Barnacle adults are sessile; most are suspension feeders with hard calcareous shells, but the Rhizocephala are specialized parasites of other crustaceans, with reduced bodies.

[9] 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 (polyproteins) and other trace components like calcium.

[9] 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.

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.

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.

Once metamorphosis is over and they have reached their adult form, barnacles continue to grow by adding new material to their heavily calcified plates.

[27][28] The sessile lifestyle of acorn barnacles makes sexual reproduction difficult, as they cannot leave their shells to mate.

Barnacles are believed to have the largest penis-to-body size ratio of any known animal,[27] up to eight times their body length, though on exposed coasts the penis is shorter and thicker.

[25] Rhizocephalan barnacles had been considered hermaphroditic, but their males inject themselves into females' bodies, degrading to little more than sperm-producing cells.

[8] 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.

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.

[8] 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 7 cm (3 in) in length.

Barnacle larvae are consumed by filter-feeding benthic predators including the mussel Mytilus edulis and the ascidian Styela gibbsi.

[39][40] A stalked barnacle in the Iblomorpha, Chaetolepas calcitergum, lacks a heavily mineralised shell, but contains a high concentration of toxic bromine; this may serve to deter predators.

In 1834, Hermann Burmeister reinterpreted these findings, moving barnacles from the Mollusca to Articulata (in modern terms, annelids + arthropods), showing naturalists that detailed study was needed to reevaluate their taxonomy.

[43] 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.

[43] He undertook this study at the suggestion of his friend the botanist Joseph Dalton Hooker, namely to thoroughly understand at least one species before making the generalisations needed for his theory of evolution by natural selection.

[1] Facetotecta Laurida Dendrogastrida Cryptophialida Lithoglyptida Iblomorpha Pollicipedomorpha Calanticomorpha Scalpellomorpha Verrucomorpha Balanomorpha Rhizocephala Over 2,100 species of Cirripedia have been described.

[52] MIT researchers have developed an adhesive inspired by the protein-based bioglue produced by barnacles to firmly attach to rocks.

[53] The stable isotope signals in the layers of barnacle shells can potentially be used as a forensic tracking method[54] for whales, loggerhead turtles[55] and for marine debris, such as shipwrecks or aircraft wreckage.

[59][60] The myth, with variants such as that the goose barnacles grow on trees, owes its longstanding popularity to ignorance of bird migration.

[64] More recently, Barnacle Bill became a "comic folktype"[65] of a seaman, with a drinking song[65] and several films (a 1930 animated short with Betty Boop,[66] a 1935 British drama,[67] a 1941 feature with Wallace Beery,[68] and a 1957 Ealing comedy[69]) named after him.

The political reformer John W. Gardner likened middle managers who settle into a comfortable position and "have stopped learning or growing" to the barnacle, who "is confronted with an existential decision about where it's going to live.