One might expect them to stick to rock (7)
I believe the answer is:
limpets
I'm afraid I can't explain this answer.
Can you help me to learn more?
(Other definitions for limpets that I've seen before include "Proverbially clingy marine gastropods" , "'Molluscs, they cling to rocks (7)'" , "Marine molluscs that cling tightly to rocks" .)