Introduction:

A barnacle is a type of arthropod constituting the subclass 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 (nonmobile) and most are suspension feeders, but those in infraclass Rhizocephala are highly specialized parasites on crustaceans. They have four nektonic (active swimming) larval stages. Around 1,000 barnacle species are currently known. The name "Cirripedia" is Latin, meaning "curl-footed".The study of barnacles is called cirripedology.

Description:

Barnacles are encrusters, attaching themselves temporarily to a hard substrate or a symbiont such as a whale, sea snake or lobster. The most common, "acorn barnacles" (Sessilia), are sessile, growing their shells directly onto the substrate. Pedunculate barnacles (goose barnacles and others) attach themselves by means of a stalk. 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. 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.

Ecology:

Most barnacles are suspension feeders; they dwell continually in their shells, which are usually constructed of six plates, and reach into the water column with modified legs. These feathery appendages beat rhythmically to draw plankton and detritus into the shell for consumption. Other members of the class have quite a different mode of life. For example, members of the superorder Rhizocephala, including the genus Sacculina, are parasitic and live within crabs. Although they have been found at water depths to 600 m (2,000 ft), most barnacles inhabit shallow waters, with 75% of species living in water depths less than 100 m (300 ft), 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.

History of Toxonomy:

Barnacles were originally classified by Linnaeus and Cuvier as Mollusca, but in 1830 John Vaughan Thompson published observations showing the metamorphosis of the nauplius and cypris larvae into adult barnacles, and noted how these larvae were similar to those of crustaceans. In 1834 Hermann Burmeister published further information, reinterpreting these findings. The effect was to move barnacles from the phylum of Mollusca to Articulata, showing naturalists that detailed study was needed to reevaluate their taxonomy. Charles Darwin took up this challenge in 1846, and developed his initial interest into a major study published as a series of monographs in 1851 and 1854. Darwin undertook this study, at the suggestion of his friend Joseph Dalton Hooker, to thoroughly understand at least one species before making the generalisations needed for his theory of evolution by natural selection.hey possess two plates which they can slide across their apertures when not feeding. These plates also protect against predation.

Fossil Records:

The oldest definitive fossil barnacle is Praelepas from the mid-Carboniferous, around 330-320 million years ago. Older claimed barnacles such as Priscansermarinus from the Middle Cambrian (on the order of 510 to 500 million years ago) do not show clear barnacle morphological traits. Barnacles first radiated and became diverse during the Late Cretaceous. Barnacles underwent a second, much larger radiation beginning during the Neogene (last 23 million years), which continues to present. In part, their poor skeletal preservation is due to their restriction to high-energy environments, which tend to be erosional – therefore it is more common for their shells to be ground up by wave action than for them to reach a depositional setting.Barnacles can play an important role in estimating paleo-water depths. The degree of disarticluation of fossils suggests the distance they have been transported, and since many species have narrow ranges of water depths, it can be assumed that the animals lived in shallow water and broke up as they were washed down-slope. The completeness of fossils, and nature of damage, can thus be used to constrain the tectonic history of regions.