<P> Commensalism is a long term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed . This is in contrast with mutualism, in which both organisms benefit from each other, amensalism, where one is harmed while the other is unaffected, and parasitism, where one benefits while the other is harmed . The commensal (the species that benefits from the association) may obtain nutrients, shelter, support, or locomotion from the host species, which is substantially unaffected . The commensal relation is often between a larger host and a smaller commensal; the host organism is unmodified, whereas the commensal species may show great structural adaptation consonant with its habits, as in the remoras that ride attached to sharks and other fishes . Both remora and pilot fish feed on the leftovers of their hosts' meals . Numerous birds perch on bodies of large mammal herbivores or feed on the insects turned up by grazing mammals . </P> <P> The word "commensalism" is derived from the word "commensal", meaning "eating at the same table" in human social interaction, which in turn comes through French from the Medieval Latin commensalis, meaning "sharing a table", from the prefix com -, meaning "together", and mensa, meaning "table" or "meal". Commensality, at Oxford and Cambridge Universities, refers to professors eating at the same table as students (as they live in the same "college"). </P>

One organism benefits at the expense of another