When the Sea Comes Alive at Night — Adults (18+)

Dusk on the reef

After sunset, colour and contrast gradually fade underwater. Shadows deepen around the rocks.

Parrotfish, which mostly feed by day, generally stop foraging and seek shelter. Some make a mucus cocoon and spend the night inside it.Soldierfish and squirrelfish often do the reverse. They spend much of the day beneath ledges or inside cavities and emerge to feed after dark.A rocky ledge can look quiet by day and be busy a few hours later. The same shelter and feeding grounds can serve different animals over the course of a day.Offshore, the change at dusk plays out across the water column.

One shelter, two visits. Imagine watching a cavity just before sunset and again an hour later. Which animals went in? Which came out?

Day–night diagram: zooplankton deeper by day and near the surface by night. — The upward arrow shows the journey towards the surface at dusk. The downward arrow shows the return to deeper water before daylight. Organisms are enlarged and depths are not to scale. © dive.explo360.fr — All rights reserved. Reproduction requires permission.

A vast migration begins each evening

The water column holds copepods, krill, larvae and countless other small drifting or swimming animals. Many spend part of the day at one depth and part at another.

In daylight, some of this zooplankton stays in deeper, dimmer water. Towards evening, the animals begin to rise. Some swim tens of metres, others hundreds.This daily change of depth is called diel vertical migration. For many animals, the climb brings them into reach of a meal.

Feeding with less light

What makes the climb worth the effort?For a small animal, the upper waters offer plenty to eat and a greater chance of being seen.Phytoplankton, including microscopic algae, use sunlight for photosynthesis. They grow mainly in the ocean’s illuminated layer, where they provide food for many of the animals that rise after dark.By day, fish that hunt plankton by sight can pick out their prey more easily near the surface. For many species, staying deeper reduces that risk.After dark, migratory animals can rise to feed in dimmer conditions. Many head down again before daylight returns.Some species barely change depth, while others travel in the opposite direction. The distance depends on conditions such as season, light, depth and the predators nearby.Taken together, these daily journeys amount to one of the largest regular movements of animal biomass on Earth.

A migration that feeds the depths

A zooplankton animal rising to feed can end up moving carbon from one part of the ocean to another.

Near the surface, phytoplankton use photosynthesis to incorporate dissolved inorganic carbon into organic matter. That carbon enters the food web when the phytoplankton is eaten.A copepod that feeds near the surface and then swims down carries some of the organic carbon from its meal with it.In deeper water, it can release some of that carbon through respiration or waste, or pass it on to another animal that eats it.This is active carbon transport. The animals move the carbon themselves as they swim between depths.Other organic matter sinks as faecal particles, dead organisms or clumps of debris.Those particles descend under gravity. Migrating animals can carry part of a meal down while they are still alive.Researchers estimate the size of this contribution to the ocean’s biological pump. Estimates vary with the region and the animals involved. How long the carbon stays away from the surface is a separate question.Animals only a few millimetres long can contribute to this transport by making the same journey night after night.

A copepod swims towards deeper water as particles sink beside it. — Two routes to deeper water. The copepod swims down carrying some of the material it ate. The organic particles on the right sink under gravity. Organisms and distances are not to scale. © dive.explo360.fr — All rights reserved. Reproduction requires permission.

Night reshuffles prey and predators

Follow a copepod. As it swims up, it leaves dark water, reaches a rich feeding area and may encounter a different set of hunters.

As zooplankton rise, they bring food within reach at new depths.

Fish, squid and other predators can feed where the small animals concentrate. Some move between depths themselves. Others swim across the same layer towards a patch with more prey.Several open-ocean squid move between deeper and shallower waters. The movements are associated with feeding and avoiding predators.Darkness changes which predators a small animal is likely to meet and how they detect it.A crustacean that is harder for one fish to see can still give itself away by disturbing the water.Some fish have eyes suited to dim light. Other hunters can detect movement or chemical cues left by prey.As visibility falls, an animal that escapes one hunter may come within reach of another.

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