From loss to life: how empty Pinna nobilis shells enhance motile macroinvertebrate habitats in soft-bottom environments

The noble pen shell ( Pinna nobilis ) is experiencing mass mortality events across the Mediterranean Sea. In the northern Adriatic, as in many other regions, soft-bottom habitats are now covered with numerous empty shells. To assess their ecological role, we conducted underwater SCUBA surveys using quadrats in 2023 and 2024. We examined the function of empty shells as refuges for motile epibenthic macroinvertebrates across vegetated and unvegetated habitats. Specifically, we compared invertebrate diversity among habitats with and without shells, assessed the influence of shell orientation on community composition, and identified taxa characteristic of each habitat type using non-invasive methods that did not displace shells. Most shells remained upright, though their density and orientation varied among the six study locations. We recorded fauna from seven classes, with malacostracans and gastropods being the most species-rich groups. Biodiversity metrics differed markedly among the four microhabitat types (sediment, sediment with shells, meadow, meadow with shells), reaching the highest values in meadows with shells and the lowest in bare sediment. Malacostracans and polyplacophorans were significantly more abundant in samples containing shells, while other taxa showed no consistent trends. Shell orientation did not affect the composition of motile fauna, but distinct assemblages were associated with specific habitat types. These findings highlight the ecological importance of P. nobilis shells as microhabitats that enhance coastal biodiversity even after the death of the organism and underline the potential negative consequences of their degradation or removal. • Empty Pinna nobilis shells provide refuge for motile macroinvertebrates. • Biodiversity peaks in seagrass meadows with empty P. nobilis shells. • Shell orientation does not influence community composition or diversity indices. • Assemblages differ among microhabitats, showing unique species–habitat associations. • Empty shells enhance habitat complexity after mass mortality.