Balanus glandula (acorn barnacle)

Found all over the mussel beds.

Acorn barnacles are everywhere on virtually any surface at the beach. Crowded acorn barnacles can rise much higher than their width. If you pop one of these barnacles off its surface, you’ll find a circular deposit of shell material, not the soft jelly residue from Semibalanus cariosus. These barnacles are particularly irritating to boat owners who discover them on their boats. To discourage barnacles from attaching to boats, some owners add cayenne pepper to the paint. Barnacles don’t care for hot pepper.