They occur in the intertidal area of exposed rocky shores in the high to mid shore area and are sometimes so densely packed that it completely covers the rock surface.
The juvenile stages of barnacles go through numerous changes - from planktonic free floating organisms - before settling into a suitable location by detecting the presence of other barnacles and suitable sites to grow into an adult.
They have armour-like wall plates all around and an opening at the top. Both these adaptations protect the animal from pounding wave action and also from prolonged exposure while the tide is out by trapping water inside. When the tide comes in the plates open and the barnacles wave their modified legs (called cirri) about in the water to filter out food. Barnacles are hermaphroditic, however they do not release eggs and sperm into the water, instead they use extendible penises to transfer sperm to receptive neighbours.