Status of Pinna nobilis populations before and after a mass mortality event in the Tabarca Marine Reserve, SE Spain

The endemic bivalve Pinna nobilis is the largest bivalve in the Mediterranean Sea and one of the largest worldwide, able to reach 1 m or more in size. During recent decades, P. nobilis populations have suffered substantial declines due to several causes of anthropogenic origin. The species situation is currently critical throughout the Mediterranean Sea due to a mass mortality event (MME) that started in late 2016. Recent findings indicate that a picornavirus had compromised the fan mussel immune system, enabling the protozoan Haplosporidium pinnae to infect immunosuppressed individuals and cause most die-offs. As a result, this bivalve has been included in the Spanish National Catalog of Threatened Species and the IUCN Red List since 2020. The present study aims to improve the knowledge of the status of P. nobilis populations between 2010 and 2023 in the Tabarca Marine Reserve, the first Spanish marine reserve. Before the MME, density reached 2.8 ind. 100 m -2 in this marine reserve. P. nobilis density varied significantly with the bottom type-depth interaction, whereas size was associated with depth and island zone. Following the MME, mortality of pen shells reached 100%. Thereafter, an increase in Pinna rudis abundance was detected, occupying the spaces left by P. nobilis after its disappearance. In addition, a putative hybrid P. nobilis × P. rudis was detected. These results contribute to the knowledge of the species population status in this marine protected area, and provide useful information, in case of recovery in the future, to facilitate the implementation of management policies.