Every day, billions upon billions of zooplankton make their way to deep water in the morning and rise to the surface as the sun sets. This process of vertical migration is carried out all over the world by marine and freshwater plankton. The reason for this daily commute is assumed to be a well synchronized routine for obtaining food at the surface of the ocean and avoiding predators that feed at the ocean’s surface during daylight hours.
Phytoplankton provide food for a wide range of sea creatures including whales, shrimp, snails, and jellyfish. They are similar to terrestrial plants in that they contain chlorophyll and require sunlight in order to live and grow. Most phytoplankton are buoyant and float in the upper part of the ocean where sunlight penetrates the water allowing photosynthesis to take place.
Zooplankton feed on phytoplankton and are thought to feed at the surface when darkness falls and move to deeper, darker water during the day to avoid predators. This daily vertical migration allows zooplankton to make the most of an abundant food source without making themselves an easy meal by remaining at the ocean’s surface during the day. Zooplankton have adopted other protective strategies as well such as transparency to avoid being seen by predators and collecting into schools or swarms.
Most plankton are not capable of purposeful swimming. That is, though they can move, their movements are incapable of overcoming the insurmountable force of waves and currents. The name plankton comes from a Greek word that means “wanderer”.
It is believed zooplankton move up and down in a water column by controlling the amount of gas in their cells. This is not a purposeful movement like an animal swimming. It is an adaptation that allows these creatures to move up and down with a minimum of effort from the ocean’s depths to the surface to feed.