barnacle

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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.

barnacle, a small crustacean that occurs in vast numbers attached to rocks, jetties, piers, etc., and on the hulls of ships and boats. The commonest on the shore are acorn barnacles (Balanus spp.), which live attached to rocks with their body enclosed within a shell formed of calcareous (limy) plates. During high tide, they open their shells and use their hairy legs to comb food particles from the water. If barnacles settle on a ship's hull in large numbers they can cause serious problems. Copper sheathing during the days of sail and now modern antifouling paints discourage fouling, but such paints are an environmental issue which has not been totally resolved. Goose barnacles (Lepas anatifera) are the worst offenders. They have long stalks, up to 10 centimetres (4 in.) long, and can grow to maturity in a matter of a couple of weeks. They normally settle on the floating debris that accumulates in slicks and appear in European waters at about the time barnacle geese (Branta leucopsis) leave on migration. This coincidence, and the superficial resemblance between their comblike legs and birds' feathers, gave rise to the medieval myth that the geese hatched from the barnacles. In 1972, the British Scientist, a BP tanker, came out of refit and sailed round the Cape to the Persian Gulf. On its return voyage the ship began to judder so violently that it had to be dry-docked in Brest. The problem proved to be a covering of 70 tonnes of goose barnacles that had grown on the ship's hull in a matter of a few weeks.

M. V. Angel

barnacle Crustacean that lives mostly on rocks and floating timber. Some barnacles live on whales, turtles and fish without being parasitic, although there are also parasitic species. The larvae swim freely until ready to become adults, when they settle permanently on their heads; their bodies become covered with calcareous plates. The adult uses its feathery appendages to scoop food into its mouth. Two main types are those with stalks (goose barnacles) and those without (acorn barnacles). Subclass Cirripedia.

bar·na·cle / ˈbärnəkəl/ • n. a marine crustacean (class Cirripedia) with an external shell, which attaches itself permanently to a variety of surfaces. ∎  used figuratively to describe a tenacious person or thing. DERIVATIVES: bar·na·cled adj.

barnacle wild goose Anas leucopsis XIII; marine crustacean XVI. ME. bernak, -ek(ke), corr, to medL. bernaca, -eca, whence app. F. barnache; barnacle(XV) corr. to F. barnacle, but may be of independent orig. ult. source unkn. The two meanings depend on an early belief that the goose was generated from a shellfish.

Barnacle Warwicks. Bernhangre 1086 (DB). ‘Wooded slope by a barn’. OE bere-ærn + hangra.

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