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The Seashore

Rocky Shores | Rocky Shore Creatures

Acorn Barnacles


Barnacles are the sharp little chaps that help you grip on the slippery seashore and give you a nasty graze on the elbow or knee when you do slip. There is a lot more to them than that however and you will discover more if you read on:

Barnacles are crustaceans, relatives of shrimps, crabs and lobsters. You usually see them at low water when they're closed up and waiting for the tide to come back in so they do not appear to do much. If you could see them at high water (or in a tank) it's a different story.

As the picture above shows they are actually very dynamic when covered by the sea. They are filter feeders and those feathery things sticking out of the top are modified legs which sweep through the water catching food (plankton and detritus) from the water. The particles are passed to the mouth which is inside the shell plates. The leg movement also sets up a current over the gills for respiration and the muscular contractions serve to circulate the animal's haemolymph ("blood") for like the tin man they have no heart. Barnacles start life as a planktonic larvae which settle out on the shore after about six weeks. The young barnacle wanders about on the shore looking for a suitable place to attach. Cracks and crevices are attractive but even better is the presence of other barnacles. Once the animal has found a good spot it stands on its front end ("head") and in an outburst of barnacly joi de vive bashes its head end on the rock. This causes glue glands to rupture and the animal is stuck down. It then grows the familiar plates and stays where it is for the rest of its life (which might be as long as 12 years) The shells of dead barnacles make favourable microhabitats for other small animals like the small winkle (Melarhaphe neritoides).


Above: Chthalamalus left, Semibalanus, right. Barnacles

Probably the commonest species (certainly around Dale Fort in West Wales) is Semibalanus balanoides. This species has separate sexes who must copulate to reproduce. You might think being glued down and surrounded by armour plating would render this difficult. Indeed it would but for the male barnacle's possession of a male organ of monstrous proportions. (Were you to multiply its length up to human proportions we are talking about roughly 20 metres). The males copulate with the females fertilizing up to 8000 eggs and the females retain the larvae until next Spring (the season not the tide) when they are released to coincide with the Spring bloom of phytoplankton (which they feed on). The mighty organ then surrenders to the way of all flesh and withers away and the males turn into females for next season. How strange and exciting is the life of the barnacle.

Elminius modestus is a 4 shell-plated species that arrived here from Australasia, in the 1940's. Unlike other barnacles it never seems to stop breeding (it is a cross-breeding hermaphrodite species) and this has resulted in a massive spread across north-west Europe. Also unlike other UK barnacles it has a tolerance of silty water and lower salanity. This means it can do well in estuaries. It is thought that the shell plates are not as tough as those of Chthamalus and Semibalanus and this is why it is not found on wave-exposed shores. It tends to live in the middle shore region of rocky shores.

Verruca stroemia Verruca stroemia. 4 unequal shell plates and a species found underneath stones in the lower shore; may also be on Laminaria holdfasts. In Scotland they have been seen to cross-fertilise in the winter to release larva in the spring when there is a peak formation of diatoms in the plankton.

Much research has been done on the principle of Competitive Exclusion with regard to the two species Chthamalus and Semibalanus. Chthamalus, a southern, warm water species, is tolerant of high temperatures and desiccation. Semibalanus, a northern species does better in cooler, wetter conditions. In the UK we see the northern limit to the Chthamalus range and the southern limit of Semibalanus. Where they overlap and exist together they compete for space on the rock. Where abiotic conditions suit Semibalanus is competitively superior (pushing Chthamalus off the rocks) but Chthamalus wins high up the shore and/or where warmer, drier conditions prevai, e.g. Pembrokeshire exposed shores.

There are several intertidal species in Britain which can be quite tricky to tell apart although a good text book like A Student's Guide to the Seashore by Fish and Fish (Cambridge 1996 2nd ed) will enable you to do it.

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