One species may live in close association with another species . If these organisms share resources this arrangement is called symbiosis . Symbiosis in which both species benef from the association is called mutualism . If the association is beneficial to one species but neither benehts nor harms the other, it is called commensalism . If one species benefits at th expense of the other then the relationship is called parasitism. An eel that attaches to the skin of a shark and feeds on its blood is an example of