Wild Wednesday - Acorn barnacle
You can find the acorn barnacle (Semibalanus balanoides), all around the coasts of Britain - they are most common barnacle to be found on our shores. Generally, a white-grey colour, with a kite-shaped opening and six shell plates.
They colonise on anything with a hard surface, including rocks, boats, leg structures on piers and other creatures, such as clams and mussels, and even whales and turtles can find they have this crustacean living on them. They stick hard to a surface by producing a brown glue (barnacle cement) from the aptly named ‘cement’ glands within their antennae. This is one of the most powerful natural glues known.
After they attach to a surface, they make little houses and filter feed small plankton and other particles when the tide is in, via their feather-like ‘limbs’ called cirri, these extend and retract as they comb the water to feed, quickly withdrawing into their protective shells if they sense a threat. This shell gives it protection from predators, and it also allows then to trap water inside, when the tide is out.
A hermaphroditic (it has both female and male sex organs) so it can cross-fertilise with its next-door neighbours. As it lives in a tight group, this comes in handy when it's time for the stationary barnacles to fertilize each other's eggs internally. The fertilised eggs remain within its shell and are released into the water to feed on plankton until they are at the right stage to find a hard surface to 'settle down’.
Its predators include snails that can drill through the shell, and some starfish that can pull the plates apart and place their stomach inside the opening to feed.
The acorn barnacle spends its whole life living on its head.
Read more here: Acorn barnacle | The Wildlife Trusts