Reproduction Surprisingly, the sexes of barnacles are separate, males having proportionately probably the largest penis in the animal kingdom. It reaches out of the shell and transfers its sperm to a female neighbour. After fertilisation the female barnacle broods its eggs until they hatch as a minute cypris larva which swims until it is ready to settle head-down on a suitable surface. The parasitic forms attach themselves to other animals. Rhizocephalans, for example, invade the tissues of crabs and other crustaceans and become visible only when a grotesque egg-sac forms under the abdomen of the host.
Ascothoracican barnacles attach to echinoderms or corals.
Acrothoracicans bore into coral or the shells of molluscs. Most are so bizarre that they are recognisable only as barnacles because they have the same larva, called a cypris. All barnacles live only in marine environments but many live in the intertidal region so spend part of their day without sea-water around them. During this time they do not feed or do anything except try to keep moist. Adult acorn barnacles live attached to rocks and other hard surfaces often in dense colonies.