Spotted Periwinkle, Littoraria pintado

On the rocks just above the water at the beach are lots of spotted periwinkles (Littoraria pintado).  A lot of these have snails in them but they are also favorite shells for hermit crabs.  We grabbed a few from next to a tide pool and dropped them in our aquarium.  The first thing they do is crawl up out of the water and wait, without moving, for days.  After a while, occasionally, they will appear in a different position a few days later.  At this point I'm guessing they crawl around under the water at night. Their cousins, the common periwinkle (Littorina littorea) of Europe, are edible.