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    A wide variety of organisms make up plankton. Many cannot be seen without a microscope. Scientists often divide plankton into two main types based on the organisms it contains. These types are phytoplankton and zooplankton. Phytoplankton consists chiefly of simple, one-celled algae. Zooplankton includes microscopic protozoans and such sea animals as copepods, water fleas, and jellyfish.

    Some organisms spend their entire life as plankton. They are called holoplankton. Other organisms, called meroplankton, spend only part of their life as plankton. The most common meroplanktonic organisms are the eggs and larvae (immature forms) of animals that live on the floors of shallow oceans. These organisms drift freely as plankton until they develop into adults and settle to the bottom. Other meroplanktonic organisms include the eggs and larvae of larger, freely swimming animals, such as fish and squid.

    Plankton plays an important part in food webs in bodies of water. Food webs are the feeding relationships between organisms. Phytoplankton forms the base of these food webs. Phytoplankton can grow using only sunlight and the minerals in water in a process called photosynthesis. Certain zooplankton eat phytoplankton. These zooplankton are eaten by larger zooplankton and by fish and other water animals. Food material produced by plankton also may sink and be consumed by bottom-dwelling organisms.

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    Plankton are not to be completely associated with krill, another type of tiny water organism which also swarm in their millions. Krill are small, shrimplike animals that live in oceans throughout the world. Krill are crustaceans, a type of animal with a shell and jointed legs. There are more than 90 species of krill. They range in length from 3/8 inch to 6 inches. Krill are abundant in plankton, the mass of tiny organisms that provides an important source of food for marine life. Blue, fin, and humpback whales eat vast amounts of krill and other planktonic organisms daily.

    Many species of krill are surface dwellers, and others live at deeper levels or migrate up or down in the water. Animals that eat krill, including whales, seals, fish, and squids, follow along.

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      Young krill hatch from eggs as larvae, immature animals that do not resemble the adults. The larvae molt (shed their shell) as they grow. Krill larvae may go through as many as 10 molts before reaching the adult stage.

    As food, krill is rich in protein. However, various problems must be overcome before krill can provide a source of protein-rich food for people. For example, efficient harvesting methods must be developed to make large-scale krill fishing economically profitable. But more importantly, researchers need to determine whether such a large catch would jeopardize the food supply of other animals that eat krill.