Definition: Any animal or plant spends a portion or all of its life cycle intimately associated with another organism of a different species is considered as symbiont or symbiote and the relationship is designated as symbiosis. By employing the term symbiosis, no intent is meant to emply the occurrence or mutual or unilateral metabolic dependency.
In order to facilitate understanding, four subordinate categories of symbiotic relationships are commonly distinguished. Under the broad heading of symbiosis, types of associations known as phoresis, commensalisms, parasitism, and mutualism should be identified.
1). Phoresis: The term phoresis means, “to carry”. During this type of relationship, that which is usually the smaller fo the two species- the phoront –is mechanically carried in or on the larger species – the host. No metabolic interaction or dependency occurs. An example of phoresis is the transport of bacteria on the legs of flies. The fly host provides only involuntary transportation for the bacteria.
2). Commensalisms: During this type of relationship, both the host and commensal “eat at the same table”. In other words, the spatial proximity of the two partners permits the commensal to feed on substance captured or ingested by the host.
A classic example of commensalisms is the unique relationship between the pilot fish Naucrates ductor and the shark. The pilot fish accompanies the shark in a free swimming manner, eating the fragments of food that become available as the shark tears apart its prey.
A similar commensal relationship is that between the remora, Echeneis remora and the shark, but the remora actually attaches itself to the shark by its dorsal fin, which is modified as a sucker. Like the pilot fish, the remora feeds on “leftover” and does not harm or benefit the shark.
3).parasitism: parasitism is defined as an intimate and obligatory relationship between two heterospecific organisms during which the parasite, usually of the two partners, is metabolically dependent on the host. The relationship may be permanent, as in case of tapeworm of intestine of mammals or very temporary in case of mosquitoes.
4). Mutualism: The fourth subcategory of symbiosis is mutualism. In this instance the mutualist and the host are mutually metabolically dependent on each other. Classic examples of this type of relationship include the lichens, which represent composites of certain species of algae synthesizes an excess of certain organic compound and the fungus utilizes these molecules. In return, the fungus provides the algae with water , minerals, and protection from desiccation and high intensities of light.