Thyrostraca, Cirrhopoda (meaning "curl-footed"), Cirrhipoda, and Cirrhipedia.
A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile (non-motile) suspension feeders, and have two nektonic (active swimming) larval stages. Around 1,220 barnacle species are currently known. The name "Cirripedia" is Latin, meaning "curl-footed".
Barnacles are encrusters, attaching themselves permanently to a hard substrate. The most common, "acorn barnacles" (Sessilia), are sessile, growing their shells directly onto the substrate. The order Pedunculata ("goose barnacles" and others) attach themselves by means of a stalk.
Most barnacles are suspension feeders; they dwell continually in their shell – which is usually constructed of six plates – and reach into the water column with modified legs. These feathery appendages beat rhythmically to draw plankton and detritus into the shell for consumption.
Other members of the class have quite a different mode of life. For example, members of the genus Sacculina are parasitic, dwelling within crabs.
Although they have been found at water depths up to 600 m (2,000 ft), most barnacles inhabit shallow waters, with 75% of species living in water depths of less than 100 m (300 ft), and 25% inhabiting the intertidal zone. Within the intertidal zone, different species of barnacle live in very tightly constrained locations, allowing the exact height of an assemblage above or below sea level to be precisely determined.
Since the intertidal zone periodically desiccates, barnacles are well adapted against water loss. Their calcite shells are impermeable, and they possess two plates which they can slide across their aperture when not feeding. These plates also protect against predation.
Barnacles are displaced by limpets and mussels, which compete for space. They also have numerous predators. They employ two strategies to overwhelm their competitors: "swamping" and fast growth. In the swamping strategy, vast numbers of barnacles settle in the same place at once, covering a large patch of substrate, allowing at least some to survive in the balance of probabilities. Fast growth allows the suspension feeders to access higher levels of the water column than their competitors, and to be large enough to resist displacement; species employing this response, such as the aptly named Megabalanus, can reach 7 cm (3 in) in length; other species may grow larger still (Austromegabalanus psittacus).
Competitors may include other barnacles, and there is (disputed) evidence that balanoid barnacles competitively displaced chthalamoid barnacles. Balanoids gained their advantage over the chthalamoids in the Oligocene, when they evolved a tubular skeleton. This provides better anchorage to the substrate, and allows them to grow faster, undercutting, crushing and smothering the latter group.
Among the most common predators on barnacles are whelks. They are able to grind through the calcareous exoskeletons of barnacles and feed on the softer inside parts. Mussels also prey on barnacle larvae. Another predator on barnacles is the starfish species Pisaster ochraceus.
Free-living barnacles are attached to the substratum by cement glands that form the base of the first pair of antennae; in effect, the animal is fixed upside down by means of its forehead. In some barnacles, the cement glands are fixed to a long muscular stalk, but in most they are part of a flat membrane or calcified plate. A ring of plates surrounds the body, homologous with the carapace of other crustaceans. These consist of the rostrum, two lateral plates, two carino-laterals and a carina. In sessile barnacles, the apex of the ring of plates is covered by an operculum, which may be recessed into the carapace. The plates are held together by various means, depending on species, in some cases being solidly fused.
Inside the carapace, the animal lies on its back, with its limbs projecting upwards. Segmentation is usually indistinct, and the body is more or less evenly divided between the head and thorax, with little, if any, abdomen. Adult barnacles have few appendages on the head, with only a single, vestigial, pair of antennae, attached to the cement gland. There are six pairs of thoracic limbs, referred to as "cirri", which are feathery and very long, being used to filter food from the water and move it towards the mouth.
Barnacles have no true heart, although a sinus close to the oesophagus performs similar function, with blood being pumped through it by a series of muscles. The blood vascular system is minimal. Similarly, they have no gills, absorbing oxygen from the water through their limbs and the inner membrane of the carapace. The excretory organs of barnacles are maxillary glands.
The main sense of barnacles appears to be touch, with the hairs on the limbs being especially sensitive. The adult also has a single eye, although this is probably only capable of sensing the difference between light and dark. This eye is derived from the primary naupliar eye.
The anatomy of parasitic barnacles is generally simpler than that of their free-living relatives. They have no carapace or limbs, having only an unsegmented sac-like body. Such barnacles feed by extending thread-like rhizomes of living cells into the host's body from their point of attachment.
Barnacles have two distinct larval stages, the nauplius and the cyprid, before developing into a mature adult.
A fertilised egg hatches into a nauplius: a one-eyed larva comprising a head and a telson, without a thorax or abdomen. This undergoes 6 months of growth, passing through five instars, before transforming into the cyprid stage. Nauplii are typically initially brooded by the parent, and released after the first moult as larvae that swim freely using setae.
The cyprid larva is the last larval stage before adulthood. It is a non-feeding stage whose role is to find a suitable place to settle, since the adults are sessile. The cyprid stage lasts from days to weeks. It explores potential surfaces with modified antennules; once it has found a potentially suitable spot, it attaches head-first using its antennules, and a secreted glycoproteinous substance. Larvae assess surfaces based upon their surface texture, chemistry, relative wettability, colour and the presence/absence and composition of a surface biofilm; swarming species are also more likely to attach near to other barnacles. As the larva exhausts its finite energy reserves, it becomes less selective in the sites it selects. It cements itself permanently to the substrate with another proteinacous compound, and then undergoes metamorphosis into a juvenile barnacle.
Typical acorn barnacles develop six hard calcareous 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.
Once metamorphosis is over and they have reached their adult form, barnacles will continue to grow by adding new material to their heavily calcified plates. These plates are not moulted; however, like all ecdysozoans, the barnacle itself will still molt its cuticle.
Most barnacles are hermaphroditic, although a few species are gonochoric or androdioecious. The ovaries are located in the base or stalk, and may extend into the mantle, while the testes are towards the back of the head, often extending into the thorax. Typically, recently molted hermaphroditic individuals are receptive as females. Self-fertilization, although theoretically possible, has been experimentally shown to be rare in barnacles.
The sessile lifestyle of barnacles makes sexual reproduction difficult, as the organisms cannot leave their shells to mate. To facilitate genetic transfer between isolated individuals, barnacles have extraordinarily long penises. Barnacles probably have the largest penis to body size ratio of the animal kingdom.
Barnacles can also reproduce through a method called spermcasting, in which the male barnacle releases his sperm into the water and females pick it up and fertilise their eggs.
The geological history of barnacles can be traced back to animals such as Priscansermarinus from the Middle Cambrian (on the order of ), although they do not become common as skeletal remains in the fossil record until the Neogene (last 20 million years). In part their poor skeletal preservation is due to their restriction to high-energy environments, which tend to be erosional – therefore it is more common for their shells to be ground up by wave action than for them to reach a depositional setting. Trace fossils of acrothoracican barnacle borings (Rogerella) are common in the fossil record from the Devonian to the Recent.
Barnacles can play an important role in estimating palæo-water depths. The degree of disarticluation of fossils suggests the distance they have been transported, and since many species have narrow ranges of water depths, it can be assumed that the animals lived in shallow water and broke up as they were washed down-slope. The completeness of fossils, and nature of damage, can thus be used to constrain the tectonic history of regions.
Barnacles were originally classified by Linnaeus and Cuvier as Mollusca, but in 1830 John Vaughan Thompson published observations showing the metamorphosis of the nauplius and cypris larvae into adult barnacles, and noted how these larvae were similar to those of crustaceans. In 1834 Hermann Burmeister published further information, reinterpreting these findings. The effect was to move barnacles from the phylum of Mollusca to Articulata, showing naturalists that detailed study was needed to reevaluate their taxonomy.
Charles Darwin took up this challenge in 1846, and developed his initial interest into a major study published as a series of monographs in 1851 and 1854. Darwin undertook this study at the suggestion of his friend Joseph Dalton Hooker, in order to thoroughly understand at least one species before making the generalisations needed for his theory of evolution by natural selection.
Barnacles are of economic consequence as they 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.
Some barnacles are considered edible by humans, and goose barnacles (e.g. Pollicipes pollicipes), in particular, are a delicacy in Spain and Portugal. 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. Indeed, the word "barnacle" originally referred to a species of goose, the Barnacle goose Branta leucopsis, whose eggs and young were rarely seen by humans because it breeds in the remote Arctic.
The picoroco barnacle is used in Chilean cuisine and is one of the ingredients in curanto.
Some authorities regard Cirripedia as a full class or subclass, and the orders listed above are sometimes treated as superorders. In 2001, Martin and Davis placed Cirripedia as an infraclass of Thecostraca and divided it into six orders:
Infraclass Cirripedia Burmeister, 1834
Goose barnacles (order Pedunculata), also called stalked barnacles or gooseneck barnacles, are filter-feeding crustaceans that live attached to hard surfaces of rocks and flotsam in the ocean intertidal zone.
Some species of goose barnacles are pelagic and are most frequently found on tidewrack on oceanic coasts. Unlike most other types of barnacles, intertidal goose barnacles depend on water motion rather than the movement of their cirri for feeding, and are therefore found only on exposed or moderately exposed coasts.
In the days before it was realised that birds migrate, it was thought that Barnacle Geese, Branta leucopsis, developed from this crustacean, since they were never seen to nest in temperate Europe, hence the English names "goose barnacle", "barnacle goose" and the scientific name Lepas anserifera (Latin anser = "goose"). The confusion was prompted by the similarities in colour and shape. Because they were often found on driftwood, it was assumed that the barnacles were attached to branches before they fell in the water. The Welsh monk, Giraldus Cambrensis, made this claim in his Topographia Hiberniae. Since barnacle geese were thought to be "neither flesh, nor born of flesh", they were allowed to be eaten on days when eating meat was forbidden by religion.
The order Pedunculata is divided into the following suborders and families:
In Portugal and Spain, they are a widely consumed and expensive delicacy known as percebes. Percebes are harvested commercially in the northern coast, mainly in Galicia, and are also imported from overseas, particularly from Morocco and Canada. The indigenous peoples of California eat the stem after cooking it in hot ashes.
Lepas anatifera, commonly known as the pelagic gooseneck barnacle or smooth gooseneck barnacle, is a species of barnacle in the family Lepadidae. These barnacles are found, often in large numbers, attached by their flexible stalks to floating timber, the hulls of ships, piers, pilings, seaweed and various sorts of flotsam.
The body or capitulum of Lepas anatifera is supported by a long, flexible stalk or peduncle. There are five smooth, translucent plates, edged with scarlet and separated by narrow gaps. The plates have growth lines parallel with their margins and a few faint radial sculpture lines. Inside the capitulum, the barnacle has a head and thorax and vestigial abdomen. A number of brown, filamentous cirri or feeding tentacles project from between the plates. The peduncle is tough and a purplish-brown colour. The capitulum can grow to a length of 5 centimetres (2.0 in) and the peduncle varies between 4 centimetres (1.6 in) and 80 centimetres (31 in).
Lepas anatifera has a cosmopolitan distribution and is found in tropical and subtropical seas worldwide. Because it is often attached to objects carried into colder seas by currents, such as the North Atlantic Drift, it is often found well away from its place of origin and in waters too cold for it to reproduce. In this way it has been recorded from Norway, the Shetland Islands, the Faeroe Islands, Iceland and Spitzbergen.
Lepas anatifera is a hermaphrodite and starts to breed when it is about 2.5 centimetres (1 in) long. Fertilisation is internal and the eggs are brooded inside the mantle for a week before emerging as free swimming nauplius larvae. After further development, drifting as part of the plankton, these settle onto floating objects.
Lepas anatifera has long been known to grow on sea turtles, but in 2008, some small specimens were found attached to an American crocodile (Crocodylus acutus) on the Pacific coast of Mexico. This crocodile species mostly inhabits mangrove swamps and river estuaries but it is salt tolerant, and is sometimes found in marine environments. In this instance, the size of the goose-neck barnacles indicated that the crocodile must have been in the sea for at least a week. This is the first time that Lepas anatifera has been recorded as an epibiont of a crocodilian.
In 13th-century England the word "barnacle" was used for a species of waterfowl, the barnacle goose (Branta leucopsis). This bird breeds in the Arctic but winters in the British Isles so its nests and eggs were never seen by the British. It was thought at the time that the gooseneck barnacles that wash up occasionally on the shore had spontaneously generated from the rotting wood to which they were attached, and that the geese might be generated similarly. Credence to the idea was provided by the tuft of brown cirri that protruded from the capitulum of the crustaceans which resembled the down of an unhatched gosling. Popular belief linked the two species and a writer in 1678 wrote "multitudes of little Shells; having within them little Birds perfectly shap'd, supposed to be Barnacles [by which he meant barnacle geese]."
The word barnacle is a slang term used in electrical engineering to indicate a change made to a product on the manufacturing floor that was not part of the original product design. A barnacle is typically used to correct a defect in the product or as a way of enhancing the product with new functionality. A barnacle is normally a quick fix that is used until the product design can be redone incorporating the barnacle into the actual product so that when manufactured, the barnacle step in manufacturing is no longer required. A barnacle may also be added in the field in order to correct a design or manufacturing defect.
The term appears to have originated from the mollusc barnacle which is an animal that attaches itself to rocks, docks, ships, whales, and other objects where it grows. A barnacle in electronics is something added to the manufactured product. Typically a barnacle on a circuit board is very noticeable, much like the mollusc variety on a rock in the sea.
While the term was originally used with electronic hardware, it has also migrated into the software industry where is it is used to describe software that is added to a system. The connotation in the software industry is that a software barnacle is malware or spyware which has been inserted into a computing system illegally.
On printed circuit boards, a barnacle may be as simple as cutting a trace, soldering a wire in order to connect two points on the circuit board, or adding a component such as a resistor or capacitor. A barnacle may also be a complex subassembly or daughterboard.
The normal development cycle for electronic hardware contains two main phases. The first phase is the development and prototype phase in which the hardware is first designed (and often simulated using a computer program such as PSpice) and the design manufactured in low quantity as prototypes for testing. The second phase is the updating of design documents based on the testing experience and the beginning of general manufacturing of the product.
During the testing phase, problems are usually found as the design and simulation tools can not duplicate some types of environmental as well as electrical circumstances in which the product may be used. During the testing phases, barnacles are often used to patch (computing) or correct the hardware so that testing can continue in the face of defects (failures or faults) found. The goals of adding barnacles at this phase are to reduce development costs by using the prototype hardware for as long as it can be used, to test hardware changes before the design documentation is updated, and to reduce development time by not requiring a new version of the prototype hardware to be manufactured.
During general manufacturing of the product, the product may sometimes be used in circumstances which the specifications indicate would be acceptable however when the product is actually used in those circumstances, a problem is encountered. Engineering will typically perform a root cause analysis in order to determine the root cause of the problem. In some cases, manufacturing changes may need to be made such as trace contaminants being introduced during some phase of manufacturing. In other cases, the problem has to do with the design of the product and a change has to be made in the product design.
When a product design change is required, often and when possible, a barnacle is designed so that existing products can be modified with the design change using the barnacle. By using a barnacle, the idea is that existing products do not need to be scrapped and replaced so in this case the use of a barnacle is an economic decision. The barnacle work may be done in the field using portable tools and components or it may require a product recall with the barnacle work being done on the manufacturing floor.