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

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