Symbiosis (Greek symbioun, “to live together”), in biology, term for the interdependence of different species, which are sometimes called symbionts. There are three main types of symbiosis, based upon the specific relationship between the species involved: mutualism, parasitism, and commensalism.

Symbiosis that results in mutual benefit to the interdependent organisms is commonly known as mutualism. 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. Another example is the relationship between most mycorrhizae and certain plants. Mycorrhizae are fungal growths on the roots of such plants as heaths, orchids, and many conifers. The fungi penetrate the roots of the plants and make soil nutriments such as nitrogen available to the plants, receiving carbohydrates in return.

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 (see Parasite). Parasites include viruses and bacteria that cause many diseases; certain protozoans that can infect plants and animals; tapeworms and flukes that infest the intestinal tracks and internal organs of animals; and external parasites such as lice and ticks. There are also parasitic plants like mistletoe that draw their nourishment from the branches of other plants.

The type of symbiosis known as commensalism is an association between two different kinds of nonparasitic animals, called commensals, that is harmless to both and in which one of the organism benefits. Many commensals are free to separate. Other commensals function together so completely that they cannot separate. They are not considered parasitic, however, because they do not harm each other. An example is a polyp found in deep water off the coast of Newfoundland and Labrador. 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. The polyp, in turn, benefits by moving about with the crab, thereby obtaining a greater food supply than it would if attached to a stationary object. Commensalism is most common among marine invertebrates, but it often occurs among land animals—for example, in the association of ants with other insects such as aphids and beetles. The association of colon bacteria with humans and other animals, especially plant-eating animals, is also a type of commensalism.