Barnacles · Sessilia · Balanidae

Acorn Barnacle

Semibalanus balanoides

Not Evaluated

It has not yet been assessed against the IUCN criteria. See full glossary

The commonest acorn barnacle of North Atlantic rocky shores, able to cover the rock completely with thousands of individuals per square metre. It settles as a larva, cements itself for life and filters plankton with its cirri every time the tide covers it. It withstands hours of drying out by closing its plates.

Size

up to 1.5 cm

Lifespan

6 years

Diet

Herbivore

Feeds mainly on plants.

Social structure

Colony

Lives in large concentrations, often for breeding reasons.

Sighting difficulty

Unknown

OceanHerbivore Colonial Ocean Crustacean

17countries 30parks 45dive sites1same family

Fun facts

Joseph Connell's experiments with this species and with Chthamalus in the 1950s showed how competition and drying out sort intertidal life into bands, and are one of the foundations of modern ecology.
It attaches to the rock with an adhesive protein that sets underwater and is being studied for developing surgical glues.
Before settling, the larva explores the rock with its antennae and chooses the spot partly by the presence of other individuals of its own species.
It is the commonest barnacle on European Atlantic coasts and forms continuous bands clearly visible on the rock.
It has the longest penis in the animal kingdom relative to body size: up to eight times its own length. Cemented to the rock and unable to move, it needs to reach its neighbours to fertilise them.
That penis adapts to wave action: on heavily exposed shores it is shorter and thicker, and in sheltered places longer and thinner.
The larva swims free and, on settling, glues itself head-first to the rock with a protein cement so powerful that it is being researched as an underwater adhesive.
It is one of the key species in the classic studies of competition for space in the intertidal, alongside the mussel and the limpet.