Bacterial-derived γ-aminobutyric acid (GABA) regulates larval settlement and metamorphosis of <i>Mytilus coruscus</i>

Bacteria play a vital role in the settlement and metamorphosis of marine invertebrates. However, the molecular mechanisms by which bacteria regulate the metamorphosis from larva to juvenile remain unknown. This study aims to investigate whether bacteria derived from <i>Mytilus coruscus</i> affect the larval settlement and metamorphosis of this species. Our results showed that all eight bacterial strains can induce settlement and metamorphosis in this mussel species. Polysaccharides and gamma-aminobutyric acid (GABA) produced by bacteria positively trigger this transformation in mussels, with GABA acting as the primary inducer. <i>Marinomonas foliarum</i> exhibited the highest GABA production and induction activity among the experimental strains. Compared with biofilms alone, the addition of a GABA synthesis inhibitor during <i>Marinomonas foliarum</i> biofilm formation reduced its induction activity by 56.02%. Our results reveal a novel role of bacterial GABA in triggering mussel larval settlement and metamorphosis.