They are marine and live in shallow and tidal waters with erosive settings. They’re sessile suspension feeders with two nektonic larval stages in their life history. About 1,220 species of barnacles are understood. 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 straight to some underwater hard substrate available. The goose barnacles attach themselves to any substrate by way of their stalks. They stay confined within their shells that are composed of six plates and reach the water column by their modified legs. These feathery modified legs beat rhythmically in order to drive the plankton and other detritus to the shell for feeding. Other members of this infraclass have accommodated different modes of life. Sacculina is completely parasitic inhabiting within crabs.
Although they can dwell up to the thickness of 600 m but 75% of the species are located 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 reside in close harmony. Since the intertidal zone desiccates so soon they are well adapted to thrive in conditions without water. The shells are composed of calcite with two plates that slide over the aperture to close it if the creature isn’t feeding. The 2 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 employ two techniques to stay protected from the predators. The first strategy is swamping where a high number of barnacles settle at the same location at a time allowing a chance to some to survive. The second technique is fast expansion.
The free living barnacles attach themselves to any hard substrate by cement glands which form the foundation of first pair of antennae so the animal is attached to the substrate in vertical position through its forehead. In some species the cement glands have been 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 various ways depending upon the species. Inside 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 tiny portion of abdomen is understood. Adults have hardly any appendages such as the antennae are either vestigial or absent, attached to the cement glands. There are six pairs of thoracic limbs which are long and feathery used for filter feeding mechanism.
They lack true heart but a sinus is present near stomach which performs the function of circulation through a series of muscles. Blood vascular system is very much reduced. Hairs are found within the limbs which are the chief sensitive structures. Adults have one eye which could detect only the feeling of light and dark. The eye is derived from the crude naupliar eye. The anatomy of these parasitic species is somewhat more straightforward. They absorb food straight from their hosts through thread-like procedures 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 missing. They stay in this phase for a period of about 6 months until it gets converted into cyprid stage. The naulpii later on change to cyprid stage which lasts from several days to weeks. At this stage 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 only the legs for feeding. When the process of metamorphosis is complete the barnacle starts adding new material in order to contribute its own growth. The shell plates don’t undergo moulting.
The ovaries are present in the base or stalk while the testes are found in the head and may extend up to the thorax. Self fertilization is normal. Sexual dimorphism is not present. It is thought that they developed during the Middle Cambrian. They were first fully studied and categorized by Charles Darwin in a series of monographs in 1851 and 1854. They’re classified as fouling animals. Some species particularly the goose barnacles are consumed as food by people of Spain and Portugal.