What Is Zooplankton?

Zooplankton
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Zooplankton may be referred to as "animal plankton" -- they are organisms that are often at the mercy of the ocean's currents, but unlike phytoplankton, are not capable of photosynthesis

Background on Plankton

Plankton are largely at the mercy of the ocean's currents, winds and waves, and don't have much (if any) mobility. Zooplankton are either too small to compete against the currents in the ocean, or are large (as in the case of many jellyfish), but have relatively weak propulsion systems.

 The word plankton comes from the Greek word planktos meaning "wanderer" or "driter." The word zooplankton incorporates the Greek word zoion, for "animal." 

Species of Zooplankton

There are thought to be over 30,000 species of zooplankton. Zooplankton can live in fresh or salt water, but this article focuses largely on marine zooplankton. 

Types of Zooplankton

Zooplankton may be classified according to their size or by the length of time they are planktonic (largely immobile).  Some terms that are used to refer to plankton include:

  • Microplankton: organisms that are 2-20 µm in size -- this includes some copepods and other zooplankton.
  • Mesoplankton: organisms that are 200 µm-2 mm in size, which includes larval crustaceans.
  • Macroplankton: organisms 2-20 mm in size, which includes euphasiids (e.g., krill) - an important food source for many organisms, including baleen whales
  • Micronekton: organisms 20-200 mm in size. Examples include some euphasiids and cephalopods.
  • Megaloplankton: planktonic organisms greater than 200 mm in size, which includes jellyfish and salps.
  • Holoplankton: organisms that are planktonic throughout their entire live, such as copepods. 
  • Meroplankton: organisms that have a planktonic stage, but grow out of it at some point, like fish and crustaceans. 

    You can see a list of marine zooplankton groups with examples, at the Census of Marine Zooplankton web site

    What Do Zooplankton Eat?

    Marine zooplankton are consumers. Instead of getting their nutrition from sunlight and nutrients in the ocean, they need to consume other organisms.  Many feed on phytoplankton, and therefore live in the euphotic zone of the ocean -- the depths in which sunlight can penetrate. Zooplankton may also be carnivorous, omnivorous or detrivorous (feed on detritus).  Their days may involve vertical migration (e.g., ascending toward the ocean surface in the morning and descending at night), which effects the rest of the food web. 

    Zooplankton and the Food Web

    Zooplankton are basically the second step of the oceanic food web.  The food web starts with the phytoplankton, which are primary producers. They convert inorganic substances (e.g., energy from the sun, nutrients such as nitrate and phosphate) into organic substances. The phytoplankton, in turn, are eaten by zooplankton, who are eaten by smaller fish and even gigantic whales. 

    How Do Zooplankton Reproduce?

    Phytoplankton may reproduce sexually or asexually, depending upon species.  Asexual reproduction occurs more often, and can be accomplished through cell division, in which one cell divides in half to produce two cells.

     

    Sources

    • Harris, R., Wiebe, P., enz, J., Skjoldal, H-R., and M. Huntley.  ICES Zooplankton Methodology Manual. Accessed May 30, 2014.
    • Marine Education Society of Australasia. Zooplankton. Accessed May 30, 2014. 
    • Morrissey, J.F. and J.L. Sumich. 2012. Introduction to the Biology of Marine Life, Tenth Edition. Jones & Bartlett Learning, LLC.  467pp.
    • Woods Hole Oceanographic Institution.  Jellyfish and Other Zooplankton.  Accessed May 30, 2014. 
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    Kennedy, Jennifer. "What Is Zooplankton?" ThoughtCo, Mar. 22, 2018, thoughtco.com/zooplankton-definition-2291632. Kennedy, Jennifer. (2018, March 22). What Is Zooplankton? Retrieved from https://www.thoughtco.com/zooplankton-definition-2291632 Kennedy, Jennifer. "What Is Zooplankton?" ThoughtCo. https://www.thoughtco.com/zooplankton-definition-2291632 (accessed May 21, 2018).