This photo was taken by Uwe Kils. It shows an amphipod, a type of plankton. The photo was taken using magnification so users can see the amphipod in detail. In reality, most amphipods are only 1 mm to 140 mm in length.
Click on image for full size
Courtesy of Wikipedia Commons


Plankton are a diverse set of marine organisms that live in both salt and fresh water. Some forms are able to move independently, but in general plankton drift with the currents in the body of water they inhabit. This is the primary way of defining plankton — they are drifters, and cannot actively swim against the current. Plankton are usually divided into three functional groups, which are separated based on function rather than genetic similarity. These groups are:

Phytoplankton—these are photosynthetic autotrophs (i.e., they can feed themselves using light from the Sun). They are typically single-celled organisms and can be either eukaryotic or prokaryotic (with or without a nucleus). They live in water that is shallow enough for sunlight to shine through. Phytoplankton are responsible for producing a large fraction of the world’s oxygen, and they serve as food for a wide variety of other organisms. Phytoplankton are generally considered the foundation of most of the world’s marine food chains.

Zooplankton—this is a broad group of heterotrophic organisms (i.e., they depend on other organisms for food) that live throughout the water. The zooplankton range in size from single-celled creatures to larger animals like jellyfish, mollusks, and crustaceans, and they feed on other forms of plankton as well as on organic wastes in the water.

Bacterioplankton—this group is composed of the bacteria that drift throughout open water. They can be autotrophic or heterotrophic. They play a large role in a number of critical geochemical cycles, where elements like nitrogen, carbon, and a variety of minerals are processed by the bacteria to keep organic and inorganic (or biologically accessible and biologically inaccessible) forms of each in the proper balance in the ocean. In this way, bacterioplankton are often thought to serve as the ocean’s recyclers.

Last modified June 1, 2010 by Lisa Gardiner.

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