Mutualism
Mutualism (from Latin mutuus meaning lent or borrowed) is a form of symbiosis, or close association between organisms of two or more species, in which both participants in the relationship benefit. Because both participants are gaining something from the relationship, this type of symbiosis is often symbolized as (+, +). Mutualism is only one of three recognized categories of symbiotic relationships. The other two are commensalism, in which one participant benefits and the other is neither helped nor harmed, and parasitism, in which one benefits and the other is harmed. Some mutualistic relationships have existed over such long periods of time that the two species have evolved simultaneously to live with each other. In some cases, the association is so close that it is obligatory; one or both of the individuals in the relationship would not be able to survive without the other. For example, in some types of lichens, a mutualistic relationship between algae and fungi have evolved so closely together that they apparently cannot survive apart from one another. Lichens are an example of mutualism because the algae photosynthesize and provide nutrients to the fungus, while the fungus protects the algae from the often harsh environmental conditions where these organisms live, by providing moisture and minerals. Another example of a mutualistic symbiotic relationship is that of the nitrogen-fixing bacteria Rhizobia living in nodules on the roots of legumous plants such as clover, alfalfa, peas, and beans.
These bacteria are able to take atmospheric nitrogen, a form that plants cannot use, and change it into a form that is usable by plants. The relationship is mutualistic because both organisms benefit; the plants gain the important nutrient nitrogen in a usable form, and the bacteria get a source of energy, usually glucose. There are numerous other examples of mutualism throughout the world. For example, large sharks often have small suckerfish known as Remora living on them. These fish attach to the surface of the shark, while it is swimming. When the shark feeds, the Remora detaches and picks up scraps. In return for the free ride and food, the Remora cleans the shark of external parasites. Termites have symbiotic protozoa living within their digestive tract. The termites themselves cannot digest the wood they consume; they rely on the protozoa. Therefore, in a mutualistic relationship, the termites provide food for the protozoa, and the termites get to use the end products of protozoan digestion.
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