Periwinkle    Littorina sp.

Poke around in the upper intertidal zone and you will find small (1/4 to 1/2 inch), dark colored snails with spiral shells.  These are called periwinkles and are found in both the Pacific Ocean from Alaska to Baja California as well on the shoreline of the Atlantic Ocean. In Puget Sound there are several different species which are hard tell apart.  Generally the Sitka periwinkle is quite broad and lives in protected areas whereas the Checkered periwinkle often has white and dark checker stripes, is slimmer and lives in more exposed areas.  There are also two other species which have been recently recognized.  Reproductive anatomy is the only way to be sure of identification.

Periwinkles feed on small algae and lichens using their file-like tongue to scrape off the plants.  They can only do this when the tide is high which is usually only for a few hours each day.  In fact they cannot tolerate constant submersion and will crawl to the surface if put an aquarium.  When the tide goes out they close up the opening in the shell using a door called an operculum.  In addition they secrete mucus that when dry attaches the shell to the surface, keeping it in place.  They are fed on by crabs, snails and sea stars.