A symbiotic relationship between differing species is one where the whole is greater than the sum of the parts. In this case, however, the organisms – of different species - only survive because they live together. They do not form a tight physical agglomeration, nor do they form the loose relationships found in co-operative or synergistic relationships. The bond is semi-permanent. The only reason the bond may be broken is when one member of the partnership dies.
Some examples include the bacteria we have in our stomachs; a Portuguese man of war, which is a symbiotic cluster of more simple organisms, with considerable added functionality.
Myxotricina paradoxa is a single celled creature which inhabits the digestive tract of the Australian termite supplying the enzymes that break down the cellulose into edible carbohydrate and lignin [used in the form of building blocks for nests]. In turn, this parasite is formed of a population of even tinier creatures living in symbiosis with one another – in effect a kind of cooperative zoo.
Furthermore, the cells which make up the human body are similarly 'made up of prokaryotic animals'. In effect, there is an underlying 'force' which drives together several creatures into assemblages of co-operating organisms.
One of the most intriguing examples of symbiotic behaviour is that between the siboglinid tube worm and bacteria that live within the worm. These worms were discovered in the late 1980s at the hydrothermal vents near the Galapagos Islands and have since been found at deep sea hydrothermal vents and cold seeps in all of the world's oceans. The worm has no digestive tract and is solely reliant on the bacteria for nutrition. The bacteria oxidize either hydrogen sulfide or methane which the host supplies to them.
There are some more unpleasant forms of symbiotic behaviour in which only one organism benefits from the relationship. Parasites like lice, or the barnacles that grow on the skins of baleen whales, or the worms that live in the digestive tracts of many animals are dependent on their host for survival, but their host clearly obtains no benefit in fact may even be killed by the association. We may feel sorry for the host under these circumstances, but we would be applying our own value judgements to the relationship. Given the objective, the result may be satisfactory – an overall increase in functionality. Effectively, therefore, the parasite gains function, whilst the host gains nothing.
Although we tend to notice the symbiotic relationships which to us seem 'unfair' on one partner, there are partnerships where one organism does not suffer from the relationship, whilst apparently not benefiting either. One partner seems to fulfil its role in an almost altruistic relationship.
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