Translation:
These are Limpets. The sign reflects how hard it is to unstick them - you can't get them off! Limpets are part of the Mollusc family which all have these conical shaped shells that are rounded at the base. It is easy to identify them with the shape of the shell. When the tide comes in their tentacles appear and the limpets start to move. They have an incredibly strong foot which attaches them to the surface of the rock. As they move they feed, scraping off algae using their extremely tough tongue. As they go, they leave a trail of slime which they then follow to return 'home'.