Barnacle covered boulders. Barnacles are arthropods, they undergo two larval stages in their life cycle. firstly as nauplius larvae they are free-floating until their second larval stage as swimming cyprids. They become juvenile barnacles once cemented to rocks or man-made structures, where they will spend the rest of their lives. The adult form filters food from the water. Photographed at Westward Ho!, Devon, UK.