What is symbiosis?

Symbiosis is a phenomenon in which two living organisms live together in such a way that they are mutually beneficial to each other. For example, the colourful clownfish live symbiotic relationship with the sea anemones. The clownfish enjoy the protection of the poisonous nettle fibres of the sea anemone and the sea anemone survives on the left-over food of the fish. 

The lichens that live in association with a fungus and an alga are also good examples of symbiotic relationships. Through the close interaction between its symbiotic partners, the lichens are able to grow in places, where they could never have existed alone.