The Cloudy Periwinkle, Littoraria nebulosa (Lamarck, 1822), is found along the coast of Southwest Florida and most of the tropical western Atlantic. They live high above the high tide line, mostly on dead tree trunks and branches, where they graze on microalgae and fungi. Periwinkles are marine gastropods that became almost completely independent of the aquatic environment. I took this photo at Cayo Costa State Park, two islands north of Captiva Island. The image shows unusually bluish-colored Cloudy Periwinkles nestled in a hollow in a dead black mangrove tree, Avicennia germinans. Read more about local mollusks and their shells in our Southwest Florida Shell Guide.