Barnacle is an arthropod belonging to the subphylum Crustacea and infraclass Cirripedia so is related to crabs and lobsters. They are marine and dwell in shallow and tidal waters with erosive settings. About 1,220 species of barnacles are known. They have a tendency to attach permanently to any hard substrate. The most common known barnacles are acorn barnacles which attach their shells straight to some submerged hard substrate available. The goose barnacles attach themselves to some substrate by means of their stalks. They remain confined within their shells that are composed of six plates and reach the water column by their altered legs. These feathery altered legs beat rhythmically in order to induce the plankton and other detritus towards the shell for feeding. Other members of this infraclass have accommodated different modes of life. Sacculina is parasitic inhabiting within crabs.
Although they can dwell until the thickness of 600 m but 75 percent of the species are found at a depth not less than 100 m and 25% species prefer to live in the intertidal zone. Within the intertidal zone that the species live in close harmony. Since the intertidal zone desiccates so soon they’re well adapted to thrive in states without water. The shells are composed of calcite with two plates that slide across the aperture to close it if the animal is not feeding. They are easily transported from one place to other by limpets and mussels. Barnacles are preyed by a range of animals. They employ two techniques to remain protected from the predators. The first strategy is swamping in which a large number of barnacles settle at one place at a time allowing a chance to some to endure. The second technique is fast growth. The common predators of barnacles are whelks which crush the shells of these animals and feed on the contents inside.
The free living barnacles attach themselves to any hard substrate by cement glands which form the foundation of first pair of antennae so the creature is attached to the substrate in upright position through its forehead. In some species the cement glands are fixed to a long muscular stalk. A ring of plates covers the animal that can be compared with the carapace of other crustaceans. In the sessile species the plates stay covered by a covering known as operculum. The plates are arranged in different ways depending upon the species. In the shell the creature remains with its limbs in upright position. Segmentation of the body is somewhat indistinct but the body is divided into head and thorax with a little portion of abdomen is known. Adults have hardly any appendages such as the antennae are vestigial or absent, attached to the cement glands. There are six pairs of thoracic limbs that are long and feathery used for filter feeding mechanism.
They lack true heart but a sinus is present near gut which performs the function of circulation through a series of muscles. Hairs are present within the limbs that are the chief sensitive structures. Adults have one eye which could detect only the sense of dark and light. The eye comes from the crude naupliar eye. The anatomy of these parasitic species is somewhat more straightforward. They lac appendages and have unsegmented sac-like body. They absorb food straight from their hosts through thread-like procedures known as rhizomes. There are two larval stages in their life history namely nauplius and cyprid. The fertilized egg hatches to nauplius larva that could be characterized by the head bearing one eye and no telson. Thorax and abdomen are missing. They remain in this phase for a period of about 6 months until it gets converted to cyprid stage. The naulpii later on change to cyprid stage which lasts from few days to weeks. At this point the barnacle starts searching for a safe place to get settled. It explores areas by its antennulues and if finds a suitable substrate it attaches through cement glands within antennules. The animal attaches with its head and then secretes a glycoproteinous material. Then it undergoes metamorphosis to become a juvenile barnacle. The adults develop hard and calcareous plates in order to protect themselves. They remain attached throughout their lives with just the legs for feeding. Once the process of metamorphosis is complete the barnacle starts adding new material so as to contribute its own growth. The shell plates don’t undergo moulting.
Major number of species is hermaphrodite but few are gonochoric or androdioecious. The ovaries are found in the base or stalk while the testes are found in the head and might extend until the thorax. Self fertilization is normal. Sexual dimorphism isn’t present. It is thought that they developed during the Middle Cambrian. They were fully studied and classified by Charles Darwin in a series of monographs in 1851 and 1854. They’re categorized as fouling animals. Some species particularly the goose barnacles are consumed as food by people of Spain and Portugal.