Commensalism

Commensalism (kumen'suliz"um) [key], relationship between members of two different species of organisms in which one individual is usually only slightly benefited, while the other member is not affected at all by the relationship. For example, some flatworms live attached to the gills of the horseshoe crab, obtaining bits of food from the crab's meals; the crab is apparently unaffected. In many cases commensalism cannot be distinguished from parasitism. (http://www.infoplease.com/ce6/sci/A0813031.html)