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The answer is Habitat. The brackish water environment is formed due to the mixing of fresh water from the river with the salty waters of the ocean. This habitat is significant for fishes that come to spawn and feed. Crabs, mosquitoes and birds can also be found in this environment.
Differentiation occurs numerous times during the development of a multicellular organism as it changes from a simple zygote to a complex system of tissues and cell types.
