Nick Upton / Flat periwinkle (Littorina obtusata / Littorina littoralis) on Toothed wrack (Fucus serratus) submerged at mid tide, near Falmouth, Cornwall, UK, August.

Alex Hyde / Common Flat Periwinkle (Littorina obtusata / littoralis) against white background. From the Isle of Skye, Inner Hebrides, Scotland, UK.

Nick Upton / Flat periwinkle (Littorina littoralis) crawling over Toothed wrack (Fucus serratus) exposed at low tide, Crail, Scotland, UK, July

Nick Upton / Flat periwinkle (Littorina littoralis) crawling over Toothed wrack (Fucus serratus) exposed at low tide, Crail, Scotland, UK, July

MYN / Jerry Monkman / Common periwinkle (Littorina littorea) Rye, New Hampshire, USA, September. meetyourneighbours.net project

Robert Thompson / Smooth / Flat periwinkle {Littorina littoralis} shells, UK

Robert Thompson / Smooth / Flat periwinkle {Littorina littoralis} shells, UK

Robert Thompson / Smooth / Flat periwinkle {Littorina littoralis} shells, UK

Adrian Davies / Flat periwinkles (Littorina littoralis) on Spiral wrack (Fucus spiralis) seaweed, UK

Sue Daly / Flat periwinkles {Littorina littoralis} Jersey, Channel Is, UK Rozel

Niall Benvie / Serrated (Toothed) wrack (Fucus serratus) with marine mollusc (Littorina littoralis) on the seaweed. Scotland, UK, Europe

Niall Benvie / Flat periwinkle (Littorina littoralis) feeds on Fucus seaweed, Scotland

Niall Benvie / Flat periwinkle shells {Littorina littoralis} Scotland, UK