At low tide, the periwinkles crawl down to the base of the stem, where algae grow best, and graze the algae off the stems. When the tide rises, however, the periwinkles crawl up the stems. Why don't they just stay down where their food is? Because blue crabs swim in with the rising water, and they like to eat periwinkles. The periwinkles crawl up the stem to stay away from the crabs, but some crabs have learned to reach up out of the water and pick off a periwinkle.