A barnacle is a crustacean glued head-first to a rock, kicking food into its mouth
Linnaeus filed barnacles with snails and clams. It took an Irish naturalist watching their larvae in 1830 to show they are crustaceans, cousins of crabs, that spend adult life cemented by the forehead to a surface, lying on their backs and sweeping the water with feathery legs.
A barnacle starts out swimming. The egg hatches into a one-eyed larva called a nauplius, which moults several times and then becomes a cyprid, a stage that does not eat at all. Its only task is to choose a home. It tests surfaces with its antennae, judging texture, chemistry, colour and the film of microbes coating them, and gregarious species favour spots near other barnacles. As its fuel runs low it grows less fussy, then glues itself down head-first with a protein cement and transforms into a juvenile that will never move again.
The adult lives inside a wall of six chalky plates capped by a four-plate lid. It has no true heart and no gills, soaking up oxygen through its legs and body surface. Simple eyes can only tell light from dark, enough to trigger a reflex that stops feeding and slams the lid shut when a shadow passes. The group's scientific name, Cirripedia, means curly-footed, after those six pairs of sweeping limbs.
Being stuck in place makes mating awkward. Most barnacles are hermaphrodites, and they solve the distance problem with the largest penis relative to body size known in any animal, up to eight times body length. In 2013 researchers found that the gooseneck barnacle can also release sperm into the water for neighbours to catch, overturning an old assumption. Other relatives took stranger routes: Sacculina and its kin are sac-like parasites that castrate crabs by threading living roots through their bodies.
Some 2,100 species exist, and the lineage dates back at least 325 million years. Three quarters live in water shallower than 100 metres, and they crowd rivals such as mussels either by settling in overwhelming numbers or by growing fast. On ship hulls they add drag and waste fuel. The very word barnacle first meant a goose: because nobody saw barnacle geese breed in the Arctic, medieval Europeans believed the birds hatched from goose barnacles.
Source: Barnacle
