Common periwinkle

The common periwinkle or winkle (Littorina littorea) is a species of small edible whelk or sea snail, a marine gastropod mollusc that has gills and an operculum, and is classified within the family Littorinidae, the periwinkles. This is a robust intertidal species with a dark and sometimes banded shell. It is native to the rocky shores of the northeastern, and introduced to the northwestern, Atlantic Ocean. The shell is broadly ovate, thick, and sharply pointed except when eroded. The shell contains six to seven whorls with some fine threads and wrinkles. The color varies from grayish to gray-brown, often with dark spiral bands. The base of the columella is white. The shell lacks an umbilicus. The white outer lip is sometimes checkered with brown patches. The inside of the shell is chocolate brown. The width of the shell ranges from at maturity, with an average length of . Shell height can reach up to , The length is measured from the end of the aperture to the apex. The height is measured by placing the shell with the aperture flat on a surface and measuring vertically. Littorina littorea can be highly variable in phenotype, with several different morphs known. Its phenotypic variations may be indicative of speciation, as opposed to phenotypic plasticity. This is of particular importance to evolutionary biology, as it may represent an opportunity to observe a transitional phase in the evolution of an organism. Common periwinkles are native to the northeastern coasts of the Atlantic Ocean, including northern Spain, France, Great Britain, Ireland, Scandinavia, Russia, and Nigeria. There have been more than 14,000 observations made available as a dataset at the Global Biodiversity Information Facility - Littorina littorea, which can be explored. More distribution information can also be found at Ocean Biographic Information System - Littorina littorea. The NBN Gateway - Littorina littorea has a distribution map over the UK and Ireland. These datasets may overlap.