The Last Frontier

sea snail
Pioneering barnacles and periwinkles (Littorina littorea) ordinarily cover the upper littoral. Their populations are regularly replenished by the young, which float in from the sea on the spring flood tides, often attaching themselves near established groups of adults. Occasionally barnacles settle where they will not be able to mature when the tide drops. Unable to maneuver down into a wetter environment, they die. Periwin­kles, however, are mobile creatures. Impelled by a complex gravity-overcoming mechanism to move up the beach, each species advances as far as it can without excessive drying, the rough periwinkle stopping at the limits of the seaweed and the small peri­winkle going on to reach the high spring tide mark.