At low tide, Periwinkles are usually wedged into crevices and cracks in rocks and even inside the empty shells of dead oysters and barnacles. Small but resilient snails, periwinkles can breathe air so they are among the most common snails seen on the rocks above the high tide line. Periwinkles graze on tiny algae growing as a film over the rocks. Once a year, males find a female for mating. Females lay their eggs in a mucus bundle in tide pools. The larvae hatch and are washed out to sea to grow.