Diversity and structure of faunal assemblages associated with mussel and oyster habitats in the Adriatic Sea, with special emphasis on non-indigenous species

Mussels and oysters are ecosystem engineers that can create habitats which harbour rich fauna, including pests and non-indigenous species. Molluscs’ aquaculture is one of the main vectors for the introduction and spread of non-indigenous species worldwide. The aim of this study was to examine diversity and structure of the faunal assemblages associated with wild and farmed populations of Mediterranean mussel Mytilus galloprovincialis Lamarck, 1819 and Pacific oyster Magallana gigas (Thunberg, 1793) in the Adriatic Sea, with special emphasis on non-indigenous species. Molluscs were sampled in spring and autumn from 3 mussel farms and 3 sites with wild mussel populations and from 2 oyster farms and 2 sites with wild oyster populations, along the coast of Emilia-Romagna region (Italy, northern Adriatic). Altogether 147 invertebrate taxa were found from mussels (114 from wild and 91 from farmed populations) and 103 taxa from oysters (47 from wild and 90 from farmed populations), with annelids, molluscs and arthropods being the most species-rich phyla, and arthropods being the most abundant group. Significant differences in diversity and structure of associated faunal assemblages were observed between farmed and wild molluscs’ habitats and between sampling seasons. Fourteen non-indigenous species were found from mussels populations (12 from wild and 8 from farmed ones; 5 molluscs, 4 crustaceans, 4 polychaete annelids and 1 ascidian) and 13 from oysters populations (10 from wild, 9 from farmed ones; 6 crustaceans, 3 molluscs, 3 polychaete annelid and 1 ascidian). Many of them are reported for the first time within these habitats in the Mediterranean Sea. This is also the first study of the fauna associated with wild populations of Pacific oysters in the Mediterranean. Our results suggest that mussels and oysters play an important role in the spread of non-indigenous species and confirm the need for a regular monitoring of their associated fauna.