Predators struggle to eat it. It is cemented to the rock. It is armoured. It is closed. The Common Acorn Barnacle is a fortress. Six plates form a wall. An operculum seals the gate. Inside, the animal waits. It is a crustacean. Not a mollusc. It looks like a shell. It acts like a stone. But it is alive. It feeds when the tide rises. It extends feathery legs. Cirri. They filter plankton from the water. Sweep. Retract. Sweep. Retract. A rhythmic dance. Invisible to the eye. Visible to the microscope. Habitat is the intertidal zone. It favours sheltered waters. Harbours. Estuaries. It competes for space. With mussels. With other barnacles. It grows in dense clusters. White patches on dark rock. It is successful. Very successful. Range is global. It originated in warm waters. Now it is everywhere. In New Zealand it is common in northern harbours. It spreads on boat hulls. It is a fouling organism. A nuisance to sailors. A success story for evolution. Diet is plankton. Microscopic organisms. It filters them from the water. It does not hunt. It waits. It trusts the tide. The tide brings food. The tide brings oxygen. The tide brings life. Without the tide, it starves. Threats are minimal. It is tough. It resists drying out. It resists predation. Only specialised snails can drill its shell. Humans scrape it off boats. But it returns. Quickly. Larvae settle. Grow. Cement. The cycle repeats. Life span is two to three years. Growth is rapid. It reaches maturity in months. Reproduction is internal. Fertilised eggs hatch into larvae. Nauplii. They swim. They drift. They settle. They cement. They become rock. Part of the landscape. Permanent. Until scraped.