Acorn barnacle

Acorn Barnacles

Acorn Barnacles ©Paul Naylor

Acorn barnacle

Scientific name: Semibalanus balanoides
Barnacles are so common on our rocky shores that you've probably never really noticed them. They're the little grey bumps covering the rocks that hurt your feet when you're rockpooling. They're actually tiny little crustaceans and are related to crabs and lobsters!

Species information


Length of shell: 2cm Average lifespan: up to 18 months

Conservation status


When to see

January to December


The most common barnacle found on our shores, the acorn barnacle lives attached to any hard substrate, including rocks, pier legs, old boats and even other animals! Their body is contained within the shell you see on the rocks, positioned upside down with their legs at the top. When the tide comes in, they open the plates of the shell and stick their legs out, using them to catch plankton and other detritus out of the water.

How to identify

One of several very similar species of barnacle, which can be hard to tell apart. The acorn barnacle is generally grey-white in colour, with a kite-shaped opening and 6 shell plates.


Found on all UK coasts.

Did you know?

Although the acorn barnacle is a hermaphrodite and has both male and female reproductive organs, individual barnacles function as either a male or a female. During the breeding season, males extend their notoriously long penises out of their shells and seek out a nearby female. After fertilisation, the larvae develop within the barnacle's body and are released into the water to feed on plankton and undergo moults until they are at the right stage to 'settle down'!