Philippe Clement / Flat periwinkle (Littorina obtusata) shells on beach showing colour variations, Brittany, France

Nick Upton / Flat periwinkle (Littorina obtusata / littoralis) yellow form crawling over rocks exposed at low tide, Cornwall, UK, March.

Nick Upton / Flat periwinkle (Littorina obtusata), yellow form crawling over Spiral wrack (Fucus spiralis). Treyarnon Bay, Cornwall England, UK. September.

Nick Upton / Flat periwinkle (Littorina obtusata), yellow form from Atlantic Ocean. Cornwall, England, UK. September.

Alex Hyde / Common Hermit Crab (Pagurus bernhardus) in Flat Periwincle shell (Littorina obtusata). Isle of Mull, Scotland, UK. June.

Sue Daly / Smooth periwinkles (Littorina obtusata) on kelp, Derrible Bay, Sark, British Channel Islands.

MYN / Carsten Krieger / Flat Periwinkle (Littorina obstusata) from rock pool, County Clare, Republic of Ireland, August. Meetyourneighbours.net project

MYN / Carsten Krieger / Flat Periwinkle (Littorina obtusata) from tide pool, County Clare, Republic of Ireland, August. Meetyourneighbours.net project

MYN / Carsten Krieger / Flat Periwinkle (Littorna obtusata) on Bladderwrack (Fucus vesiculosus) from tide pool, County Clare, Republic of Ireland, December. Meetyourneighbours.net project

Christophe Courteau / Flat periwinkle {Littorina obtusata} Brittany France

Christophe Courteau / Flat periwinkle {Littorina obtusata} Brittany France

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

Alex Hyde / Selection of gastropods commonly found in rockpools, clockwise from top left: Periwinkle (Littorina compressa / nigrolineata), Dogwhelk (Nucella lapillus / Thaius lapillus), Common Flat Periwinkle (Li...

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

Alex Mustard / 2020VISION / Flat periwinkle (Littorina obtusata) feeding on kelp frond, Loch Carron, Ross and Cromarty, Scotland, UK, April

Philippe Clement / Shells of the Flat periwinkle (Littorina obtusata) showing colour variation, Brittany, France

Robert Thompson / Periwinkle {Littorina obtusata} camouflaged on seaweed, Ballyhenry Point, Strangford Lough, County Down, Northern Ireland, UK, September

Philippe Clement / Flat periwinkle (Littorina obtusata) shell on beach, Mediterranean, France

Philippe Clement / Flat periwinkle (Littorina obtusata) shell, Mediterranean, France

Philippe Clement / Flat periwinkle (Littorina obtusata) shell showing the aperture, Mediterranean, France