Slipper shell

Crepidula fornicata is a species of medium-sized sea snail, a marine gastropod mollusc in the family Calyptraeidae, the slipper snails and cup and saucer snails. It has many common names, including common slipper shell, common Atlantic slippersnail, boat shell, quarterdeck shell, fornicating slipper snail, Atlantic.

The common slipper shell is a small sea snail that stacks in chains on rocks, shells, and other hard surfaces in shallow coastal waters. Native to the western Atlantic, it has become an invasive species along European coasts.