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The rocky shores of most coastlines are liberally dotted with clusters of barnacles (phylum Arthropoda, class Crustacea). Few people take any notice of these animals, despite their common occurrence. Barnacles are exclusively marine animals: some 900 species have been identified worldwide. Many are tiny organisms measuring just a few centimeters in diameter, while others such as the South American Balanus psittacus may reach a height of 9 in (23 cm) and a diameter of 3 in (8 cm). Some of the smallest barnacles are parasitic, burrowing into mollusc shells and corals. The majority, however, are free-living animals that occur in distinct parts of the shoreline: while most species live within the intertidal range, some are limited to the low tide mark, while others are adapted to living in the spray zone, which only the highest tides can reach. A few species are even adapted to living in deep water.

There are two main types of barnacles—acorn and goose. Acorn barnacles are generally recognized by their squat, limpet-like appearance and extremely tough outer covering made up of five calcareous plates, which surround and protect the soft body cavity. With muscular contractions these plates can be opened or closed, depending on the state of the tide: at full tide, the plates are pushed outwards to allow the barnacle to feed, but as the tide withdraws, the barnacle closes its shell once again, leaving just a tiny opening for oxygen to enter. Thus enclosed in their shells, barnacles can resist drying at low tide.

Goose, or stalked, barnacles differ in appearance by having a long stalk (peduncle), the base of which is attached to the substratum and the main part of the body (capitulum) poised at the other end. The latter is enclosed in a toughened carapace, similar to that of acorn barnacles, while the peduncle is muscular and capable of movement.

Rocks are not the only substrate that attract barnacles. Some species attach to intertidal grasses, while others fix onto the shells of crabs or other molluscs such as clams, where they may help camouflage the host animal, and some even become attached to active-swimming species such as marine turtles or even the fins or other body parts of whales. Floating timber and flotsam, marine buoys, piers and ship's keels are also convenient anchoring points for many barnacles.

Adult barnacles remain in the same position for their lifetime. Being literally stuck in one place might prove an obstacle to many species, but barnacles have overcome this problem by having a larval dispersal phase. Most barnacles are hermaphroditic—each individual having both male and female reproductive organs and, while self-fertilization may take place in some instances, the normal pattern is for cross fertilization. Barnacles have extremely long male reproductive organs, some of which measure more than 30 times the length of the animal's body. The advantage of this is that although barnacles usually live in crowded conditions, they may also be able to reach other, more distant animals and fertilize them. Using this means to reach other barnacles, sperm are deposited in neighboring animals and the eggs brooded for about four months in a special sac within the mantle cavity. When they hatch, the tiny larvae will be released to the ocean where they drift with the currents. As many as 13,000 larvae may be released by a single individual. These larvae feed and mature through a series of six stages, following which they are ready to settle—a critical time in the life of the barnacle. As the larvae settles, it attaches itself to some substrate by means of cement glands located in the base of the first antennae. It then undergoes a period of metamorphosis in which the existing larval carapace becomes covered with interlocking calcareous plates.

Locked in that position for the remainder of its life, the barnacle has evolved a simple, but effective means of feeding. When covered with water, the barnacle extends six pairs of curved, hairy legs (cirri) from the body cavity into the water column. Here they are able to trap tiny plankton and small crustaceans directly from the water. As food is captured, the cirri are withdrawn into the mouth where particles are cleaned off and the cirri unfolded once again to continue feeding. Rhythmic beating of the cirri also creates a gentle flow of water down towards the mouth, further enhancing the chances of obtaining additional food.

In spite of their small size barnacles are of considerable economic importance, particularly for shipping, as high densities of barnacles on a ship's keel can reduce its speed by as much as one-third. Similar agglomerations may form on the legs of oil rigs, piers and other semi-permanent features, causing considerable damage. A great deal of research and money has been invested in the design of anti-fouling paints, which would deter barnacles from settling in the first place. Many of these products, however, have had a negative effect on the marine environment, causing poisoning among some species.

See also Arthropods.

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