What remain of the ecological role of the ecosystem-engineer species Pinna nobilis (Bivalvia: Pteriomorphia) after the last (2016-2022) Mass Mortality Event (MME)? A North Adriatic Sea case study

The fan mussel Pinna nobilis is the largest endemic bivalve of the Mediterranean Sea, which inhabits coastal ecosystems from the transitional to the deep infralittoral waters, characterized by sandy bottoms and extensive seagrass meadows. P.nobilis plays important ecological roles: being a filter-feeder species, it ingests large amount of Phytoplankton, Zooplankton, particulate and suspended organic matter and seagrass detritus playing a key role in energy transfer flow from water column to the sediments. Furthermore, because of its large shell strongly anchored to the sediment, P.nobilis acts as an ecosystem-engineer species and it represents a stable hard substrate for a wide variety of macrobenthic species. The fan mussel, for long exploited and threatened directly and indirectly by humans, recently has also faced an epidemic Mass Mortality Event (MME) that led it almost to the local extinction. Since 2016, mortality rates close to 100% have been reported throughout the Mediterranean Sea, including the North Adriatic area and the Gulf of Trieste, which hosted one of the densest populations. The numerous empty shells found in the gulf, still intact and firmly anchored in the sediment, have continued to serve as substrate, have even become shelters for many species, and also gave rise to complex 3D-structures that host rich and diverse communities of benthic organisms. Hence, it appears that the ecosystem-engineering role of the bivalve might persist despite the animal death. Therefore, this study aims to assess the biogenic capacity of dead specimens of P.nobilis as “local biodiversity enhancers”, through the analysis of the associated benthic community in terms of composition and structure, and whether it might be also affected by the size of individuals.