A Post from the Past 
Your eye can be attracted by small bright spots of colour among the seaweed as you walk along the shore. A closer inspection often reveals living flat periwinkles Littorina obtusata Linnaeus. These delightful little molluscs are only about a centimetre across and can be seen at all times of the year – even on a cold February day.
The colour of the shell is very varied but mostly bright. The shells can be any colour from cream, to yellow, orange and brown. Sometimes there is a faint pattern as well. I remember a long time ago finding hundreds of empty seashells of this type on the beach at Longis Bay in Alderney, one of the Channel Isles; they were spread out multi-coloured and glistening on the wet sand like so many small jewels. I still have a jar of those same shells on my mantlepiece today.
The little flat periwinkle shown in the photograph above was spotted at Kimmeridge Bay in Dorset grazing on a frond of the brown seaweed known as Toothed Wrack or Fucus serratus Linnaeus. You can see the two antennae which it was using to feel its way forward over the weed. The flesh is the same colour as the shell.
Although the shore still had certain types of seaweeds surviving through the winter, like the Fucus that the periwinkle was eating, many types had died back in the colder weather – or at least were not very visible to the casual observer.
In the summer, at the edges of rock pools in the mid shore area you can find, for example, a small tufted seaweed with a pink or purple colour. The many segments that make up its stem and branches are stiffened with calcium. The picture below shows the remaining white chalky ‘skeleton’ of one of these calcareous seaweeds – all that is left at the end of the cold season: it is called Corallina officinalis Linnaeus – it doesn’t have a common name. The whole bead-like structure is only a couple of centimetres high. It is a strange organism to look at – it reminds me of those jointed wooden hands that artists use; or even some sort of construction toy for toddlers.
© Jessica Winder and Jessica’s Nature Blog, 2009
6 Replies to “Flat periwinkles & calcareous seaweed at Kimmeridge in February”
“Flat” is a strange name to give the periwinkle considering its more spherical form, don’t you think? Where does that come from?
The common term ‘flat’ refers to the fact that the apical spire of the shell is flattened and not pointed as usually found in other types of gastropod mollusc like the common and the rough periwinkle.
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Lovely insight. I have a couple of specimens with the very same chalky white ‘skeletons’ attached. I have never stopped to try and identify them – and now i know its Corallina. Thankyou
Thank you, Coleshed. I am glad that the post filled a gap for you. (It was one of the first I ever wrote but has been hidden from public view for many years).