Monday, 14 September 2009

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...


  1. That's a fascinating theory - I reckon you must be right. Limpets on the Sandown breakwaters are encrusted with barnacles and sometimes tiny forests of algae. I've never considered the consequences of the barnacled territory around them.

  2. Interesting thought. I notice that there's another much smaller limpet above and to the left of the big one, which appears to be partially smothered by barnacles and maybe even (fatally?)glued down to the rock by them. The other player in this conundrum is the dog whelk, that feeds on barnacles and is most likely the cause of all those empty 'volcanoes' in the picture. Sometimes you find dead limpets on the beach with a bunch of seaweed attached, that was probably instrumental in wrenching a limpet from its rock as the weed was battered in a storm - a case of a little bit of attached weed being camouflage for a limpet, too much being a fatal encumbrance. being attached to a limpet is probably the safest place for a juvenile seaweed, until it grows too large and brings about the demise of its anchor.

  3. Hi Rob. I suppose part of our problem is that we only get to see these communities when the tide is out and all the beasts are tucked up tight. Its difficult, unless you take up diving, to have much chance to observe behaviour. Rock pools offer some insight but there must be so much that we just don't really know.

  4. Hi Phil. There's another aspect to the barnacle-v-limpet contest which I've just discovered. Limpets graze on newly settled cypris larvae of the barnacles. So if it moves fast enough it might be able to hold back the encroaching hoards - for a while at least!

    I read a report once about the Torrey Canyon oil tanker break up which said that the oil spill killed off all the limpets and the following year what had been clean inter-tidal rocks were absolutely smothered in seaweed growth. Just shows the phenomenal scale of limpet grazing.