| Barnacle | |
|---|---|
 | |
| Chthamalus stellatus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Thecostraca |
| Subclass: | Cirripedia Burmeister, 1834 |
| Infraclasses | |
| |
| Diversity[1] | |
| ~2115 species | |
| Synonyms | |
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Barnacles arearthropods of thesubclassCirripedia in the subphylumCrustacea. They are related tocrabs andlobsters, with similarnauplius larvae. Barnacles are exclusively marineinvertebrates; many species live in shallow and tidal waters. Some 2,100species have been described.
Barnacle adults aresessile; most aresuspension feeders with hard calcareous shells, but theRhizocephala arespecialized parasites of other crustaceans, with reduced bodies. Barnacles have existed since at least the mid-Carboniferous, some 325 million years ago.
In folklore,barnacle geese were once held to emerge fully formed fromgoose barnacles. Both goose barnacles and theChilean giant barnacle are fished and eaten. Barnacles are economically significant asbiofouling on ships, where they cause hydrodynamicdrag, reducing efficiency.
The word "barnacle" is attested in the early 13th century asMiddle English "bernekke" or "bernake", close toOld French "bernaque" andmedieval Latinbernacae orberneka, denoting thebarnacle goose.[2][3] Because the full life cycles of both barnacles and geese were unknown at the time, (geese spend their breeding seasons in the Arctic) a folktale emerged that geese hatched from barnacles. It was not applied strictly to the arthropod until the 1580s. The ultimate meaning of the word is unknown.[3][4]
The nameCirripedia comes from theLatin wordscirritus "curly" fromcirrus "curl"[5] andpedis frompes "foot".[6] The two words together mean "curly-footed", alluding to the curved legs used in filter-feeding.[7]
Most barnacles are encrusters, attaching themselves to a hard substrate such as a rock, the shell of a mollusc, or a ship; or to an animal such as a whale (whale barnacles). The most common form,acorn barnacles, aresessile, growing their shells directly onto the substrate, whereasgoose barnacles attach themselves by means of a stalk.[8]
Barnacles have a carapace made of six hard calcareous plates, with a lid or operculum made of four more plates. Inside the carapace, the animal lies on its stomach, projecting its limbs downwards. Segmentation is usually indistinct; the body is more or less evenly divided between the head andthorax, with little or noabdomen. Adult barnacles have few appendages on their heads, with only a single, vestigial pair of antennae attached to the cement gland. The six pairs of thoracic limbs are calledcirri; these are feathery and very long. The cirri extend to filter food, such asplankton, from the water and move it towards the mouth.[9]
Acorn barnacles are attached to the substratum by cement glands that form the base of the first pair ofantennae; 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. These glands secrete a type of natural quickcement made of complexprotein bonds (polyproteins) and other trace components likecalcium.[10]: 2–3 This natural cement can withstand a pulling strength of 5,000 lbf/in2 (30,000 kPa) and a sticking strength of 22–60 lbf/in2 (200–400 kPa).[9]
Barnacles have no trueheart, although a sinus close to theesophagus performs a similar function, with blood being pumped through it by a series of muscles.[11] The blood vascular system is minimal.[12] Similarly, they have nogills, absorbingoxygen from the water through the cirri and the surface of the body.[13] The excretory organs of barnacles are maxillary glands.[14]
The main sense of barnacles appears to be touch, with the hairs on the limbs being especially sensitive. The adult has three photoreceptors (ocelli), one median and two lateral. These record the stimulus for the barnacle shadow reflex, where a sudden decrease in light causes cessation of the fishing rhythm and closing of the opercular plates.[15] The photoreceptors are likely only capable of sensing the difference between light and dark.[16] This eye is derived from the primarynaupliar eye.[17]
Barnacles pass through two distinct larval stages, thenauplius and the cyprid, before developing into a mature adult.
Afertilised egg hatches into a nauplius: a one-eyed larva comprising a head and atelson with three pairs of limbs, lacking a thorax or abdomen. This undergoes six moults, passing through fiveinstars, 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 usingsetae.[18][19] All but the first instars are filter feeders.[20]
![Nauplius with fronto-lateral horns[21]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aCirripedia_nauplius.png)
The cypris larva is the second and final larval stage before adulthood. In Rhizocephala and Thoracica an abdomen is absent in this stage, but the y-cyprids (post-naupliar instar) has three distinct abdominal segments.[22] It is not a feeding stage; its role is to find a suitable place to settle, since the adults aresessile.[18] The cyprid stage lasts from days to weeks. It explores potential surfaces with modifiedantennules; once it has found a suitable spot, it attaches head-first using its antennules and a secretedglycoproteinous cement. Larvae assess surfaces based upon their surface texture, chemistry, relative wettability, color, and the presence or absence and composition of a surfacebiofilm; swarming species are more likely to attach near other barnacles.[23] As the larva exhausts its energy reserves, it becomes less selective in the sites it selects. It cements itself permanently to the substrate with another proteinaceous compound, and then undergoesmetamorphosis into a juvenile barnacle.[23]
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Typicalacorn barnacles develop six hard calcareous plates to surround and protect their bodies. For the rest of their lives, they are cemented to the substrate, using their feathery legs (cirri) to capture plankton. Once metamorphosis is over and they have reached their adult form, barnacles continue to grow by adding new material to their heavily calcified plates. These plates are notmoulted; however, like allecdysozoans, the barnacle moults itscuticle.[24]
Most barnacles arehermaphroditic, producing both eggs and sperms. A few specieshave separate sexes, or haveboth males and hermaphrodites. 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 moulted hermaphroditic individuals are receptive as females. Self-fertilization, although theoretically possible, has been experimentally shown to be rare in barnacles.[27][28]
The sessile lifestyle of acorn barnacles makessexual reproduction difficult, as they cannot leave their shells to mate. To facilitate genetic transfer between isolated individuals, barnacles have developed extraordinarily longpenises. Barnacles possess the largest penis-to-body size ratio of any known animal,[27] up to eight times their body length, though on exposed coasts the penis is shorter and thicker.[26] The mating of acorn barnacles is described as pseudocopulation.[25][29]
The goose barnaclePollicipes polymerus can alternatively reproduce by spermcasting, in which the male barnacle releases his sperm into the water, to be taken up by females. Isolated individuals always made use of spermcasting and sperm capture, as did a quarter of individuals with a close neighbour. This 2013 discovery overturned the long-held belief that barnacles were limited to pseudocopulation or hermaphroditism.[25]
Rhizocephalan barnacles had been considered hermaphroditic, but their males inject themselves into females' bodies, degrading to little more than sperm-producing cells.[30]
Most barnacles are filter feeders. From within their shell, they repeatedly reach into the water column with their cirri. These feathery appendages beat rhythmically to drawplankton and detritus into the shell for consumption.[8][31]
Although they have been found at water depths to 600 m (2,000 ft),[8] most barnacles inhabit shallow waters, with 75% of species living in water depths less than 100 m (300 ft),[8] and 25% inhabiting theintertidal zone.[8] Within the intertidal zone, different species of barnacles live in very tightly constrained locations, allowing the exact height of an assemblage above or below sea level to be precisely determined.[8]
Since the intertidal zone periodicallydesiccates, barnacles are well adapted against water loss. Their calcite shells are impermeable, and they can close their apertures with movable plates when not feeding.[32] Their hard shells are assumed by zoologists to have evolved as ananti-predator adaptation.[33]
One group of stalked barnacles has adapted to a rafting lifestyle, drifting around close to the water's surface. They colonize every floating object, such as driftwood, and like somenon-stalked barnacles attach themselves to marine animals. The species most specialized for this lifestyle isDosima fascicularis, which secretes a gas-filled cement that makes it float at the surface.[34]
Other members of the class have an entirely different mode of life. Barnacles of thesuperorderRhizocephala, including thegenusSacculina, areparasitic castrators of other arthropods, including crabs. The anatomy of these parasitic barnacles is greatly reduced compared to their free-living relatives. They have no carapace or limbs, having only unsegmented sac-like bodies. They feed by extending thread-like rhizomes of living cells into their hosts' bodies from their points of attachment.[35][16]
Goose barnacles of the genusAnelasma (in the orderPollicipedomorpha) are specialized parasites of certain shark species. Their cirri are no longer used to filter-feed. Instead, these barnacles get their nutrients directly from the host through a root-like body part embedded in the shark's flesh.[36]
Barnacles are displaced bylimpets andmussels, which compete for space.[8] 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.[8] 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 namedMegabalanus, can reach 7 cm (3 in) in length.[8]
Competitors may include other barnacles. Balanoids gained their advantage over the chthalamoids in the Oligocene, when they evolved tubular skeletons, which provide better anchorage to the substrate, and allow them to grow faster, undercutting, crushing, and smothering chthalamoids.[37]
Among the most commonpredators of barnacles arewhelks. They are able to grind through the calcareous exoskeleton and eat the animal inside. Barnacle larvae are consumed by filter-feedingbenthic predators including themusselMytilus edulis and theascidianStyela gibbsi.[38] Another predator is the starfish speciesPisaster ochraceus.[39][40] A stalked barnacle in the Iblomorpha,Chaetolepas calcitergum, lacks a heavily mineralised shell, but contains a high concentration of toxicbromine; this may serve to deter predators.[41] Theturbellarian flatwormStylochus, a serious predator ofoyster spat, has been found in barnacles.[42] Parasites of barnacles include many species ofGregarinasina (alveolate protozoa), a few fungi, a few species oftrematodes, and a parasitic castratorisopod,Hemioniscus balani.[42]
Barnacles were classified byLinnaeus andCuvier asMollusca, but in 1830John Vaughan Thompson published observations showing the metamorphosis of the nauplius and cypris larvae into adult barnacles, and noted that these larvae were similar to those of crustaceans. In 1834,Hermann Burmeister reinterpreted these findings, moving barnacles from theMollusca toArticulata (in modern terms, annelids + arthropods), showing naturalists that detailed study was needed to reevaluate their taxonomy.[43]
Charles Darwin took up this challenge in 1846, and developed his initial interest into a major study published as a series ofmonographs in 1851 and 1854.[43] He undertook this study at the suggestion of his friend the botanistJoseph Dalton Hooker, namely to thoroughly understand at least one species before making the generalisations needed for his theory ofevolution bynatural selection.[44] TheRoyal Society notes that barnacles occupied Darwin, who worked from home, so intensely "that his son assumed all fathers behaved the same way: when visiting a friend he asked, 'Where does your father do his barnacles?'"[45] Upon the conclusion of his research, Darwin declared "I hate a barnacle as no man ever did before."[44][46]
The oldest definitive fossil barnacle isPraelepas from the mid-Carboniferous, around 330-320 million years ago.[1] Older claimed barnacles such asPriscansermarinus from theMiddle Cambrian, some510 to 500 million years ago,[47] do not show clear barnacle morphological traits, thoughRhamphoverritor from theSilurianCoalbrookdale Formation of England may represent astem-group barnacle. Barnacles firstradiated and became diverse during the LateCretaceous. Barnacles underwent a second, much larger radiation beginning during theNeogene and still continuing.[1]
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The followingcladogram, not fully resolved, shows thephylogenetic relationships of the Cirripedia withinThecostraca as of 2021.[1]
Over 2,100 species of Cirripedia have been described.[1] Some authorities regard the Cirripedia as a fullclass orsubclass. In 2001, Martin and Davis placed Cirripedia as an infraclass ofThecostraca, and divided it into six orders:[48]
In 2021, Chan et al. elevated Cirripedia to a subclass of theThecostraca, and the superorders Acrothoracica, Rhizocephala, and Thoracica to infraclass. The updated classification with 11 orders has been accepted in theWorld Register of Marine Species.[1][49]
Barnacles are of economic consequence, as they often attach themselves to man-made structures. Particularly in the case of ships, they are classified asfouling organisms. The number and size of barnacles that cover ships can impair their efficiency by causinghydrodynamicdrag.[50]
![Barnacles on a ship. The resulting biofouling creates drag, slowing the ship and reducing its fuel efficiency.[50]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aBarnacles_on_a_Ship.jpg)
The flesh of some barnacles is routinely consumed by humans, including Japanese goose barnacles (e.g.Capitulum mitella), andgoose barnacles (e.g.Pollicipes pollicipes) are a delicacy inSpain andPortugal as well.[51] The Chilean giant barnacleAustromegabalanus psittacus is fished, or overfished, in commercial quantities on the Chilean coast, where it is known as thepicoroco.[52]
MIT researchers have developed an adhesive inspired by the protein-based bioglue produced by barnacles to firmly attach to rocks. The adhesive can form a tight seal to haltbleeding within about 15 seconds of application.[53]
The stableisotope signals in the layers of barnacle shells can potentially be used as a forensic tracking method[54] forwhales,loggerhead turtles[55] and formarine debris, such asshipwrecks or aircraft wreckage.[56][57][58]
One version of thebarnacle goose myth is that the birds emerge fully formed from goose barnacles.[59][60] The myth, with variants such as that the goose barnacles grow on trees, owes its longstanding popularity to ignorance ofbird migration.[61][62][63] The myth survived to modern times throughbestiaries.[64]
More recently, Barnacle Bill became a "comic folktype"[65] of a seaman, witha drinking song[65] and several films (a 1930 animated short with Betty Boop,[66]a 1935 British drama,[67] a1941 feature with Wallace Beery,[68] anda 1957 Ealing comedy[69]) named after him.
The political reformerJohn W. Gardner likened middle managers who settle into a comfortable position and "have stopped learning or growing" to the barnacle, who "is confronted with an existential decision about where it's going to live. Once it decides... it spends the rest of its life with its head cemented to a rock".[70]
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Delivered to McKinsey & Company, Phoenix, AZ November 10, 1990
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