It is an obligatory physically close and mutually beneficial relationship that occurs between two interacting organisms. Both the interacting organisms fail to show normal growth in the absence of this association. Examples:
Lichens: They are dual organisms formed of an algae and Fungus.Fungus provides the new materials to the algae and algae prepares the food using the raw materials and the pigments.
Termites and Trichonympha: Trichonympha campanula lives in the hind gut of the termites and it secretes cellulose. The cellulose enzyme helps in digestion of cellulose. The digested product is utilized by Trychonympha and the termites.
Ruminants and cellulose digestion: The rumen part of the chambered stomach of ruminant contains a number of cellulose metabolizing bacteria like Ruminococcus albus , Ruminococcus flavefaciens.
Symbiotic Nitrogen fixation: Rhizobium species lives in roots of legumes forming nodules inside the bacteria forming nitrogen fixation.
Mycorrhiza: It is mutually beneficial association between roots of plants and fungi like Rhizopogon, Amanita,, Inocyte, Rhizoctonia.
Yucca and Yucca moth: Yucca is pollinated by moth called Pronuba yuccasella. Insect deposits egg in the ovary of the yucca flower. On entering the flower, moth undergoes rotational movement. This deposits pollen grain over the hollow stigma of the same flower.