Barnacle
A barnacle is a type of arthropod constituting the 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 (nonmotile) 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".
Description:
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.[2] The order Pedunculata ("goose barnacles" and others) attach themselves by means of a stalk.[2]
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 carinolaterals, and a carina.[3] 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 their heads, with only a single, vestigial pair of antennae, attached to the cement gland. The six pairs of thoracic limbs are referred to as "cirri", which are feathery and very long, being used to filter food, such as plankton, 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 their carapaces. 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.[4] This eye is derived from the primary naupliar eye.[5]
Life cycle:
Barnacles have two distinct larval stages, the nauplius and the cyprid, before developing into a mature adult.
Nauplius:
A fertilised egg hatches into a nauplius: a one-eyed larva comprising a head and a telson, without a thorax or abdomen. This undergoes six 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.[7]
Cyprid:
The cyprid larva is the last larval stage before adulthood. It is not a feeding stage; its role is to find a suitable place to settle, since the adults are sessile.[7] 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 other barnacles.[8] 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.
Adult:
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 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 not moulted; however, like all ecdysozoans, the barnacle itself will still molt its cuticle.
Sexual reproduction:
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.[10]
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.[12]
Ecology:
File:Semibalanus balanoides upernavik 2007-07-05.ogg
Semibalanus balanoides feeding
Most barnacles are suspension feeders; they dwell continually in their shells, which are usually constructed of six plates,[2] and reach into the water column with modified legs. These feathery appendages beat rhythmically to draw plankton and detritus into the shell for consumption.[13]
Other members of the class have quite a different mode of life. For example, members of the genus Sacculina are parasitic, dwelling within crabs.[14]
Although they have been found at water depths to 600 m (2,000 ft),[2] most barnacles inhabit shallow waters, with 75% of species living in water depths less than 100 m (300 ft),[2] and 25% inhabiting the intertidal zone.[2] 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.[2]
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 apertures when not feeding. These plates also protect against predation.[15]
Barnacles are displaced by limpets and mussels, which compete for space. They also have numerous predators.[2] 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.[2] 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;[2] other species may grow larger still (Austromegabalanus psittacus).
Competitors may include other barnacles, and disputed evidence indicates balanoid barnacles competitively displaced chthalamoid 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 the latter group.[16]
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.[17] Another predator on barnacles is the starfish species Pisaster ochraceus.[18][19]
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