Danger barnacles!

8th February 2021

Due to lock down I have been unable to get to the beach for a while and so this blog is based on some photographs I took on an earlier visit and some bookwork. It is about barnacles.

Barnacles (Latin name Cirripedia) are those tiny, ivory coloured, volcano shaped shells you see attached, often in large numbers, to rocks along the shore. Each is only a few millimetres across, but their numbers can be as the stars in the sky, colouring the rocks and becoming part of the landscape. I like them if that is not a silly thing to say. I am not quite sure why. They seem more like a form of rock than a form of life; hard, immovable, and apparently inert, yet as much a part of the shore as the sea and the sand.

Barnacles (and a few limpets) encrusting rocks in the intertidal zone.

Although they may appear featureless, get down on your knees and really look, perhaps with a magnifying glass, and you will see there is more to them than you might have thought.  Each shell is made up of several plates1 that form a small chamber, though often the plates are fused together and the joints hard to see. The chamber has a flat top which is closed by more plates making a pair of tiny gates. These gates meet along an intricate wiggly line which looks a little mysterious, almost as if the gates are sealed with a hieroglyph or magic symbol.  

But these gates do not need a spell to open them, just a covering of sea water. When the tide is out and the barnacle is above water the gates shut, providing protection from predators and preventing the creature inside drying out, but when submerged the gates open, allowing the barnacle to feed by extending a number of feathered legs called cirri which capture tiny particles of plankton suspended in the sea water. 

If you look carefully at different groups of barnacles up and down the shore you may find some that look subtly different; particularly the detailed shape of the plates and the line along which the gates meet. This might be because you are looking at a different species of barnacle, however barnacle identification is not easy, and you will need to look closely.

Two different species of barnacle. Left: Semibalanoides balanoides, Right: Chthalamus montagui (I think!) 

But be careful! Even as you crouch down to look you should know that you are potentially in danger and that although small and apparently lifeless, barnacles can be hazardous. It is a well documented fact that at least one fully grown man was trapped and held captive by barnacles, not for a few minutes, but for a full eight years. 

This man was Charles Darwin, the world-famous scientist and author of ‘The Origin of Species’ who, having found an unusual type of barnacle while voyaging on the Beagle thought he would spend ‘some months, perhaps a year’ 2 sorting out the then chaotic classification of the world’s barnacle species. Six years and many hundreds of pages of detailed scientific publication later he would write, to a friend (with still two years’ work to go):

“I am at work on the second vol. of the Cirripedia, of which creatures I am wonderfully tired: I hate a Barnacle as no man ever did before, not even a Sailor in a slow-sailing ship”.3,4

So, what is this creature that can engage such a brilliant and hardworking mind for eight years?  What exactly is a barnacle? 

Even before Darwin gave them his attention it had taken science quite a long time to decide what barnacles are and what class of animal they belong to. Maybe we can see why.  At a casual glance barnacles look like tiny limpets, which also have hard, roughly conical shells and, at the times we generally see them are stationary and appear rigidly attached to the rocks.  Barnacles and limpets however are completely different types of animal; limpets are molluscs, which are creatures like snails, that have soft unsegmented bodies, whereas barnacles have a segmented body and a hard exoskeleton. Even so barnacles were classified as a type of mollusc until it was discovered that they pass through two free-swimming larval stages5 before they permanently attach to the rocks. It was the observation of these larval stages that led to the correct classification of barnacles in 1834, as belonging to Crustacea, the same grouping as crabs, shrimps and lobsters.    

There are two main categories of barnacle, the type we have been talking about, which attach directly to the rocks and are known as sessile barnacles and a type that attach via a stalk, which are known as pedunculate barnacles. There are over a thousand species of barnacle worldwide but around the UK we have just two or three species of pedunculate barnacles, which are usually found washed ashore attached to floating driftwood or plastics and, depending on whether you count invasive species or not, six or nine species of sessile barnacle.  However, differentiating even such small numbers of species can be tricky.  In the words of Charles Darwin again:

Whoever attempts to make out from external characters alone, without disarticulating the valves [Darwin called the shell plates valves], the species, (even those inhabiting one very confined region, for instance the shores of Great Britain) will almost certainly fall into many errors”6

The difficulty seems to arise because there is a large variation in the appearance of individual barnacles, even within one species. Books such as ‘A Student’s Guide to the Seashore’7 offer a step through guide to barnacle identification, but as alluded to by Darwin, differentiating between some species requires scraping the barnacle off the rock and dismantling it. Personally, I think I would rather live with a little ambiguity in my identifications.  

If we think classifying barnacles as molluscs was something of a scientific blunder it is as nothing to what was believed in the middle ages about a type of pedunculate barnacle known as a goose barnacle.  This type of barnacle, which was found washed up on British shores attached to driftwood, was thought to be the embryo of a black and white goose known as a barnacle goose. Because this goose only winters in the UK it was never seen to breed and as migration was not understood at that time an explanation for its seasonal appearance was lacking.  The shape and colour of the floating goose barnacle looks, if you have a good imagination, somewhat like the beak, head, and neck of the goose and so the one was assumed to be the embryo of the other.  The full supposed life cycle involved a barnacle goose tree (remember the barnacles were always found attached to wood), on which the goose barnacles grew and from which the geese hatched.

One form of the wonderful barnacle goose tree8.

There is an Anglo Saxon riddle that accompanies this story:

My beak was close fettered, the currents of ocean,
running cold beneath me. There I grew in the sea,
my body close to the moving wood.
I was all alive when I came from the water,
clad all in black, but a part of me white.
When living, the air lifted me up,
the wind from the wave, and bore me afar,
up over the seal’s bath. Tell me my name

I have to say, I find this an attractive story and sometimes wonder if it might not have been fun to have lived in less scientifically constrained times, when one could be free to entertain poetic ideas like geese hatching from barnacle trees. But then maybe there is a poetry of a different and more powerful sort in the image of a man working patiently for eight years to understand these tiny creatures, while all the time devising the theory that would explain the growth, not of the barnacle tree, but of another tree, the evolutionary tree of life, among the branches of which not only barnacles and geese but man and all other creatures would be shown to have their place.


  1. Some UK species of barnacle have four plates, and some have six.
  2. Letter to: J. D. Hooker   [2 October 1846]
  3. Letter to: W. D. Fox, [24 October 1852]
  4. This and other wonderful excerpts from Darwin’s letters at: https://www.darwinproject.ac.uk/darwins-bad-days
  5. Observations of the ‘naupliar’ and nonfeeding ‘cypris’ stages of the barnacle’s life cycle were published in 1830 by the British military surgeon and marine biologist John Vaughan Thompson.
  6. From:  A Monograph on the Cirripedia, by Charles Darwin, London: Ray Society, 1854.
  7. ‘A Student’s Guide to the Seashore.’ J. D. Fish & S. Fish (Third Edition) (301-311)
  8. From Topographia Hibernica British Library MS 13 B VIII Ray Oaks, CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons)
  9. Anglo-Saxon Riddles of the Exeter Book  (1963) translated by Paull Franklin Baum (Creative Commons Attribution-ShareAlike License)

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