To anchor in seashore habitats, mussels fabricate adhesive byssus fibers mechanically reinforced by protein-metal coordination mediated via 3,4-dihydroxyphenylalanine (DOPA) – providing a well-established role model for bio-inspired design of smart metallopolymers and underwater glues. However, currently, the mechanism by which metal ions are integrated as cross-links during byssus formation is completely unknown. Here, we investigated the byssus formation process, combining traditional and advanced methods to identify how and when metals are incorporated into the material. We discovered that mussels concentrate and store iron and vanadium ions in intracellular metal storage particles (MSPs) complexed with previously unknown catechol-based storage molecules. During thread formation, stockpiled secretory vesicles containing concentrated fluid proteins are mixed with MSPs within a complex microfluidic-like network of interconnected channels where they coalesce forming protein-metal bonds within the nascent byssus. These insights are important for bio-inspired materials design, but also from a biological and chemical perspective – the active accumulation and utilization of vanadium is extremely rare in nature.