A semipost on semibalanus

Oh no I hear you sigh - he's on about barnacles again. True, but just a quickie.....

Next time you tramp on top of a load of barnacles, thinking only of the welcome underfoot grip they give you on slippery rocks, you might just like to ponder their fascinating life cycle.

For a great introduction and a couple of wonderful pictures of the nauplius and cypris larval stages try this... and find out along the way why 13 is unlucky for some.

Trapped by the Barnacles

Standing and staring at barnacles the other day set me off on another puzzler. Barnacle larvae settle out of the plankton onto a substrate they think is just the job and grow into barnacles proper. They pick up chemical clues from barnacles that have already settled - hence their appearance in tightly packed colonies. If the rock they settle on is actually a limpet I don't suppose they notice the difference, and anyway it will be of no detriment to the barnacle as it gets piggy-backed about. But what about the limpet? Well, I guess a few barnacles on your back are neither here nor there and may even be useful camouflage against a passing oystercatcher. Perhaps it's the ones around you that you need to worry about?

Limpets are grazers and when the tide comes in they meander slowly off from their home base rasping off algal growth from the rock surface as they go before returning to exactly the same home spot. They have nowhere else to go but that spot as it's the only place on which their shell is a perfect fit for the rock shape, enabling them to batten down the hatches when the tide goes out and avoid drying up.

What is puzzling me is how the limpet in the photo makes a living as it is almost completely hemmed in by barnacles with its grazing territory severely compromised.

I reckon it's doomed in a barnacle prison and wonder if the barnacle-free spaces nearby are where its mates have starved, died and dropped off. If I am right then eventually the barnacles should see off all the limpets and take over the whole patch.

This could all be complete rubbish of course! I await your pearls of wisdom...

Grabbed by the Barnacles

After a holiday pounding the London streets with the family, what better way to unwind than to ponder the barnacle.

From a distance, the intertidal rocks to the north of Bamburgh are a lovely caramel colour but get close up and you see that the rock is in fact almost black. The caramel effect is made by barnacles - millions upon millions of them. What particularly grabbed me though, is that when you look more closely, it is mainly the upper parts of the bigger barnacles that have the colouration.

The bases of the shell plates are white, as are the smaller ones. Why?

I have to say that I have struggled to find a definitive answer but a passing reference (in a google hit that frustratingly wouldn't actually open) hinted at colouration due to 'algal infection'. If this is true, why white at the base and why only the larger barnacles?

Here's my theory then, for what its worth. As a barnacle grows so the external shell plates have to enlarge to accommodate the bigger beast inside. In this species, new shell material is secreted along the base of each shell-plate. Perhaps then, the new stuff stays white for a while until the algal colouration takes effect, by which time its been pushed further up the shell by more new (and white) growth below. Hey presto, white bottoms and caramel tops. If this is true then the young barnacles -the smaller ones- should be all white (they are - see photo) and a barnacle that has finished growing should be all caramel (see the larger barnacle at the bottom of the photo). I rest my case.

This species is Semibalanus balanoides, the most widespread and successful intertidal barnacle in the British Isles. Until the mid 70s it was known as Balanus balanoides until cut down to size and renamed by the barnacle boffins.

Stood on and ignored by many a beach visitor it may be, but I reckon that any creature that spends the first part of its life floating about as a piece of plankton, then one day sticks itself by the head to a rock, turns itself into a hermaphrodite crustacean in a rock-hard shell stuck down with one of the toughest natural adhesives there is, catches its dinner with its feet and has, proportionately, one of the largest male appendages in the animal kingdom (which incidentally drops off and then re-grows each year) is worthy of a bit more attention than that.....