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The Definition of Zooplankton

Plankton is a catchall term for any living thing that floats freely in the ocean. Plankton is, for the most part, at the mercy of ocean currents. The two major types of plankton are phytoplankton and zooplankton. Phytoplankton is tiny algae and plants. Zooplankton is any small animal that lives all or part of its life as plankton. However, zooplankton has many different types, including almost every major animal group, from protists to tiny vertebrates.

Size Types
- Free-living bacteria are among the smallest plankton.
  
  One of the major ways that zooplankton is classified is by the plankton size. The small side of the scale starts with nanoplankton, which ranges from two um to 20 um. Next comes microplankton, which ranges from 20 um to 200 um. After microplankton comes mesoplankton, which ranges from .2 mm to 200 mm. Last comes the macroplankton, which includes any plankton larger than 200 mm, up to 8 inches. For comparison, 1 um is about four hundred-thousandths of an inch.
Development Types
- Many crustaceans live as plankton during their larval stages.
  
  Zooplankton can also be classified according to their life cycles. Two main categories of zooplankton are in this classification scheme. The first is holoplankton, which lives its entire life out as plankton. The second is meroplankton, which includes animals that only live part of their life, usually a larval stage, as plankton. Many invertebrates or fish live part of their life as meroplankton before settling out and growing into their adult forms.
Holoplankton
- Krill are among the largest plankton.
  
  Holoplankton includes various animals, bacteria and protists. Flagellates, a type of bacteria, are both holoplankton and nanoplankton. Flagellates are bacteria that swim with a tail-like appendage. Various worms, including arrow worms, are adapted to a life in the current with fins along their tiny bodies. Cnidarians -- jellyfish and their relatives -- can run the gamut through the various types of plankton, all the way up to macroplankton. Krill are among the largest plankton, at 3 cm, making them macroplankton.
Meroplankton
- Clownfish live as planktonic larva before growing into their adult forms.
  
  Many ocean and freshwater animals live part of their life as plankton. In addition to the Cnidarians that live as holoplankton, some like coral and anemones may have a short, free-swimming larval state. Some ocean snails, called gastropods, have a planktonic stage to their life. In freshwater, some insects lay eggs in water, which hatch into planktonic larva. And even some vertebrates like fish get in on the act, living as plankton before growing into their adult forms.

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