If you get on your hands and knees to peer closely at the acorn barnacles high up on rocky shores, especially in the splash zone, you may be surprised to see tiny gastropod molluscs on them, between them, and even sheltering inside their empty cases. Some of these molluscs look like smooth, dark grey grape pips. They are Small Periwinkles, Melarhaphe neritoides Linnaeus, formerly known as Littorina neritoides.
Small Periwinkle shells have a flat-sided spire and pointed apex. The main shell body whorl is black or brown but the colour is variable and light-hued specimens do occur. The surface of the shell is generally smooth but the body whorl may have spiral banding or vertical stripes. The whole shell measures no more than 9mm high by 7mm across and many are a lot smaller. These seashore creatures feed on detritus and black lichen. Black lichen can grow in very small patches on barnacle shells or as more extensive areas on the rocks. At Winspit in Dorset where these photographs were taken, the appearance of the barnacle shells has been altered by the presence of endo-lithic lichens so that they have many pits and lace-like sculpturings. The pits are the place where the fungus part of the lichen produces spores. It is possible to see these as small black dots in some of the pits.
You can read more about this endolithic lichen and pitted barnacle shells in an earlier post: Pitted barnacle shells at Bran Point.
Also present in these photographs is the larger Rough Periwinkle – Littorina saxatilis (Olivi) – which tends to be yellow and is covered with sharp spiral ridges. You can read more about Rough Periwinkles in an earlier post: Rough Periwinkles at Kimmeridge Bay.
Revision of a post first published 17 December 2009
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