Ecosystem services from shellfish reefs as a nature-based solution: a global evidence synthesis to guide restoration and policy

1. Context. Shellfish reefs, comprising oysters, mussels, clams, and mixed bivalves, act as ecosystem engineers and nature-based solutions (NbS), providing supporting, regulating, provisioning, and cultural ecosystem services (ES). Yet, despite rapid growth in restoration practice, the translation of ES evidence into policy and management remains uneven across regions and taxa. 2. Objectives. We systematically synthesized global evidence to (i) classify ES delivered by shellfish, (ii) link restoration approaches and techniques to reported ES, (iii) quantify policy mentions and governance frameworks, and (iv) identify regional and taxonomic gaps constraining applied uptake. 3. Methods. We quantitatively assessed how research on shellfish ES and restoration has evolved over the past two decades, identifying biases and missing linkages between ecological evidence, ecosystem service assessment, and policy uptake. This approach enables a systematic evaluation of progress toward integrating restoration outcomes into governance frameworks. 4. Results. Evidence is dominated by regulating and supporting services, particularly water filtration, nutrient removal, habitat provision, and coastal protection, while provisioning and cultural services remain limited. About 75% of studies address restoration, mainly active or hybrid approaches (substrate addition, seeding, engineering, community engagement), with monitoring reported in only one-sixth. Roughly three-quarters cite a policy framework, dominated by regulatory and governmental instruments, with market-based tools emerging. Stakeholder engagement appears in about one fifth of studies, often tied to cultural services. Research and policy linkages remain weak for mussel and clam systems and across the Global South. 5. Synthesis and applications. To enhance the policy relevance of shellfish-based NbS, restoration should integrate socio-ecological goals through hybrid approaches, standardized monitoring, and participatory design. Linking restoration outcomes with market-based and regulatory instruments, such as nutrient credits and green-infrastructure standards, can strengthen implementation. Expanding research and investment in under-represented regions of the Global South would enable shellfish reefs to deliver equitable, multi-benefit outcomes for biodiversity, coastal protection, and human well-being.