Common Barnacle

The common barnacle, also called the common rock barnacle or northern rock barnacle and known scientifically as Semibalanus balanoides, is a widespread boreo-arctic acorn barnacle. It grows up to 15 millimeters across, with six greyish wall plates surrounding a diamond-shaped operculum, and unlike most other barnacles has a membranous rather than calcified base; when covered by water it extends feathery cirri to filter feed, and closes its operculum when exposed to air to avoid drying out. It lives in the intertidal zone of northwestern Europe, from Svalbard to northwestern Spain, and both coasts of North America, reaching Cape Hatteras in the Atlantic and British Columbia in the Pacific, often dominating rocky shores from very sheltered to very exposed sites and tolerating the reduced salinity of estuaries.