Animals Atlantic Ocean Bar Harbor Coast Walk CoastWalk Common Periwinkle Gulf of Maine January Littorina littorea Littorina MDI Maine New England Northern Rock Barnacle PhSh Places Periwinkle Sea Semibalanus balanoides algae-covered alive barnacle barnacles beach biology coast coastal coastline gastropod littoral zone low tide marine mollusc mollusk nature periwinkle periwinkles rock rocks rocky seashore shell shore shoreland shoreline small snail snails stone stones stony tidal wildlife