Mutualistic Symbiosis
Mutualistic Symbiosis is a type of mutualism in which individuals interact physically, or even live within the body of the other mutualist. Frequently, the relationship is essential for the survival of at least one member.
Example: Lichens are a fungal-algal symbiosis (that frequently includes a third member, a cyanobacterium.) The mass of fungal hyphae provides a protected habitat for the algae, and takes up water and nutrients for the algae. In return, the algae (and cynaobacteria) provide carbohydrates as a source of energy for the fungus.