The group of animal plankton includes small heterotrophic organisms that inhabit water ecosystems. Heterotrophic organisms receive energy by consuming other living organisms.  

Some animal plankton, such as ciliates, are unicellular protists. The group of animal plankton also includes small multicellular crustaceans such as copepods and water fleas.

The smallest animal plankton are only 50 micrometres in diameter, whereas the largest are a few millimetres in diameter. When you gather animal plankton in a test tube, you can distinguish small movements even without a microscope!

Most animal plankton move by flowing along water currents. Some animal plankton can also swim by using their antennas or cilia.

Animal plankton use smaller phytoplankton or other small organisms as their source of energy. They themselves are also the food of other organisms. Especially small or young fishes like to eat animal plankton.

The group of animal plankton is often considered to contain only small, microscopic animals. However, our waters are inhabited by a number of larger species that can be considered plankton. The most famous of these is the jellyfish found in the Baltic Sea. Large jellyfish can swim, but most of the time they flow along water currents.

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The jellyfish is a cnidarian. It can swim by contracting its jelly-like body cavity.