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.....