Northern Rock Barnacle

Adult S. balanoides grow up to 15 millimeters in diameter, and are sessile, living attached to rocks and other solid substrates. They have six greyish wall plates surrounding a diamond-shaped operculum. The base of the shell is membranous in Semibalanus, unlike other barnacles which have calcified bases. When the tide rises to cover the barnacles, the operculum opens, and feathery cirri are extended into the water to filter food from the seawater. When the tide falls, the operculum closes again to prevent desiccatio