Latin name: Semibalanus balanoides
Global Distribution: Atlantic, Pacific
UK Distribution: All coasts of Britain & Ireland, but sometimes is absent or rare in south-west Cornwall, the Isles of Scilly and south west Ireland
Size: 15mm diameter
Diet: Zooplankton
Habitat: Rocky shores (inter-tidal zone)
Sources: Spatial variation in size and density of adult and post-settlement Semibalanus balanoides: effects of oceanographic and local conditions
Battle of the Barnacles: Catalyst 2008

The Semibalanus Balanoide is one of the most common species on our shores. Barnacles are incredibly important creatures from an ecological perspective and are one of the true stalwarts of the coastal zone. Their abundance and distribution can help highlight long-term climatic condition, oceanic food supply and local wave patterns. There are over 1,000 species of barnacle world-wide, but fortunately for the amateur rock-pooler there are only four species in the UK to contend with. Semibalanus balanoides can be identified most easily by a diamond-shaped opening which it uses to feed.

One of three other species is an invasive non-native species (Elminius modestus or acorn barnacle) which can outcompete native barnacles in some situations. The barnacle is a cross fertilizing hermaphrodite which famously has one of the longest penises relative to body size. It may live for up to 8 years, depending on its position on the shore. Barnacles are considered edible by some people, especially in Japan, but it is not common to eat them in the UK. There is some excellent literature on the usefulness of barnacles as an ecological indicator.

Fun facts

  • A new species of water bear, micro animals that can seemingly withstand anything nature can throw at it, was found on the surface of this barnacle species.
  • When environmental conditions are ideal for S. balanoides they can remove smaller barnacle species by growing underneath them and levering them off the seabed.
  • They can be castrated by the parasitic isopod, Hemioniscus balani
Photo credit: By MichaelMaggs – Own work, CC BY-SA 3.0,