General Parasitology - Types of Animal Associations
Organisms frequently associate together, often closely. There are a number of "motives" for these associations, including protection, nutrition, and as an aid to the dispersion (both geographically and temporally) of the organism. There are four main ways that animals of different species may be associated together; Symbiosis, Mutualism, Commensalism and Parasitism. These classifications however, on closer inspection, may become blurred, one type taking on the aspects of another, for example over time as the relationship evolves. However, as a general guide these terms are still very useful.
Here both associates are dependent on each other. Examples being the association of flagellate protozoa in the gut of termites, where termites are dependent on the protozoa breaking down their foodstuffs, and the protozoa are dependent on the termites as host organisms. Another good example here which is often cited is the association between clown fish and anemones in tropical reefs, where the fish derives food and protection from the anemones and is dependent on anemone for protection whilst the anemone does not appear gain anything by 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.
Other more well known example are found with the lichens, symbiotic organisms composed of fungi and algae, and the flagellate protozoans found in the gut of termites.
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 organism purely as a means of dispersal. For example bacteria or fungal spores on the legs of fly's, or coelenterates and barnacles on the carapaces of marine crustaceans. This particular form of symbiosis is sometimes called Phoresis.
Here the associates may or may not be dependent on each other for their existence, but both benefit when they are associated. A good example of this occurs with the association of sea anemones on the backs of crabs. Both gain something from the association (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 (particularly 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 point of view of 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 point of view of the caterpillar, this association is required for its survival (i.e. the association is obligate, or obligatory). This illustrates that these definitions may become blurred, and, over time, one form of association may evolve into another.
Again neither associate is dependent on the other for its existence, but in this case only one of the partners benefits when they are associated, 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 closer inspection elements of mutualism or parasitism may become apparent.
Here one of the associates live 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 be generally not very pathogenic. Parasitism is carried out by many organisms, the main groups including viruses, bacteria, protozoa (these usually being endoparasitic), and various metazoan groups (multicellular eukaryotic animals), these being 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 generally only refers to the study of infection with eukaryotic protozoan, and invertebrate metazoan parasites, not bacteria, viruses or the higher chordate parasites, even though these are parasites in the true sense.