Dark glistening masses of common edible mussels, Mytilus edulis Linnaeus, smother the Carboniferous limestone of Spaniard Rocks and the island of Burry Holms at the north end of Rhossili Bay, Gower. This bivalved mollusc thrives here, particularly where the limestone has dissolved into peculiar shapes providing endless new facets for the mussels to colonise.
We tend to think of mussels as having blue/black shells – at least, that is what we normally see if moules mariniere is ordered, or we find shells on the beach. You can see from these pictures that the living mussels are quite varied in appearance. At least, the immature individuals show lighter colours and stripeyness. I also think the degree of exposure to wave action erodes the brown outer horny periostracum layer, and settlement intensity and competition for food may affect shell thickness and therefore colour.
Where the mussels do not form a continous layer on the rocks, interesting patterns develop. Bare limestone can be coated with green surface algae. Cream coloured acorn barnacles compete for settlement space with the mussels. Where mussels and barnacles occur in abundance, dog whelks are sure to be found in vast numbers because they just love to eat these encrusting shelled animals. They often bore a hole into the shell to get at the meats.
I am always amazed at the variety of colour and pattern in dog whelks. White or yellow through all shades to brown. Plain or striped. Thick shelled or thin. Pointed apices or blunt. Surprisingly, not edible like the mussels they feed on. Not having tried to eat them, I’m not sure why dog whelks are considered unpalatable.
The photographs show that the rocks are covered in several generations of mussels. You can tell this from the relative sizes. New seasons of young mussels prefer to settle and fix themselves by their fine byssus threads to surfaces where mussels already grow. Having said this, sometimes there are so many larval mussels that they settle on an incredible range of surfaces whether or not adult mussels are colonisers there. I have found hundreds of thousands of minute mussels attached to strands of green seaweeds round the corner from Rhossili in Broughton Bay.
Rock surfaces in the right zone for survival can be almost entirely covered with mussels. This is usually the lowest level on vertical or near vertical surfaces.
On a final note, I will return to this subject as mussels are of great economic importance to the local economy, especially in years when, for one reason or another, the cockle populations that are normally commercially exploited off north Gower shores, fail.
Revision of a post first published 14 May 2011
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Jessica's Nature Blog 
Dog whelks will eventually ruin the mussel beds..caswell as well as mumbles islands used to have good crop of mussels,all left there now dog whelks and Perry winkles!
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Thank you, Geoff. Perhaps the dog whelks will become fewer as the mussels disappear and the dog whelks only have barnacles to feed on. Then the mussels will return and the predation cycle will start again.
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