Symbiosis
Symbiosis, a close association between two different organisms. Symbiosis is a Greek word that means “living together.” In one type of symbiosis, known as mutualism, both partners benefit from the association. Some biologists restrict the term symbiosis to those relationships that are mutualistic; that is, they use the words symbiosis and mutualism as synonyms. However, many biologists consider other types of relationships, such as commensalism and parasitism, as kinds of symbiosis. All three types are discussed in this article.
Mutualism takes place between certain species of ants and aphids. Aphids give off a sweet liquid called honeydew which the ants eat. In turn, the ants protect the aphids from their enemies. Another example of mutualism is provided by the lichen—an organism made up of algae and fungi. Algae make food for the lichen and the fungi provide protection and obtain water and minerals. Some biologists consider the partnership to be much more beneficial to the fungi and therefore not a true example of mutualism.
If a symbiotic relationship benefits only one of the partners but does not harm the other (called the host) it is called commensalism. There are barnacles that attach themselves to whales, tiny crabs that live inside oysters, and plants such as Spanish moss and various orchids that grow on other plants. The barnacles are benefited by being carried to areas where food is plentiful. The crabs are given shelter, and the moss and orchids are provided with support. In none of these examples is the host harmed.
If a symbiotic relationship can be harmful to the host, it is called parasitism. The organism that preys on the host is called a parasite. Many fungi and bacteria are parasites on plants and animals, often causing disease or the death of the host. Many worms, such as tapeworms and hookworms, are parasites of other animals, including humans. They too can cause disease or death. .)