What Is Zooplankton?

There are two basic forms of plankton: zooplankton and phytoplankton. Zooplankton (also known as "animal plankton") can be found in both saltwater and freshwater. There are estimated to be over 30,000 species of zooplankton.

Ocean Plankton

Ocean plankton are, for the most part, at the mercy of the vital forces of the seas. Having little or no powers of mobility, plankton are either too small to compete against ocean currents, waves, and wind conditions, or when large—as in the case of many jellyfish—lacking in sufficient propulsion to initiate movement on their own. 

Types and Classifications of Zooplankton

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Understanding the Definition of Plankton

By Jennifer Kennedy

Some species of zooplankton are born as plankton and remain so for their entire lives. These organisms are known as holoplankton and include such tiny species as copepods, hyperiids, and euphausids. Meroplankton, on the other hand, are species that begin life in a larval form and progress through a series of life stages to evolve into gastropods, crustaceans, and fish.

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 2-20 µm in size which includes some copepods and other zooplankton.
• Mesoplankton: Organisms 200 µm-2 mm in size, which includes larval crustaceans.
• Macroplankton: Organisms 2-20 mm in size, which includes euphausiids (such as krill), an important food source for many organisms, including baleen whales. 
• Micronekton: Organisms 20-200 mm in size, which includes some euphausiids and cephalopods.
• Megaloplankton: Planktonic organisms greater than 200 mm in size, which includes jellyfish and salps.
• Holoplankton: Organisms that are planktonic throughout their entire lives, such as copepods. 
• Meroplankton: Organisms that have a planktonic stage, but mature out of it, such as some fish and crustaceans. 

Zooplankton's Place in the Food Web

Marine zooplankton are consumers. Rather than getting nutrition from sunlight and nutrients via photosynthesis like phytoplankton, they must consume other organisms in order to survive. Zooplankton may also be carnivorous, omnivorous, or detrivorous (feeding on waste). 

Many species of zooplankton live in the euphotic zone of the ocean—the depths at which sunlight can penetrate—feeding on phytoplankton. The food web begins with the phytoplankton, which are primary producers. Phytoplankton convert inorganic substances including energy from the sun and nutrients such as nitrate and phosphate into organic substances. The phytoplankton, in turn, are eaten by zooplankton, who are consumed by ocean creatures ranging in size from smaller fish and gastropods to gigantic whales. 

The days for many species of zooplankton often involve vertical migration—ascending toward the ocean surface in the morning when phytoplankton are more plentiful, and descending at night to escape predation. Since zooplankton generally comprise the second step in a food web in which they dwell, this daily ascent and descent has an impact on the rest of the species they feed on, and in turn, those that feed on them.

Zooplankton Reproduction

Zooplankton may reproduce sexually or asexually, depending upon species. Asexual reproduction is more common for holoplankton and can be accomplished through cell division, in which one cell divides in half to produce two cells, and so on. 

Sources

• Harris, R., Wiebe, P., enz, J., Skjoldal, H-R., and M. Huntley.  ICES Zooplankton Methodology Manual.
• Marine Education Society of Australasia. Zooplankton.
• 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. 
• Census of Marine Zooplankton 
• "Zooplankton." Center for Coastal Studies Provincetown.