also run into layers of active predators, which lure, track, or chase nearby prey. Many important commercial species, like tuna and swordfish, also feed on migrating animals, and some species of sharks make deep foraging dives into the twilight zone itself.

How can vertical migration affect global climate?

Animals living in the twilight zone remove huge amounts of carbon from the upper ocean as they feed on plankton near the surface. When they migrate back into deeper waters, they bring that carbon down with them. Some are eaten by other animals living in the twilight zone—and as those animals defecate, the carbon in their waste sinks to the bottom where it remains for millenia.

How do we plan to study vertical migration?

New technology such as Deep-See will help measure the true biomass of organisms—migrating and otherwise—living in the ocean twilight zone. Other robotic submersibles, like Mesobot, can track individual organisms autonomously throughout the ocean, giving scientists new insight on the behavior of individual migrating species. Together, these flagship tools are helping tease out the light-following behavior of twilight zone residents, and can measure and record previously unknown details about the largest animal migration on the planet.

Researchers can also look directly at fish samples to gain insight on how life in the twilight zone behaves. By dissecting these samples, scientists can study gut contents, reproductive organs, and structures called ear stones, which can help determine the age of individual fish and how fast they grow.

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