Plankton are a floating mass of life formed by billions of tiny living organisms. Some plankton are as big as large jellyfish whereas some are so tiny that they are not visible even under a microscope. Some common plankton are algae, bacteria, protozoan, etc. Two main categories of plankton are plant like, phytoplankton and animal like, zooplankton. Young crustaceans like shrimps, crabs, lobsters, crayfish and water fleas are the examples of zooplankton. Young molluscs such as snails and clams are plankton in the early stages of their life.