A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacean, and is hence distantly related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have two nektonic larval stages. Barnacles are encrusters, attaching themselves permanently to a hard substrate. Shown is a developed adult barnacle.