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A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea and is hence distantly related to crabs and lobsters. Some authorities regard Cirripedia as a full class or subclass, and the orders listed at right are sometimes treated as superorders. Around 1,220 barnacle species are currently known. The name "Cirripedia" is Latin, meaning "curl-footed".
Barnacles were first fully studied and classified by Charles Darwin who published a series of monographs in 1851 and 1854. Darwin undertook this study at the suggestion of his friend Joseph Dalton Hooker, in his quest to further his ongoing development of the theory of evolution by natural selection .
Barnacles have two larval stages. The first is called the nauplius, which spends its time as part of the zooplankton, floating wherever the wind, waves, currents, and tides may take it, whilst eating and molting. This lasts for about two weeks (and five larval instars) until the second stage is reached. At this point the nauplius metamorphoses into a non-feeding, more strongly swimming cyprid larva. The cyprids settle down in an area where environmental cues indicate a safe and productive environment.
When an appropriate place is found, the cyprid larva cements itself headfirst to the surface and then undergoes metamorphosis into a juvenile barnacle. Typical barnacles develop six hard armour plates to surround and protect their bodies. For the rest of their lives they are cemented to the ground, using their feathery legs (cirri) to capture plankton and gametes when spawning. They are usually found in the intertidal zone.
Once metamorphosis is over and they have reached their adult form, barnacles will continue to grow, but not moult. Instead, they grow by adding new material to the ends of their heavily calcified plates.
Like many invertebrates, barnacles are hermaphroditic and alternate male and female roles over time. Barnacles have the longest penis in the animal kingdom, in proportion to their body length [1].
Barnacles often attach themselves to man-made structures, sometimes to the structure's detriment. Particularly in the case of ships, they are classified as fouling organisms. Other members of the class have quite a different mode of life. For example, members of the genus Sacculina are parasitic on crabs.
Some barnacles are edible by humans, and the gooseneck barnacle (Pollicipes polymerus), in particular, is treasured as a delicacy in Greece, Spain, and other Mediterranean countries. The resemblance of this barnacle's fleshy stalk to a goose's neck gave rise in ancient times to the notion that geese, or at least certain seagoing species of wild goose, literally grew from the barnacle. Most notably, the wild Barnacle Goose (Branta leucopsis), whose eggs and young were rarely seen by humans because it breeds in the remote Arctic, got its popular name because it was imagined to grow from gooseneck barnacles.
This article follows Martin and Davis in placing Cirripedia as an infraclass of Thecostraca and in the following classification of cirripedes down to the level of orders:[2]
Infraclass Cirripedia <small>Burmeister, 1834</small>