They are marine and dwell in shallow and tidal waters with erosive settings. About 1,220 species of barnacles are known. The title Cirripedia comes from a Latin word meaning curl footed. They tend to attach permanently to any hard substrate. The most common known barnacles are acorn barnacles which attach their shells directly to some underwater hard substrate available. The goose barnacles attach themselves to any substrate by way of their stalks. They remain confined within their shells that are made up of six plates and get to the water column by their modified legs. These feathery altered legs beat rhythmically in order to drive 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.
Vast majority of the species inhabit shallow waters. Even though they can dwell up to the depth of 600 m but 75 percent of the species are found at a depth 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 are well adapted to thrive in conditions without water. The shells are made up of calcite with two plates which slide over the aperture to close it when the creature is not feeding. The two plates also protect the animal from the predators. They are easily transported from one location to other by limpets and mussels. Barnacles are preyed by a number of animals. They use two techniques to stay protected from the predators. The first strategy is swamping in which a large number of barnacles settle at the same place at a time allowing a chance to some to survive. The second technique is fast expansion. Some species of starfish also feed them.
The free living barnacles attach themselves to any hard substrate by cement glands which form the foundation of first set of antennae so the creature is attached to the substrate in upright position through its forehead. In certain species the cement glands are fixed into a long muscular stem. A ring of plates covers the animal which could be compared with the carapace of other crustaceans. In the sessile species the plates remain covered by a covering known as operculum. The plates are arranged in different ways depending upon the species. In the shell the animal 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 tiny part of abdomen is understood. Adults have very few appendages such as the antennae are either 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 close to gut which performs the role of circulation through a collection of muscles. Gills are absent so oxygen is absorbed either through the limbs of by the thin membrane of the carapace. Hairs are found within the limbs that are the principal sensitive constructions. Adults have a single eye which can detect only the sense of light and dark. The eye is derived from the crude naupliar eye. The anatomy of the parasitic species is somewhat simpler. They lac appendages and have unsegmented sac-like body. They consume food straight from their hosts through thread-like processes called rhizomes. There are two stages in their life history namely nauplius and cyprid. The fertilized egg hatches into nauplius larva that could be characterized by the head bearing a single eye and no telson. Thorax and abdomen are lacking. They stay in this phase for a period of about 6 months before it gets converted into cyprid stage. They are cared by parents and are release as free swimming larvae just after the first moult. The naulpii later on change to cyprid stage which lasts from several days to weeks. At this stage the barnacle begins searching for a safe place to get settled. It explores areas by its antennulues and when finds a suitable substrate it attaches through cement glands within antennules. The creature attaches with its head and then secretes a glycoproteinous substance. Then it undergoes metamorphosis to be a juvenile barnacle. The adults develop hard and calcareous plates so as to protect themselves. They remain attached throughout their lives using only the legs for feeding. Once the process of metamorphosis is complete the barnacle begins adding new material in order to contribute its growth. The shell plates don’t experience moulting.
Major number of species is hermaphrodite but few are gonochoric or androdioecious. The ovaries are present in the base or stalk while the testes are found in the head and might extend up to the thorax. Self fertilization is normal. Sexual dimorphism isn’t present. It’s thought that they developed during the Middle Cambrian. They were fully studied and categorized by Charles Darwin in a series of monographs in 1851 and 1854. They’re classified as fouling animals.