Population status of the Critically Endangered fan mussel Pinna nobilis in Thau Lagoon (France)

Abstract

Since 2016, the noble pen shell Pinna nobilis, a unique and emblematic giant bivalve endemic to the Mediterranean Sea, has been affected by mass mortality events (MMEs), primarily due to the pathogen Haplosporidium pinnae. To date, all known populations located in the open sea have been decimated by epizootics, and P. nobilis is currently classified as a Critically Endangered species. The last refuge areas with P. nobilis populations are now in coastal lagoons. This study assesses the status, size, density, and spatial distributions of P. nobilis populations in Thau Lagoon (Occitany, France). At the end of January 2024, 1931 live individuals were recorded at 30 different surveyed sites. Our surveys indicate various size-class distributions ranging from young recruits to adults. Examining the spatial distribution of P. nobilis revealed high-density populations in specific areas (up to 25 ind. 100 m-2). P. nobilis is present from the edges of the lagoon to the central and deeper zones (up to 9 m deep). Although Thau Lagoon experienced a localized MME at the end of 2020, driven by the presence of H. pinnae, this study reveals the occurrence of new individuals in previously infected areas, indicating signs of population recovery and potential resilience. All these observations suggest that Thau Lagoon represents one of the very last sanctuaries for P. nobilis in the Mediterranean Sea.