Barnacle are sedentary, highly modified crustaceans resembling conical pyramids. Barnacles live by using long, feathering appendages to sweep the surrounding water for small, free-floating organisms. The critical resource for barnacles is a place to stay. Barnacles attach to rocks, ships, shells, whales, and just about anywhere else they can gain a foothold. The barnacle gains a place to live and the organism is not harmed by the presence of the barnacles. What type of interaction are the barnacles an example of?