INTRODUCTION:

The phenomenon of symbiosis touches all aspects of life.  It operates in a profound way on the intracellular organization of eukaryotic cells while, at the other end of the biological spectrum, its principles are apparent in the realms of population biology.  It is found in all forms of life, from simple bacteria through to birds and mammals.  While it is often said that parasitism is the most common 'way of life', this distinction really belongs to symbiosis which, in its broadest context, embraces not only parasitism but also mutualism, commensalism, aegism and, ultimately, all of the eukaryotic life forms (e.g. mutualism operates in 90% of land plants and virtually all mammalian and insect herbivores would starve without their cellulose digesting symbionts).

TERMINOLOGY:

The term "symbiosis" (plural - "symbioses") is derived from the Greek sumbiosis meaning "living together".  The first usage is attributed to the German mycologist, Anton de Bary, who used the term in a paper on lichens titled "The phenomenon of symbiosis".  De Bary's original use of the term defined symbiosis in the broadest possible way as "the living together of differently named organisms".  He included in his definition all cases of intimate associations between species, such as epiphytes growing on trees, insects pollinating flowers, parasitism, mutualism and commensalism.  Confusion arose over the following years and biologist began to equate "symbiosis" with "mutualism (see definition below).  Today, most biologists have reverted back to the original broad definition: symbiosis is the living together of different organisms, usually in close association with one another, to the benefit of at least one of them.  The partners are referred to as symbionts.

Because of the broad nature of the symbiosis definition, sub-categories have been introduced to describe the different associations.  These interactions often exert a major effect on both the ecology and evolution of organisms:
   

Parasitism:	In its simplest form, parasitism is a one-sided association in which one of the symbionts (the parasite) benefits at the expense of the other (the host).  It is then possible to narrow this further and describe parasitism as an intimate, protracted, obligatory association between two species in which the smaller (the parasite) is metabolically dependent upon the larger (the host).
Mutualism:	This describes those situations in which both symbionts derive reciprocal benefit from the association.  Strictly speaking, mutualism describes any mutualistic interaction occurring between individuals, whether intra- or inter-specific, and can include such behaviour as cooperative hunting, cooperative defence, and reciprocal altruism.
Commensalism:	This literally means "dining together" and is used to describe those associations in which the symbionts share a food resource without harming one another.  If the sharing of food significantly affects one of the partners in the association, the relationship shifts towards kleptoparasitism.
Aegism:	This term has recently been introduced to describe those associations in which one symbiont is afforded protection by the other (the host) without the host being harmed.  This can be further sub-categorized as:        Epizoism/Epihytism:  In which one symbiont lives on the outer surface of the other        Endoecism:  In which one symbiont habitually shelters in the burrow of the other        Inquilinism:  In which one symbiont lives within the other with the host being harmed        Phoresis:  In which one symbiont uses the transporting potential of the other

The categories listed above are not mutually exclusive: a given association may contain elements of two or more of these categories at any given time; there may be frequent transitions from one category to another as the dynamics of the association change (see below).