Bivalve animals such as mussels and pearl oysters secrete proteinaceous byssus from the foot to attach themselves to solid surfaces finder seawater. Although the biomolecules of mussel byssus have been extensively studied, how they form insoluble threads underwater from soluble protein precursors and how they produce hierarchical microscale threads from biomolecules remains unclear. Here, using the pearl oyster Pinctada fucata as a model, metal ions rather than pH are found to play critical roles in the solidification process of the byssus. Particularly, Ca2+ can induce self-assembly of the foot proteins and the byssal proteins, resulting in aggregate formation. At a concentration of 10 mM, protein fibers with a diameter of approximately 26 mu m and a length ranging from 50 to 400 mu m, were formed. Moreover, the fibers are composed of 60-90 nm nanospheres, reminiscent of the ultrastructure sizes of threads. It is proposed that Ca2+ can interact with byssal proteins such as thrombospondin-1 containing protein through phosphorylated serine and/or von Willebrand factor type A domains. This study provides insight into how the byssus forms from the soluble protein molecules into insoluble threads underwater and may inspire further biomaterial design for underwater use.