The following points highlight the four main terms that have been widely used for various types of heterogenetic association: 1. Phoresis 2. Amensalism 3. Commensalism 4. Mutualism and Symbiosis.
Term # 1. Phoresis:
Phoresis is a term used for a particular type of heterogenetic association in which one individual merely provides shelter, support or transport for another organism of a different species. The classical example is Fierafier (fish) which lives within the respiratory trees of holothurians or occasionally in starfish.
The holothurians appear to be undisturbed by the presence of the small fish. They are relatively helpless and are readily attacked and devoured by other species.
Term # 2. Amensalism:
It is a condition or state in which the mere presence of a species causes difficulties or hardship of another species though the former species in heterogenetic association is neither harmed or helped. For example, Penicillium in cultured condition produces certain toxic substances which inhibits the growth of many kinds of bacteria.
The cultivated Penicillium, however, is not affected by the presence of the bacteria neither it gains by the death of the bacteria.
Term # 3. Commensalism:
The term, commensalism, literally means ‘eating at the same table’, and there are a number of classical examples of this type of loose association between animals of different species.
Commensalism is considered to be a type of loose association in which two animals of different species live together without either being metabolically dependent on the other, although one or both species may receive some benefit from the heterogenetic association, i.e., it exists between two members of different species which live together and any one out of the two becomes benefited and the other is not helped or harmed.
The one which derives benefit in such relationship is called commensal. A large number of insects live as commensals in the nests of birds. This kind of association is very common in marine habitat. The oysters keep small crabs as commensals in their mantle cavities. Some small marine fishes live as commensals in the cloacal respiratory trees of sea- cucumbers. Waste scraps of food and shelter are gained by the commensals.
Term # 4. Mutualism and Symbiosis:
A hetero-specific association is that in which both the species are benefitted. The heterogenetic association where two different species have benefited, has been referred to as mutualism by some authors and Symbiosis by others. The literature on the definition of these words is confused. Smith (1974) has given a clear cut distinction between mutualism and symbiosis.
Mutualism is derived from the Latin word mutus (= exchanged) whereas symbiosis comes from the Greek word symbiousn (= to live together). The term symbiosis can thus broadly include all the different types of relationship that exist in nature. According to Smith (1974), “Symbiosis is an association of two or more dissimilar organisms regardless of benefit or lack of them”.
By usage, it has come to be restricted to associations of a special kind in which the participating species are dependent on each other for existence. For example the association between wood-eating termites and hyper-flagellates in their intestine is of the symbiotic type of parasitism.
The Protozoan Trichonympha lives in the digestive tract of termite. Trichonympha secretes some sort of enzymes which hydrolyse the cellulose in termite’s diet of wood. It has been seen from experiments that if the termite is free from the presence of Trichonympha, it dies due to starvation. This protozoan inside the digestive tract of termite is so adjusted that it cannot survive outside the digestive tract.
Mutualism is an heterogenetic association in which both the members of different species are benefitted but this hetero-specific association is not obligatory for existence. An example of an association which can be considered to be mutualistic is that between Hydra viridis (Cniderian) and Zoochlorella (alga) which lives within its endodermal cells.
This alga produces oxygen which Hydra utilizes and Zoochlorelle makes use of nitrogenous waste products of Hydra for its synthetic process.