Provides adhesiveness to the mussel's foot. Mussels produce one of the strongest water insoluble glues. The mussel's adhesive is a bundle of threads, called a byssus, formed by a fibrous collagenous core coated with adhesive proteins (By similarity). Produced by the byssal gland. Almost exclusively composed of repeats of the decapeptides APPPAWTAWK and ATPKPWTAWK. The identity of the hexosyl group at the C-2 position in the modified tryptophan residues has not been confirmed, but UV absorbance data suggests that it is mannose.