There are two commonly used definitions of symbiosis (from the Greek syn = together, bio = life) which differ in biologically relevant ways that can be confusing if not clarified beforehand (Wilkinson 2001). The first definition is the more broad, encompassing all close long-term associations between different [species of] organisms (sensu de Bary 1879). The second is more specific, referring only to those close long-term associations that are beneficial to both, or all, of the organisms involved (e.g. Woodhead 1915, cited by Wilkinson 2001). Here, we follow the original definition, further classifying the type of symbiosis depending on whether the relationship has a net benefit or detriment to one or the other organism, as follows. (For simplicity, we represent symbioses as involving two organisms, although they may involve more.)


Organism 1
Organism 2 Type of symbiosis
beneficial Mutualism
no effect Commensalism
detrimental Parasitism



It is possible to define the relationships between symbiotic organisms further (Smith & Douglas 1987). For example, in any pair, the larger organism is the host and the smaller organism the symbiont. Ectosymbionts occur on the outside of the host. Endosymbionts occur inside the host and may be intracellular or extracellular. Symbiosis may be obligate, if an organism cannot survive and reproduce without its partner, or facultative. Symbioses also may differ in their specificity, from being highly specific (always involving the same strains or subspecific taxon) to very general (involving organisms within a class, phylum, or larger grouping). The symbiosis may also be classified according to the mode of interaction (e.g. genetic, metabolic, behavioral) and a donor and recipient identified for each resource.
Although predation could be defined as a kind of parasitism (the predator benefits to the detriment of the prey) and competition is an association that is detrimental to both organisms involved, they are not traditionally described as symbioses because they are not generally long-term associations (Smith & Douglas 1987), and in the case of competition also need not be close. Predation and competition, therefore, are discussed elsewhere.
Many symbioses can be found involving scyphozoans. Follow the links to mutualism, commensalism, and parasitism to find out more.


Prepared by M. N Dawson