Types Of Parasitic Association

Symbiosis

   Here both organisms are dependent on each other. Examples being the association of flagellate protozoa in the gut of termites, where termites are dependent on the protozoa for breaking down their foodstuff, and the protozoa are dependent on the termites as host organisms.

Another good example here which is often cited is the association between clownfish and anemones in tropical reefs; where the fish is dependent on anemone for protection and food while the anemone does not appear to gain anything from the association, except possibly cleaning. However, it has been observed that in some cases, in the absence of the fish partner the anemones tend to disappear from their reef home, indicating a true symbiotic rather than a mutualistic or commensal relationship.


Another well-known example is found with the lichens, symbiotic association composed of fungi and algae. These associations may become very close, and it is thought that the eukaryotes as a group evolved as a result of such an association. Intracellular organelles such as the mitochondria and chloroplasts appear to have their origin as intracellular symbiotes of early eukaryotes, (some extremely primitive eukaryotes, such as the intestinal parasite Giardia lamblia, lack these
organelles).

Other forms of symbiosis may be much less close, for example, an organism that uses another plant purely as a means of dispersal. For instance, bacterial or fungal spores on the legs of flies, or coelenterates and barnacles on the carapaces of marine crustaceans. This particular form of symbiosis is sometimes called Phoresis.

Mutualism

   Here the associates may or may not be dependent on each other for their existence, but both benefit when they are associated. An excellent example of this occurs with the association of sea anemones on the backs of crabs. Both gain from the association 10(the anemone providing some food for the crab, which in turn gives extra motility to the anemone), but both can survive on their own.


   Another less well-known example is found between certain species of ants and the caterpillars of some of the Lycaenidae butterflies (notably the 'Blues'), where the caterpillar is protected by the ants within their nests, in return for which the caterpillar secretes a honeydew which the ants collect. In this case, from the ant, it benefits from the association but does not appear to need it, (i.e., the association is facultative, or opportunistic).
However, from the caterpillar, this association is required for its survival (i.e., the association is obligatory). This illustrates that these definitions may become blurred, and, over time, one form of association may evolve into another.

Commensalism

  Neither organism is dependent on the other for its existence, but in this case, only one of the partners benefits from the association, the other being unaffected. An
example of this, found in humans, are the non-pathogenic obligate commensal protozoa such as the amoebae Entamoeba gingivalis, commonly found in the
mouth, feeding of bacteria, dead epithelial cells and food particles. Purely commensal relationships tend to be rather rare, as on a closer inspection element of mutualism or parasitism may become apparent.

Parasitism

Here one of the associates lives either partly or wholly at the expense of the other associate, the other partner (the host organism) not gaining anything from the association. This association may give rise to extreme pathology in the host, or the parasitism may generally be not very pathogenic.

Dependency is carried out by many organisms, the main groups including viruses, bacteria, protozoa (these usually being endoparasitic), and various metazoan groups (multicellulareukaryotic animals), these were mostly groups of helminths (often endoparasitic), and arthropods (usually ectoparasitic), as well as some higher organisms, such as ectoparasitic lampreys and hagfish. Generally, however, for partly historical reasons, the term parasitology only refers to the study of infection with the eukaryotic protozoan, and invertebrate metazoan parasites, not bacteria, viruses or the higher chordate parasites, even though these are parasites in the real sense.