Sometimes two different plants, or a plant and an animal, live together for their mutual advantage. This is symbiosis.

Lichens, for example, that grow on rocks and walls, are made up of a fungus and an alga (a simple green plant) living within it. The fungus protects the alga to some extent, and may be able to store water in dry periods, while the alga is able to photosynthesize and so provide food for the fungus. Other minute algae live in corals.

Commensalism is a less close association than symbiosis. It is a food-sharing relationship, but sometimes one party is the provider, receiving little or nothing in return.

The hermit crab has no hard shell but lives in the abandoned shell of another creature such as a whelk. Quite frequently a sea anemone grows on this shell, and is able to eat leftover food that drifts upwards from the hermit crab's meals. Sometimes a worm shares the crab's borrowed home as well as its sources of food. Another example of commensalism is the association between a shark and the pilot fish that swims ahead of it feeding from the remains of its much larger partner's meals.