Nick Upton / Group of Rough periwinkles (Littorina saxatilis) with various colours and patterns sheltering in limestone rock crevice high on the shore at low tide, alongside encrusting Montagus's stellate barnacle...

Nick Upton / Rough periwinkles (Littorina saxatilis) with varied colours and patterns sheltering in limestone rock crevice high on the shore at low tide, with several Montagus's stellate barnacles (Chthamalaus mon...

Robert Thompson / Periwinkles {Littorina saxatilis} and {Littorina littorea} and Barnacles exposed on rock at shoreline, Killough, County Down, Northern Ireland, UK, December

Robert Thompson / Periwinkles {Littorina saxatilis} on rock, Strangford Lough, County Down, Northern Ireland, UK, September

Philippe Clement / Rough periwinkle (Littorina saxatilis) showing foot inside shell, Belgium

Philippe Clement / Rough periwinkle (Littorina saxatilis) shell showing aperture, Normandy, France

Philippe Clement / Rough periwinkle (Littorina saxatilis) shell, Belgium

Philippe Clement / Rough periwinkle (Littorina saxatilis) shell, Normandy, France

Robert Thompson / Rough periwinkle (Littorina saxatilis) Killard Point, NNR Ballyhornan, County Down, Northern Ireland.

Robert Thompson / Rough periwinkle (Littorina saxatilis) Killard Point, NNR Ballyhornan, County Down, Northern Ireland.