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. They can live in salt and fresh water. Although some forms are able to move independently, most plankton drift with the water currents. This is actually the main way of defining plankton — they are drifters, and cannot actively swim against the current. Plankton are usually divided into three groups:

Phytoplankton—these are photosynthetic autotrophs (that is they can feed themselves using light from the Sun). They are typically single-celled organisms and can have a nucleus or not. They live in water that is shallow enough for sunlight to shine through. Phytoplankton are responsible for producing a lot of the world’s oxygen, and they serve as food for many other organisms. Phytoplankton are considered the foundation of most of the world’s marine food chains.

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

Bacterioplankton—this group is composed of the bacteria that drift in open water. They can be autotrophic or heterotrophic. They play a large role in important geochemical cycles, like the nitrogen and carbon cycles. 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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