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Zooplankton are tiny aquatic animals that cannot move against currents in the water. Zooplankton are either herbivorous, feeding on phytoplankton, or carnivorous, feeding on other zooplankton. They themselves are fed upon by other zooplankton, fish, and even whales. Zooplankton is the vital transition between marine primary production (phytoplankton) and large animals (fish).

Zooplankton, like all plankton, exist in the epipelagic zone of the ocean. It is possible for these creatures to move up and down in the water, (diel vertical migration). However, if the organism sinks too low, it will not be able to reach a suitable height, and will be washed out of the system.

Some types of marine zooplankton include: Meroplankton (organisms that spend only their larval stages as plankton)

Holoplankton (organisms that spend their entire lives as plankton)

Zooplankton are also found in freshwater systems, and play very much the same role in the food web as described by Zarah. The functional classification of different types of zooplankton is different in lakes and rivers. Instead of being grouped into holo- and meroplankton, limnologists tend to refer to zooplankton as being either pelagic or benthic. Also, fish larvae are not normally grouped with other zooplankton in freshwater systems; they are considered independently.

One of the principle differences between marine and freshwater zooplankton is their mean size. In lakes and rivers, it is rare to find zooplankton species larger than 5 millimeters in total length, while this is common in marine systems. However, in the freshwater environment we commonly encounter densities several orders of magnitude greater than in marine systems. They generally consume phytoplankton, but can consume other zooplankton and bacteria. In freshwater systems, the larger zooplankton species can swim against all but the strongest currents, and also perform diel vertical migration.

Most freshwater zooplankton species are crustaceans (the notable exceptions being the rotifers and some rare species of freshwater jellyfish). Some common freshwater zooplankton taxonomic groups are:

  • Daphnia -- These are some of the largest zooplankters, and are commonly called water fleas
  • Copepods -- The three types are cyclopoids (predatory), calanoids (herbivorous) and harpactacoids (benthic)
  • Rotifers -- there are a phenomenal number of species of these small zooplankton (generally < 250 μm)
  • Sididae -- These are large benthic crustaceans
  • Mysis -- A freshwater shrimp species

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