It’s hard to take a trip to a rocky coastline in New England without noticing hundreds of common periwinkles (Littorina littorea). Despite the name “common”, these gastropods are not actually native to New England. They were introduced in the mid-19th century, likely through ballast water in ships from Europe (its native range). Since then, the common periwinkle has become well established in eastern North America and is now the most common marine snail along our coastline. Although most abundant on rocky substrates and common in tidepools, it is a habitat generalist and can also occur on muddy or sandy bottoms. In favorable habitats, it can reach densities of up to 200-800 individuals per square meter.

Common periwinkles are often found in high densities on rocks. 

Although most common on rocks, periwinkles can inhabit sandy and muddy areas. 

Common periwinkles are masters of the intertidal zone, and are well adapted for long periods of time both in and out of water. As denizens of the mid- to upper-intertidal zone, they not only require a way to breathe when the water recedes during low tide, but they must also avoid desiccation caused by exposure to wind and sun. To survive through such difficult conditions, they close themselves into their shells with their operculum, retaining moisture in their gills to allow continued breathing, and excrete a sticky mucous that hardens, firmly attaching the animal to the rock it’s on.

Periwinkles cling tightly to rocks during low tide.

Periwinkles use their operculum to seal up the opening to their shell not only to avoid desiccation, but also to avoid predators. 

When the water returns, periwinkles use their muscular foot to hold securely onto the substrate when waves crash over the rocks. Their foot is divided into a right and a left half, which the snail moves alternately in order to propel itself forward over the surface in search of food. Their stalked tentacles are sensory organs that are used to see and taste, directing the snail to the best patches of algae for grazing. Although their diet consists largely of algae, they are also known to feed on small newly settled barnacles and other invertebrates. Fascinatingly, although they are small in size, the constant scraping action of the radula of many individual snails can eventually lead to rock erosion.

Periwinkles use their stalked tentacles to search for algae and other suitable food.