Zooplankton forms the second trophic level in an ecosystem. These are heterotrophic planktons that play an important role in the transfer of energy in the aquatic food chain. These can be found in both, marine and freshwater ecosystems. Most of the zooplankton are found in the upper part of the oceans where maximum sunlight reaches to sustain the phytoplankton that form the first trophic level of the aquatic food chain and are also the food for many zooplankton. Crabs, shellfish, jellyfish are a few examples of zooplankton. The microzooplankton includes the unicellular protozoans that feed on the phytoplankton cell present in the oceans.