ABSTRACT
Living organisms are always connected in some way to other individuals of the same species and to individuals of other species living in the same area. Many of these established interactions are known as direct relationships (i.e. sex recognition, competition, predation). In these cases, the involved organisms are known as free-living. However, some species have become closely associated, often to mutual benefit (frequently referred to as “symbiosis”). These relationships usually convey a high degree of specificity, to the extent that at least one of the involved partners can no longer be considered a free-living organism.
