Acorn barnacle biology, When the tide rises, the plates covering the aperture open, and the thoracic appendages (known as 'cirri') are extended into the water current and used to filter particles of food from the water (2). During winter, barnacles do not feed, but rely on stored reserves (3). This barnacle is a hermaphrodite; individuals, although possessing both male and female reproductive organs, function as a male or a female (3). There is a single breeding season during autumn (2). Functional males extend the penis, which is much longer than the body, out of the shell wall and seeks a nearby functional female (3). After fertilisation, the embryos are stored within the barnacle's body, until they develop into 'naupilus' larvae. These are released into the water from February to May, and live in the water column feeding on plankton for several weeks. They undergo six moults, before developing into a second larval form known as a 'cyprid' larvae. This stage is specialised for seeking a suitable site for settling. They search the substrate with their antennae; once a suitable site has been found they release a substance that fastens them to the rock. This typically occurs in spring and early summer. They then undergo metamorphosis into the adult form (2). Sexual maturity may be reached in the first year after settlement, but it usually takes 2 years (3).