Acorn barnacle  (Semibalanus balanoides)

Common and widespread (2).

Barnacles are well-known intertidal organisms. They were not firmly accepted to be crustaceans until the 1830s; before this time, the calcareous shell and sessile habit of the adults resulted in confusion with molluscs (2). Our current knowledge of barnacles is largely based on an 8-year period of research by Charles Darwin (3). Adult acorn barnacles are sessile; they have a cone shaped shell-wall comprising a number of calcareous pates. In this species, Semibalanus balanoides, the shell-wall consists of 6 greyish-white plates. The opening at the top of the 'cone' is diamond-shaped, and can be covered by movable plates when the tide goes out. This protects the animal inside from desiccation (2).

Found in the north-east Atlantic from Spitzbergen to north-west Spain, as well as the Pacific coasts of North America reaching south to British Columbia, and on the Atlantic coast south as far south as Cape Hatteras. It is common around all coasts of Britain, but is rare or absent from the south-west of Cornwall and the Isles of Scilly (3).

You can view distribution information for this species at the National Biodiversity Network Gateway.

Inhabits rocky shores of all exposures, and forms the distinct greyish-white 'barnacle zone' on the shore with species of Chthamalus barnacles. When the two occur together, Semibalanus balanoides tends to occur lower down the shore than Chthamalus. Fewer barnacles occur where seaweeds dominate the shore, as the sweeping action of the seaweed fronds removes newly settled barnacles from the rock (2).

When the tide rises, the plates covering the aperture open, and the thoracic appendages (known as 'cirri') are extended into the water current and used to filter particles of food from the water (2). During winter, barnacles do not feed, but rely on stored reserves (3).

This barnacle is a hermaphrodite; individuals, although possessing both male and female reproductive organs, function as a male or a female (3). There is a single breeding season during autumn (2). Functional males extend the penis, which is much longer than the body, out of the shell wall and seeks a nearby functional female (3). After fertilisation, the embryos are stored within the barnacle's body, until they develop into 'naupilus' larvae. These are released into the water from February to May, and live in the water column feeding on plankton for several weeks. They undergo six moults, before developing into a second larval form known as a 'cyprid' larvae. This stage is specialised for seeking a suitable site for settling. They search the substrate with their antennae; once a suitable site has been found they release a substance that fastens them to the rock. This typically occurs in spring and early summer. They then undergo metamorphosis into the adult form (2). Sexual maturity may be reached in the first year after settlement, but it usually takes 2 years (3).

Not currently threatened.

No conservation action has been targeted at this species.

There may be further information about this species available via the National Biodiversity Network Gateway.

For more on this species see the MarLIN (Marine Life Information Network) species account, available from:
http://www.marlin.ac.uk/species/Semibalanusbalanoides.htm