An emergency situation for pen shells in the Mediterranean: The Adriatic Sea, one of the last Pinna nobilis shelters, is now affected by a mass mortality event.

We identified areas with high individual densities of the pen shell, Pinna nobilis, in two areas along the Croatian Adriatic coast. The surveys carried out in 2018 and 2019 showed population densities of approximately 9 to 13 individuals/100 m2. However, in 2019 a mass mortality event (MME) causing 36% to 100% mortality of this bivalve species was observed in the surveyed Croatian bays. The parasite Haplosporidium pinnae was identified by histological and molecular methods in all affected sampled individuals, while Mycobacterium sp. and Gram negative bacilli were detected in some affected and live bivalves. This finding constitutes the first record of these pathogens affecting P. nobilis in the middle Adriatic, confirming the continuous spread of the disease. Previously, the Adriatic water body was considered to be a natural shelter against the MME caused by pathogens in pen shell populations because of its distinct ecological features. The Adriatic Sea is a semi-closed water body with the largest continental shelf in the Mediterranean Sea, and due to its geomorphology and bathymetry, it is a sea with distinct characteristics. Monitoring plans and further studies in the Adriatic bays are now a priority for mitigating the high risk of extinction and working toward the conservation of this protected species.