Fissurella apertura, also known as keyhole limpet, is a species of sea snail with an oval-shaped opening on the top of its shell. Synonyms for Fissurella apertura include limpets, keyhole snails, and slit shells. The snail's unique structure allows them to cling tightly to rocks in intertidal zones, where they feed on algae and other small organisms. The characteristic keyhole-shaped opening helps to regulate water flow, and the snail's strong foot facilitates movement and adhesion. Fissurella apertura is also an important bioindicator of environmental change, with researchers using the snail to monitor the effects of ocean acidification, climate change, and pollution on marine organisms.