Pinna nobilis dead shells as a receptacle of a diverse and rich mollusc community. A case study from the northern Adriatic Sea

Pinna nobilis is the largest endemic bivalve of the Mediterranean Sea that plays key ecological roles in coastal ecosystems. As a suspension feeder, it facilitates the transfer of energy and matter from the water column to the sediment, promoting benthic-pelagic coupling and benefiting infaunal communities. In addition, the fan mussel is considered as an ecosystem engineer and catalyst for enhancing local biodiversity by creating new suitable substrates and providing shelter for a diverse epibenthic community. Recently, P.nobilis has experienced an epidemic mass mortality that brought it close to the brink of local extinction in several Mediterranean regions, including the northern Adriatic Sea and the Gulf of Trieste. Despite this significant loss, the numerous empty shells of P. nobilis, that remained intact and firmly anchored in the sediment, continued to serve as substrate giving rise to complex 3D-structures that harbour and thrive rich and diverse benthic communities. The aim of this study is to evaluate the persistence of the ecological role of empty P. nobilis shells as biodiversity enhancers by investigating the associated epibenthic community. As an initial step, the study conducted a structural analysis of mollusc community living associated with P. nobilis shells collected from four different sites in the Gulf of Trieste. A total of 96 mollusc species characteristic of both infralittoral and sublittoral hard and soft bottoms, were identified in thirteen dead specimens of the fan mussel. In addition, significantly high diversity metrics were observed, suggesting that a rich mollusc community inhabits both the outer surface and inner cavities of the fan mussel shells, and that the shells now freed from the animal may also host some soft-bottom mollusc species. These findings confirm that the role of the bivalve as an island of biodiversity persists after the death of the animal.