Rough keyhole limpet (Diodora aspera) shells have a radial pattern and a large circular hole at the top (apex) of the shell. This circular opening was used as part of the respiration process. In a living rough keyhole limpet fresh oxygenated water is drawn under the shell and over the gills, while the waste water is expelled from this top opening creating a flow of clean water through the animal. For defense, this limpet can extend its mantle upward covering the shell margin to help provide protection against sea stars. The rough keyhole limpet grows to about three inches (7.6 cm) long, with a range from northern Alaska to northern Mexico.