



Anatomy



In the familiar barnacles the antennae are used as an attachment organ and there are six pairs of thoracic legs. The body is surrounded by pairs of fixed calcareous plates and is protected by another pair of plates which close the opening. Barnacles are divided into two groups: the acorn barnacles (Balanomorpha) in which the plates attach directly to the rock, and goose barnacles (Lepadomorpha) which are attached on the end of a stalk.



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.





Parasitic Barnacle Relatives



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.





Ecology



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. Some are specialists attaching themselves to turtles or whales. Many coastal species grow no more than a few millimetres tall but some deep-water species are 80 mm. A net of six pairs of long legs and cirri, protrudes from the cavity in which the animal lives and sweeps through the sea water to filter microscopic planktonic cells for food.





Distribution



Barnacles live on hard surfaces at all latitudes at all depths from the intertidal zone to the deep sea.





InfoZone.

InfoZone at Museum Victoria can help you only with questions about crustaceans from southeastern Australia. It can not advise on the care of pet hermit crabs or on crustaceans from other regions.