A barnacle is an animal. It is a kind of mollusk that forms on different hard surfaces like piers, boats, and on rocks. They are considered mostly a nuisance to the shiping industry and private boat owners. Barnacles are filter feeders, which means that they suck in water and filter out different things like plankton to use as food. The barnacles send out little polyps that land on other hard surfaces and start to create hard shells, forming new barnacles. invertebrates | barnacle