Surviving the collapse: unprecedented densities of the critically endangered fan mussel Pinna nobilis in a Mediterranean coastal lagoon.

The fan mussel Pinna nobilis, the largest bivalve endemic to the Mediterranean Sea, has experienced a near-total collapse since 2016 due to mass mortality events (MMEs) primarily caused by the protozoan parasite Haplosporidium pinnae. Now listed as Critically Endangered, the species is considered functionally extinct across most of its native range. However, recent surveys in the Thau Lagoon (southern France) revealed a large resilient population, with total abundance previously estimated at over 100,000 individuals. Within this population, exceptionally high densities were recorded along the Tocs sandbank system and the adjacent Quilles sector. Densities were quantified by freediving-based 100 m² belt transects (50 m × 2 m; n = 89) across 15 stations. Our study reports unprecedented densities, with a global mean of 78.6 ± 8.7 individuals/100 m² and a maximum of 511 individuals/100 m², surpassing previously reported values. Extrapolation within a conservative 180-ha core survey area yields a total abundance exceeding 1.4 million individuals. Field observations indicate the presence of various life stages, from juveniles to large adults, suggesting ongoing recruitment and long-term survival. Together, these findings highlight the potential conservation importance of Thau lagoon as a refuge for P. nobilis and identify the Tocs-Quilles area as a major density hotspot within this lagoon refuge.