How do mussels attach themselves to rocks and piers?

Mussels attach themselves to rocks, piers, and other solid surfaces using proteinaceous threads known as byssus threads.12

These threads are secreted by a specialized gland located on the animal's foot. By producing these strong, anchor-like fibers, mussels are able to colonize hard substrates on wave-swept shores and often form dense clusters. While this byssal attachment is a characteristic feature retained by adult true mussels (family Mytilidae) and the family Dreissenidae, it allows them to remain securely fixed to their environment despite external pressures.12