COMMENSALISM
Commensalism is a loose, usually facultative (neither dies if the relationship ends), and interspecific association between a larger "host" and a smaller "commensal," without any metabolic dependency. Only the smaller commensal derives benefits, such as sharing space, substrate, defense, shelter, transport, and/or food, and does so without helping or harming the host. In some cases, the interaction is obligatory for the commensal. Escherichia coli, and a variety other of bacteria, are harbored comfortably in the large intestine of people to feed on nutrients that are outgoing at any rate. There is no obvious harm or benefit to the host. (Although some benefits have been recognized as contributed by certain bacteria that live within the human digestive system, such as aiding immunity, synthesizing certain vitamins, and fermenting complex indigestible carbohydrates(O'Hara and Shanahan 2006)). Entamoeba gingivalis is also a commensal in the human mouth. Commensalism means "eating at the same table," graphically conveying a situation whereby the commensal is able to feed on food that is not being consumed by the host—offering benefit to the commensal without impacting the host.
Examples of metazoan commensals include barnacles on a whale and epiphytes on a tree. Sedentary adult barnacles attach to a whale, or other large moving objects, and get renewed sources of food and oxygen supply without giving causing obvious harm to the carrying host. Similarly, epiphytic plants like orchids and bromeliads grow perched on sturdier trees for better exposure to sunlight and for absorbing moisture and nutrients from the air.
Even though innumerable examples of commensalism have been recognized, a closer inspection suggests that true commensal relationships tend to be rather rare. For example, the pinworm in the large intestine of human beings has been considered as a commensal relationship. In reality, at the time of the pinworms reproduction, it intentionally causes itching in the anal region. Shrimp has been considered to be commensal on the sea cucumber, using it for passive transport. From time to time, the shrimp leaves the host for food. But it also has been pointed out that the commensal may feed on the parasites of the host body surface and also provide protection to the host because of its warning coloration. Thus, commensalism in many cases appears in reality to fall into the categories of either mutualism or parasitism.
