They are the microscopic plants and animals.
These tiny plants and animals float freely with the movement of ocean and are drifter.
They are extremely small in size and are mainly concentrated in the upper layers of water-where they can absorb sunlight and mineral nutrients.
Planktons are both phytoplankton (plant) and zooplankton (animal).
(i) Phytoplankton: All marine creatures are directly or indirectly dependent on phytoplanktons.
Phytoplanktons are found only where conditions are favourable for their reproduction and growth.
For example, their reproduction is closely related to availability of sunlight, optimum temperatures, salinity and nutrients.
A common example of phytoplankton is the diatom. Diatom is a unicellular plant covered with siliceous deposits. It forms a major part of diet of a number of sea-animals.
(ii) Zooplanktons are microscopic animal creatures.
They include tiny jelly fish, arrow worms, small crustaceans and copepods.
Different zooplankton lives under different conditions of temperatures and salinity. They depend on phytoplankton for their food.