Commensalism is the relationship between individuals of two species of which one is benefited and the other is almost unaffected, i.e.9 neither benefited nor harmed. Sucker fish (Echeneis, Remora) attaches to the underside of a shark by means of its sucker (modified dorsal fin) located on its head. It rides along fixed to the shark, but occasionally detaches itself long enough to capture the fragments falling off from the shark's prey. Symbiosis is an interaction in which both the interacting species are benefited. Predation is the interaction between two species, one of which captures, kills and eats up the other. Parasitism is an association of two organisms of different sizes and species in which the smaller one is benefited and the larger one is harmed.