Barnacle
Barnacles are arthropods of the subclass Cirripedia in the subphylum Crustacea. They are related to crabs and lobsters, with similar nauplius larvae. Barnacles are exclusively marine invertebrates; many species live in shallow and tidal waters. Some 2,100 species have been described.
Barnacle adults are sessile; most are suspension feeders with hard calcareous shells, but the Rhizocephala are specialized 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 from goose barnacles. Both goose barnacles and the Chilean giant barnacle are fished and eaten. Barnacles are economically significant as biofouling on ships, where they cause hydrodynamic drag, reducing efficiency. In culture, Barnacle Bill became a comic folktype of a seaman, appearing in several films and a drinking song.
Etymology
The word "barnacle" is attested in the early 13th century as Middle English "bernekke" or "bernake", close to Old French "bernaque" and medieval Latin bernacae or berneka, denoting the barnacle goose. Because the full life cycles of both barnacles and geese were unknown at the time, 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.The name Cirripedia comes from the Latin words cirritus "curly" from cirrus "curl" and pedis from pes "foot". The two words together mean "curly-footed", alluding to the curved legs used in filter-feeding.
Description
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. The most common form, acorn barnacles, are sessile, growing their shells directly onto the substrate, whereas goose barnacles attach themselves by means of a stalk.Anatomy and physiology
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 upwards. Segmentation is usually indistinct; the body is more or less evenly divided between the head and thorax, with little or no 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 called cirri; these are feathery and very long. The cirri extend to filter food, such as plankton, from the water and move it towards the mouth.Acorn 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. These glands secrete a type of natural quick cement made of complex protein bonds and trace components like calcium.
Barnacles have no true heart, although a sinus close to the esophagus performs a 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 the cirri and the surface of the body. 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 has three photoreceptors, 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. The photoreceptors are likely only capable of sensing the difference between light and dark. This eye is derived from the primary naupliar eye.
Life cycle
Barnacles pass through two distinct larval stages, the nauplius and the cyprid, before developing into a mature adult.Nauplius larva
A fertilised egg hatches into a nauplius: a one-eyed larva comprising a head and a telson with three pairs of limbs, lacking a thorax or abdomen. This undergoes six moults, 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. All but the first instars are filter feeders.Cypris larva
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 has three distinct abdominal segments. It is not a feeding stage; its 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 suitable spot, it attaches head-first using its antennules and a secreted glycoproteinous cement. Larvae assess surfaces based upon their surface texture, chemistry, relative wettability, color, and the presence or absence and composition of a surface biofilm; swarming species are more likely to attach near other barnacles. 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 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 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 moults its cuticle.Sexual reproduction
Most barnacles are hermaphroditic, producing both eggs and sperm. A few species have separate sexes, or have both 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.The sessile lifestyle of acorn barnacles makes sexual reproduction difficult, as they cannot leave their shells to mate. To facilitate genetic transfer between isolated individuals, barnacles have developed extraordinarily long penises. Barnacles possess the largest penis-to-body size ratio of any known animal, up to eight times their body length, though on exposed coasts the penis is shorter and thicker. The mating of acorn barnacles is described as pseudocopulation.
The goose barnacle Pollicipes 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.
Rhizocephalan barnacles had been considered hermaphroditic, but their males inject themselves into females' bodies, degrading to little more than sperm-producing cells.
Ecology
Filter feeding
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 draw plankton and detritus into the shell for consumption.Species-specific zones
Although they have been found at water depths to, most barnacles inhabit shallow waters, with 75% of species living in water depths less than, and 25% inhabiting the intertidal zone. 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.Since the intertidal zone periodically desiccates, barnacles are well adapted against water loss. Their calcite shells are impermeable, and they can close their apertures with movable plates when not feeding. Their hard shells are assumed by zoologists to have evolved as an anti-predator adaptation.
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 some non-stalked barnacles attach themselves to marine animals. The species most specialized for this lifestyle is Dosima fascicularis, which secretes a gas-filled cement that makes it float at the surface.
Parasitism
Other members of the class have an entirely different mode of life. Barnacles of the superorder Rhizocephala, including the genus Sacculina, are parasitic 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.Goose barnacles of the genus Anelasma 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.
Competitors
Barnacles are displaced by limpets and mussels, which compete for space. 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 in length.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.