How the reef changes at dusk

As the light fades, parrotfish leave their daytime feeding grounds and look for shelter. Some species spend the night inside a cocoon they make from mucus. Soldierfish and squirrelfish usually head the other way, out of shelter and into the open to feed.

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.
Some small animals rise in the evening and descend in the morning. The arrows show daily vertical migration, rising at dusk and descending at dawn. © dive.explo360.fr — All rights reserved. Reproduction requires permission.

A vast migration begins each evening

Copepods and other zooplankton can stay in deeper water during the day and rise in the evening. Near the surface, they find food such as phytoplankton, which includes tiny algae.

A migration that feeds the depths

Phytoplankton use light to turn dissolved carbon into part of their own bodies. A copepod that eats them and swims down carries some of that carbon into deeper water. It can release carbon there through respiration or waste, or pass it on to a predator that eats it. Sinking particles carry carbon down under gravity, too.

A copepod swims towards deeper water as particles sink beside it.
Small animals carry their food downwards. Leftover material also sinks through the water. Active transport by migrating animals complements the sinking of particles into deeper waters. © 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.

Concentrations of zooplankton attract fish and squid. Some change depth and others swim towards places rich in prey. Even when a small animal is hard to see, a predator can detect the movement it makes in the water.

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