Build it and they will come: habitat and spatial drivers shape fish assemblages and ecological functions on shellfish reefs

Shellfish reefs support biodiversity, fisheries and coastal protection but are globally degraded. Effective shellfish reef conservation and restoration requires an understanding of the drivers of assemblage structure and ecological functions that underpin reef resilience. We investigated the spatial and habitat characteristics that shape fish communities, their functional diversity, and key ecological functions, including shellfish recruitment, herbivory, and carnivory at 10 intertidal shellfish reefs in Moreton Bay, central eastern Australia. Fish assemblages on shellfish reefs were shaped by habitat and spatial factors, particularly live oyster abundance, oyster size variation, and proximity to seagrass. Fish functional diversity, richness and evenness, and metrics indexing total ecosystem functioning all peaked at sites with higher live oyster abundance (400 live individuals/m 2 ), lower mud cover (<25%), and lowest intertidal area nearby (<5 hectares within 500 m). Habitat characteristics, such as reef vertical relief, reef patch size and muddy sediment presence combined with spatial isolation from connected habitats to strongly influence ecological functions. Specifically, herbivory was higher at sites with higher vertical relief (from 0.7 to 1.7 g/72hr), shellfish recruitment was highest at sites within larger patches (from 60 to 180 recruits/m 2 ), and carnivory was higher at reefs less isolated from nearby subtidal channels and with less muddy sediment (from 0 to 25 g/hr). These results highlight the importance of habitat quality and spatial context in underpinning the environmental health of shellfish reefs. Conserving and restoring larger, more complex reefs in highly connected seascapes can enhance ecological functions and biodiversity, boosting management success. • Shellfish reefs are under threat globally leading to calls for their restoration • Habitat and spatial context strongly shape shellfish-reef fish communities • Reef complexity and connectivity drives fish diversity and ecosystem functioning • Herbivory, carnivory and recruitment vary with reef structure and seascape setting • Restoring larger, complex, connected reefs enhances biodiversity and resilience