Symbiosis is a term which is used to describe the close and often long-term relationship between different species of organisms. The closeness implied by the definition of symbiosis is not always beneficial to both organisms as one organism may benefit from the relationship at the expense of the other.
Mutualism is the relationship between organisms from different species where both organisms derive a benefit from their close association. An example of this type of relationship is between human beings and the microorganisms in their gut that help them digest their food. Another type is the relationship between human beings and dogs where the dogs provide companionship and protection in return for food, care and shelter.
A parasitic relationship is a symbiotic relation where one organisms benefits from the relationship to the detriment of the other. In other words, the presence of one organism diminishes the other. The interesting thing about parasitic symbiosis is that while one organism is harmed by the presence of the other, the parasite is not harmed, but rather gains some kind of benefit from the relationship. A good example of a parasitic relationship is that of mosquitoes that feed on mammals. They gain nourishment from sucking the blood of their hosts and cause irritation by their annoying buzzing, sickness from transferring parasites and pain from the site of their bites.
Commensalism is used to describe a symbiotic relationship where one organism benefits and the other one is neither harmed nor benefits from the relationship. An example is a spider which spins its cobweb on a tree. The tree is unaffected by the activities of the spider while the spider uses it to build its "house." Another type of commensalism is between cattle egrets and livestock as they forage in the fields, feeding on the insects that are stirred up by the movement of the animals.
Endosymbiosis and Ectosymbiosis
Endosymbiosis is used to define any symbiotic relationship where one symbiotic organism lives within the tissue of the other. An example of this type of relationship is with the rhizobia, which is nitrogen-fixing bacteria that live inside the root nodules on legume roots. Ectosymbiosis or exosymbiosis refers to the type of symbiotic relationship where one organism lives on the body surface of the host. An example is the barnacles that live on the body of the baleen whales by attaching themselves to their jaws.
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