The juveniles pick their place to settle because they can detect their adults by chemical means. Adult rock barnacles are protected by four, six or eight calcareous plates, which form a volcano-like cover. The top entrance is covered by another two plates. When feeding, these two top plates open and basket-like cirri limbs wave into the oncoming current of water and direct food into the mouth. Barnacle Reproduction and Life Cycle Barnacles do not release their gametes into the sea, but are able to fertilise one another.
Barnacles are hermaphrodytic , which means that a single animal has both male and female parts.
When ready to reproduce, an adult barnacle uncoils its long tubular penis and extends it out through the operculum to search for a nearby receptive neighbour. When the sperm is transferred, the fertilised eggs are brooded within the shell of the receiver adult until they develop into a nauplius larvae. A single adult barnacle may release over 10,000 larvae. The numerous naupli are then released into the water as plankton . As the nauplius grows and develops it goes through several moult stages, until it reaches the cyprid larvae stage.