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Introduced Pacific oysters provide biodiversity benefits in Australia's Port River

The study—led by Brad Martin, a Flinders University Ph.D. candidate, with colleagues from Flinders' College of Science and Engineering—assessed the effects of introduced, reef-forming Pacific oysters (Magallana gigas), within a temperate Australian mangrove-dominated estuary (the Port River-Barker Inlet Estuary) and how they influenced both intertidal fish and invertebrate communities, and fish foraging behavior.

"The introduction of non-native foundation species such as Pacific oysters can significantly alter ecological communities and functions, but through this research we found that these Port River oyster reefs are providing habitats for coastal animals," says Martin.

in NeoBiota, the study shows both positive and negative ecological impacts of these "invaders" on gray mangrove forest biodiversity in the Port River and how they are influencing local estuarine biodiversity. This includes:

• Oysters had no discernible impact on mangrove pneumatophores (aerial roots), but doubled the densities of mangrove seedlings.
• Pacific oyster habitats supported 2x greater richness and 8x greater biomass of mobile invertebrates (including crabs, worms and snails) compared to other habitats. This means there is a richer diversity of prey for fish to feed on, and that oysters are supporting a greater range of invertebrates.
• Video surveys showed that the oyster habitats generally supported greater fish richness, abundances and foraging behavior. Fisheries-targeted black bream showed a preference for the oyster habitat, while juvenile King George whiting were common to mangroves with and without oysters. Oyster shells were also essential for native oyster blennies which used the shells for nest sites and shelter through high and low tides.
• Other non-native species including European shore crabs, crested oystergobies and mud creepers were highly associated with Pacific oyster habitats.

Despite the Pacific oyster's status as an exotic introduced species, the researchers found that these novel oyster reefs generally benefited biodiversity, including fisheries-targeted species, but also other non-native species and declared marine pests.

"This demonstrates that in high densities, Pacific oysters can modify ecological communities when introduced into mangrove-dominated estuaries," says Martin. "Given that South Australia historically had subtidal native oyster reefs, these intertidal non-native oyster reefs likely represent novel ecosystems but may offer similar benefits."

Given the biosecurity focus on shellfish reef management in Port River—which includes a ban on the removal of wild Pacific oysters to protect SA's shellfish industry from the deadly POMs virus—Martin says this case study improves knowledge around the ecological impacts of wild Pacific oysters, and may indicate possible benefits applicable to native shellfish reefs and restoration efforts in South Australia.

This work complements outcomes from a recent by Murdoch University, highlighting the complex ecological impacts that can arise from invasive shellfish.

It is the first Australian study on how invasive Pacific oyster reefs impact fishes, and the first globally of such reefs in mangrove systems.