What are byssal threads made by mussels?

Byssal threads, also known as byssus, are proteinaceous threads secreted by a gland located in the foot of certain bivalve mollusks, including marine mussels (family Mytilidae) and freshwater species like the zebra mussel.12

These threads allow mussels to attach themselves securely to solid objects, such as rocks, ship hulls, or one another, enabling them to colonize hard surfaces on wave-swept shores. While the byssus is a larval feature in many bivalves, true mussels retain and enlarge these threads into adulthood. In this context, the mussel's foot muscles are often redefined as byssal retractors, which help pull the shell firmly against the substrate.123