A cluster of living specimens of the globular Rough Periwinkle, Littorina saxatilis (Olivi), huddling in a small rock crevice on the beach at Kimmeridge Bay, Dorset – part of the Jurassic Coast. They can be various colours but the green hue on some of these is only due to a surface coating of microscopic algae. This species of gastropod mollusc is generally small being only 18 by 14 mm at most. There are three or four rounded whorls and a large rounded aperture or opening by which the animal can withdraw into the shell. The surface may be smooth but it frequently has sharp ridges along the spiral coil of the shell – giving it the rough texture from which it gets its common name.
Rough periwinkles live on the upper shore, often among Pelvetia and Fucus spiralis; also on mudflats and areas of low salinity.
A very similar gastropod mollusc is the Black-lined Periwinkle (Littorina nigro-lineata Gray). However, it is on maturity a lot larger: upto 30 by 27 mm. The aperture is a different elongate-oval shape; there are five or six whorls; and the spiral ridges are more flat strap-like compared to the sharp ridges of the Rough Periwinkle. Additionally, in the colour of the shell in the grooves or striae between the spirals is usually deep brown and it is this feature that gives it the specific name nigrolineata.
Black-lined Periwinkles live on the mid part of rocky shores among Egg Wrack and Fucus seaweeds, right up to the barnacle zone.
Some empty shells of the Black-lined Periwinkle, not actually from Kimmeridge but elsewhere in the English Channel, are shown below to highlight the difference in shape, surface sculpturing and texture to that of the Rough Periwinkle.
For more information about Kimmeridge Bay see the Purbeck Marine Wildlife Reserve Web Site.
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