Their favorite places are on rocks and pilings of piers. Barnacles grow in six neatly fitted plates that overlap to form a ring. A sliding door of four additional plates closes to protect the animal when the tide is out. With the door plates shut, it is fairly safe from its enemies. The door opens to let the barnacle feed when the tide comes in.
Barnacles eat plankton.
Acorn Barnacles, Tetacalita Squanosa , are shaped like tiny volcanoes. They are attached to the rock by their natural glue on the bottom of their shell. When the tide washes over the barnacles it is time for lunch. The barnacle simply opens up and sticks out huge hairy feet and comb plankton out of the water. Acorn Barnacles have a brown leathery skin over their limestone shell.