The Common Periwinkle is a small snail with a smooth, spiral shell that lives on rocks and in rockpools along the Irish coast. It feeds mainly on algae, scraping it off the rocks with a rough tongue, which helps keep the rock surfaces clean and stops algae from taking over. Periwinkles are important for the health of the rockpool because they help control algae growth, making space for other plants and animals to live. They are harmless and often serve as food for crabs, birds, and fish. Their presence shows a healthy and balanced rockpool environment.