barnacle, common name of the sedentary crustacean animals constituting the subclass Cirripedia. Barnacles are exclusively marine and are quite unlike any other crustacean because of the permanently attached, or sessile, mode of existence for which they are highly modified. Typical barnacles attach to the substrate by means of an exceedingly adhesive cement, produced by a cement gland, and secrete a shell, or carapace, of calcareous (limestone) plates, around themselves. Colonies of such barnacles form conspicuous encrustations on wharves, boats, pilings, and rocky shores. They range in length from under 1 in. (2.5 cm) to 30 in. (75 cm). Their shells are commonly yellow, orange, red, pink, or purple, sometimes with striped patterns. Because of their sedentary life and enclosing shells, barnacles were thought to be mollusks until 1830, when their larval stages were discovered. Much of what is known about barnacles is the result of research by Charles Darwin, who published a monumental work on the subject in the 1840s.

Shelled and Shell-less Barnacles

Barnacles with a calcareous shell (order Thoracica) include the gooseneck barnacles, which are attached to the substrate by means of a stalk, or peduncle, and the acorn, or rock, barnacles, which are attached directly to the substrate. The stalk of gooseneck barnacles is simply an elongation of the attached end of the animal's body. In some gooseneck barnacles the stalk as well as the body is covered by calcareous plates; in others it is a naked leathery or horny structure. A gooseneck barnacle found in large numbers on ships and pilings is Lepas, which has a leathery stalk and flattened shell and looks like a small clam attached by its siphon.

Balanus is an acorn barnacle commonly found on rocks; it has a thick conical shell attached at its wide base, with an opening at the top. As in many of the acorn barnacles, the plates of the surrounding carapace form an impenetrable wall, and the opening is equipped with two movable plates that can be pulled down to close off the body completely.

In both gooseneck and acorn barnacles the feathery legs of the animal may sometimes be seen protruding through the carapace opening. When the animal feeds, these jointed legs, called cirri, sweep organic particles and minute planktonic organisms toward the mouth, which is located deeper inside the shell. The attached end of the animal is its anterior, or head region: the barnacle has been described as a shrimplike animal standing on its head in a limestone house and kicking food into its mouth with its feet. Barnacles lack gills; gas exchange occurs through the cirri and the body wall. Some shelled barnacles are commensal, attaching themselves to living animals such as whales, porpoises, turtles, crustaceans, and echinoderms. The gooseneck barnacle Conchoderma may be found growing on the acorn barnacle Coronula, which grows on the skin of whales.

Besides the shelled barnacles there are naked barnacles (orders Ascothoracica and Rhizocephala), which live on, and in some cases parasitize, other invertebrate animals. There are also shell-less boring barnacles (order Acrothoracica), which live inside holes that they drill in shells and corals.

Reproduction

Although nearly all other crustaceans have separate sexes, most barnacles are hermaphrodites, with cross-fertilization between adjacent individuals being the rule. Some species, however, have dwarf males, which are parasitic on female or hermaphroditic individuals. The fertilized egg develops into a free-swimming larva, called a nauplius larva, of the basic crustacean type, with paired antennae. This form then molts to become a cypris, or bivalve, larva, which eventually attaches itself to a suitable substrate by its first pair of antennae and undergoes metamorphosis into an adult.

Economic Significance

Barnacles are economically significant because they settle on ship hulls and harbor installations; the resulting encrustation of the ships greatly increases friction, diminishing speed and increasing fuel consumption. Ships are treated with plastic coating or with antifouling paints containing copper or mercury to prevent or diminish encrustation.

Classification

Barnacles are classified in the phylum Arthropoda, subphylum Crustacea, class Cirripedia.

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 order to understand at least one species before making the generalisations needed for his theory of evolution by natural selection .

Life cycle

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 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 goose barnacles (e.g. Pollicipes polymerus) are treasured as a delicacy in Portugal, Spain, and Greece 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.

Classification

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 :

Infraclass Cirripedia Burmeister, 1834

• Superorder Acrothoracica Gruvel, 1905
• Order Pygophora Berndt, 1907
• Order Apygophora Berndt, 1907
• Superorder Rhizocephala Müller, 1862
• Order Kentrogonida Delage, 1884
• Order Akentrogonida Häfele, 1911
• Superorder Thoracica Darwin, 1854
• Order Pedunculata Lamarck, 1818
• Order Sessilia Lamarck, 1818

Synonyms

Other names for this group of crustaceans include Thyrostraca, Cirrhopoda (meaning "tawny-footed"), Cirrhipoda, and Cirrhipedia.

References

External links

• Rock barnacle at Aquascope
• Barnacles from the Marine Education Society of Australasia
• Barnacles in Spain Article on barnacles in Spain, and their collection and gastronomy.
• Newcastle University's barnacle and biofouling information site.