Semibalanus balanoides is the most widespread intertidal barnacle in the British Isles. It may grow up to 15 mm in diameter and has 6 calcified grey-white shell plates. It may be distinguished from other barnacles by the presence of a diamond shaped opercular aperture and a membranous shell base. The barnacle feeds on zooplankton when immersed, by extending the thoracic appendages (cirri). It is a cross fertilizing hermaphrodite and may live for up to 8 years, depending on its position on the shore.
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