Mangrove-associated Pacific oysters (Magallana gigas) influence estuarine biodiversity

Estuarine ecosystems are often characterised by endemic foundation organisms which facilitate ecosystem interactions and processes established over millennia. Introduction of non-native foundation species can significantly alter ecological communities and functions. Here, we assessed the effects of introduced, reef-forming Pacific oyster Magallana gigas, within a temperate Australian mangrove-dominated estuary. Specifically, we investigated whether mangrove-attached M. gigas oyster reefs influenced intertidal fish and invertebrate communities, and fish foraging behaviour. We measured and contrasted the benthic structure and faunal communities of fringing bare sediment, Avicennia marina mangrove and Pacific oyster M. gigas reef habitats using a combination of quadrats, fyke nets and remote unbaited video (RUV) surveys. Pacific oyster reefs showed no impacts on pneumatophore density or morphology, but were positively associated with higher seedling densities. Furthermore, invertebrate and fish community metrics (e.g. richness, biomass, length) were typically higher in M. gigas reefs compared to the other habitats. However, several mobile, non-native species were predominantly detected in M. gigas reefs, including exotic gobies and European shore crabs Carcinus maenas, a declared marine pest. Overall, we found that M. gigas reefs in fringing mangrove forests were associated with biodiverse faunal communities, including fisheries-targeted species, but also facilitated other non-native species. These outcomes highlight some of the ecological pros and cons of non-native oyster reefs and the complexity of managing estuaries globally where Pacific oysters increasingly co-occur with endemic habitat-forming species.