Barnacles uncovered

On New Year’s day I was drawn to the sea for a walk to clear the toxins of the previous nights festivities. The long sandy beach at Slapton eventually made way for some rocks where we rested to enjoy a lunch of leftover Scotch eggs and shortbread. Whilst relaxing, my eyes became focussed on a more macro world of animals on the rocks as the tide went out.....

 

Acorn barnacles with limpets

....Most people have had the painful experience of barnacle encrusted rocks whilst rock pooling at the seaside. These creatures commonly coat rocks alongside other shelled animals, but are in fact very different in origin. Molluscs such as limpets and periwinkles are relatives of snails using a muscular foot to move and graze the rock surfaces whilst carrying their protective shells with them. The barnacles that hurt our feet are the static adult forms that await the tide to come in to feed, using feathery legs (cirri) that emerge when their opercular plates are opened, like a scene from Dr No. These cirri are thrown out and back like thin clawing hands, netting plankton and detritus. It is however the young barnacles that give themselves away as crustaceans, relatives of crabs, prawns and shrimps. The free-swimming larvae travel with other sea plankton, moulting several times before settling down to a ‘fixed’ life on a rock. They literally cement themselves down head first once they have selected a spot, ideally rough and shaded, orientating themselves across the current to maximise feeding (Yonge, 1976).

The commonest forms of barnacle on British shores are various types of acorn-barnacles. The species are distinguished by the number of shell plates (six in British spp as opposed to four with the Australian invader, Elminius), and the shape of the opercular openings which vary from oval to kite to diamond (Oakley, 2010). The image is probably of Chthamalus sp with kite shapes. One of the commonest British species is Semibalanus balanoides, which has a fascinating sex life - To overcome the reproductive challenge of separate sexes, each glued down and surrounded by armour plating, the male organ is gigantic (equivalent to 20 metres in human terms) – once the males have completed copulating with the females fertilizing up to 8000 eggs the mighty organ withers away and the males turn into females for next season (The Seashore, 2013).

Oakley, J. (2010) Seashore Safaris. Cardiff: Graffeg Books

The Seashore (2013) [online] www.theseashore.org.uk  [Accessed January 10^th 2013]

Yonge, C.M. (1976) The Sea Shore (The New Naturalist series). London: Collins