Semibalanus balanoides is a filter feeder, using its thoracic appendages, or cirri, to capture zooplankton and detritus from the water. If there is a current, then the barnacle holds its cirri stiffly into the flow, but when there is no current, the barnacle beats its cirrirhythmically.
Although they have no eyes, barnacles are aware of changes in light, and withdraw into their shells when threatened.
You can find clusters of these hard, white, cone-like houses on rocks, dock pilings, boats, and even on mussels.
Biologists have observed barnacles in the laboratory taking as long as an hour to choose a location. In nature, barnacles may take days to find a suitable spot, investigating one area, then allowing the currents to carry them to another.