Semibalanus balanoides is a common and widespread boreo–arctic species of acorn barnacle. It is common on rocks and other substrates in the intertidal zone of north-western Europe and both coasts of North America.

Adult S. balanoides grow up to 15 millimetres (0.6 in) in diameter, and are sessile, living attached to rocks and other solid substrates. They have six greyish wall plates surrounding a diamond-shaped operculum.^[2][3] The base of the shell is membranous in Semibalanus, unlike other barnacles which have calcified bases.^[2]When the tide rises to cover the barnacles, the operculum opens, and feathery cirri (modified thoracic appendages)^[4] are extended into the water to filter food from the seawater. When the tide falls, the operculum closes again to prevent desiccation; the reduction from the primitive condition of eight wall plates to six is believed to decrease water loss even further by reducing the number of sutures through which water can escape.^[5]

S. balanoides is found in the intertidal zone in the world’s northern oceans. Its distribution is limited in the north by the extent of the pack-ice and in the south by increasing temperature which prevents maturation of gametes.^[2] The mean monthly temperature of the sea must drop below 7.2 °C (45.0 °F) for it to breed.

In Europe, S. balanoides is found on Svalbard and from Finnmark^[6] to north-west Spain but excluding part of the Bay of Biscay. It is common throughout the British Isles, except in parts of Cornwall, the Scilly Isles and south-western Ireland. On the North American coast of the Atlantic Ocean, it reaches as far south as Cape Hatteras, North Carolina, and on the Pacific coast, it reaches as far south as British Columbia.^[2] S. balanoides is the most common and widespread intertidal barnacle in the British Isles, and the only intertidal barnacle of the north-east coast of North America.^[7]

S. balanoides can be the dominant species of rocky shores, where it grows in a range of situations, from very sheltered to very exposed. It is typically found lower on the shore than another barnacle, Chthamalus montagui, although with some overlap. S. balanoides can tolerate salinities down to 20 psu, allowing it to colonise parts of estuaries.^[2] On semi-exposed shores, S. balanoides may form a patchwork with patches of seaweeds, such as Fucus serratus, and limpets; the fronds of the seaweeds brush the barnacle larvae from the rocks, allowing limpets to colonise it instead. At the southern limit of its range, including Cornwall, S. balanoides is replaced by Chthamalus montagui.^[3] Although capable of living in the sublittoral zone, S. balanoides tends to be restricted to the intertidal by predation and by competition from species such as the blue mussel, Mytilus edulis, and the algae Ascophyllum nodosum and Chondrus crispus.^[7]

(From Wikipedia, June 3rd, 2019)