Wild Wednesday - Acorn barnacle
The acorn barnacle, scientific name, Semibalanus balanoides, is the most intertidal (refers to the coastal area exposed to air at low tide and submerged at high tide), widespread barnacle around the coasts of Britain. These tiny crustaceans are usually grey/white, with six hard shell plates and a distinctive diamond - or kite-shaped opening.
They settle on almost any hard surface, including rocks, groynes, pier posts, harbour walls, shells and boats. Once attached, they stay put for life, fixing themselves in place with a powerful natural glue produced from special cement glands.
Although they look like tiny shells, barnacles are animals related to crabs and lobsters. When the tide is in, they open their plates and use feathery limbs called 'cirri' to filter plankton and other food particles from the water. When the tide goes out, they close tightly to reduce water loss and protect themselves from drying out.
Acorn barnacles are hermaphrodites, meaning each individual has both male and female reproductive organs, but they still need nearby neighbours to reproduce. After fertilisation, the larvae are released into the sea, where they drift as part of the plankton before settling on a solid surface and developing their familiar cone-shaped shells.
Their predators include dog whelks, which can drill through the shell, and some starfish, which can prise the plates apart to feed.
Amazingly, the acorn barnacle spends its adult life fixed in place, living upside down on its head.
In Arun District, acorn barnacles can be found anywhere the shore offers firm surfaces for them to cling to. This includes Littlehampton Harbour, the lower River Arun, coastal defences, groynes and rocky or shell-covered stretches near Climping, Bognor Regis and Pagham. They are part of the rich mix of coastal wildlife that depends on Arun's beaches, estuary edges and nearby marine habitats, alongside wider local conservation work supporting biodiversity and our recovering shorelines.
Read more here: Acorn barnacle | The Wildlife Trusts