Marine Wildlife Encyclopedia
Common Periwinkle Littorina littorea
The common periwinkle has a black to dark gray, sharply conical shell and slightly flattened tentacles, which in juveniles also have conspicuous black banding. The sexes are separate and fertilization occurs internally. Female common periwinkles release egg capsules, containing two or three eggs, directly into the water during the spring tides. The eggs hatch into free-swimming larvae that float in the plankton for up to six weeks. After settling and metamorphosing into the adult form, it takes a further two to three years for the adult common periwinkle to fully mature. It feeds mainly on algae, which it rasps from the rocks. Recently, the common periwinkle was accidentally introduced into North America, where its selective grazing of fast-growing algal species has considerably affected the ecology of some rocky shores.