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...