Some types of barnacle live in the deep oceans, but many more thrive on the narrow intertidal band of rocky shores, where they have evolved special adaptations to surviving periods of low tide when the water retreats and they are exposed to the air. At this time, the biggest threat to the creatures' survival is water loss, so barnacles have special plates that seal them within their own calcareous 'shell' until the tide rises once more. There are two main groups of barnacle. Acorn barnacles cement their cone or volcano-like shells directly to an object, while goose barnacles attach with a long stalk.