Review of the fauna associated with wild and farmed mussels and oysters in the Mediterranean

Mussels and oysters are important ecosystem engineers which modify the physical and chemical characteristics of the environment and create habitats that support highly diverse associated communities. In the Mediterranean Sea, the native Mediterranean mussel Mytilus galloprovincialis and the European flat oyster Ostrea edulis, together the introduced Pacific cupped oyster Magallana gigas, are among the most important habitat builders and farmed molluscs. The widespread disappearance of natural beds of mussels and flat oysters, driven by multiple anthropogenic disturbances, disease outbreaks, and harmful invasive species, has led to the loss of the diverse communities they support. Moreover, the proliferation of harmful invasive species is exacerbated by mollusc aquaculture, which represents one of the main vectors for the introduction and spread of non-native species (NNS) in the Mediterranean. The aim of this review is to compile the first comprehensive database of macrofauna associated with wild and farmed M. galloprovincialis, O. edulis and M. gigas in the Mediterranean Sea, based on an extensive literature review. Specifically, we aimed to identify differences in the diversity of associated fauna among the three mollusc species, assess variability of the fauna between wild and farmed systems, highlight associated non-native species, reveal knowledge gaps, and provide directions for future research. Overall, we recorded 782 taxa, with polychaetes, molluscs, arthropods, and ascidians representing the most species-rich groups. Fauna associated with mussels is the most extensively studied and consequently the most diverse, with a considerably higher number of taxa reported from wild mussel habitats (647) than from mussel farms (204), likely reflecting higher environmental heterogeneity in natural contexts. By contrast, the higher diversity of fauna reported from farmed European oysters (163 taxa) compared to wild ones (50 taxa), is consistent with greater research effort focused on aquaculture systems. Fauna associated with naturalized populations of Pacific oysters (M. gigas) remains severely understudied, with only a few species reported to date. Information on fauna associated with molluscs is particularly lacking along much of the African coastline. Future research should therefore prioritize these understudied habitats and regions. To date, a total of 52 non-native species have been reported from mussel and oyster habitats, with arthropods, molluscs and ascidians comprising the majority. Mussels host the highest number of NNS. Reports of NNS from mollusc habitats are increasing, particularly in mussel and oyster farms, suggesting that research efforts should focus on the detection and monitoring of NNS in Mediterranean aquaculture systems. Particular attention should be given to key groups such as non-native ascidians and shell-boring polychaetes, which are known pests and parasites of molluscs. This review provides a baseline for monitoring changes in communities associated with mussels and oysters in the Mediterranean. It may also serve as a reference point for the implementation of management strategies aimed at conserving and restoring mollusc habitats, as well as limiting the spread of non-native and pest species in aquaculture systems.