An example of mutualism is the coexistence of certain species of algae and fungi that together compose lichens. Their close association enables them to live in extreme environments, nourished only by light, air, and minerals. Living separately, the alga and fungus would not survive in such conditions. In parasitism, also known as antagonistic symbiosis, one organism receives no benefits and is often injured while supplying nutrients or shelter for the other organism. The type of symbiosis known as commensalism is a food-sharing association between two different kinds of nonparasitic animals, called commensals, that is harmless to both and in many cases is mutually advantageous.
Many commensals are free to separate.
Other commensals function together so completely that they cannot separate. They do not harm each other. An example is a polyp found in deep water off the coast of Newfoundland. It attaches itself to the shell of a certain species of hermit crab and, by budding, covers the entire shell with a colony that dissolves the original shell. Because the colony grows at the same rate as the crab, it furnishes continuous protection, and the crab does not shed its shell at periodic intervals as it normally would.