To study the impact of ferric ions on the ability of Shewanella marisflavi ECSMB14101 biofilms to induce larval settlement and metamorphosis in Mytilus coruscus, single bacterial biofilms cultivated with different ferric ion concentrations were used to induce larval settlement and metamorphosis. The effects of ferric ion concentration on the inducing activity of larval settlement and metamorphosis, bacterial density, extracellular polymeric substances, and cytochrome C content of S. marisflavi biofilms were analysed. Results showed that among the seven groups with ferric ion concentrations of 0, 1, 10, 20, 30, 40, and 50 μmol/L, the biofilm of S. marisflavi cultivated at a concentration of 20 μmol/L exhibited the highest inducing activity. Furthermore, at this concentration, both the bacterial density of the biofilm and the levels of extracellular proteins and cytochrome C reached their peak among all seven tested concentrations. Extracellular polysaccharides and lipids were not significantly affected by ferric ion concentration. It was shown that under the effects of ferric ions, S. marisflavi biofilms regulate the larval settlement and metamorphosis in M. coruscus by modulating the production of extracellular proteins, including cytochrome C. This research offered a solid theoretical foundation and served as a valuable reference for the in-depth investigation of the molecular regulatory mechanisms underlying larval settlement and metamorphosis in M. coruscus. Additionally, it contributes to the understanding of interactions between shellfish and bacteria.