In 2019, the status of the Mediterranean fan mussel Pinna nobilis was elevated to
‘Critically Endangered’ on the IUCN Red List, in response to the pandemic caused by the parasite
Haplosporidium pinnae. Identifying refuge habitats, free from parasites, is critical to the survival
of the mussel. The distribution of P. nobilis was investigated along the Occitan coast (Northwest
Mediterranean Sea) because of the presence of a unique lagoonal system that may provide potential
refuges. Interviews with users and managers were conducted to identify target zones where
the species was sighted. In situ surveys were carried out to define the main aggregations of fan
mussels and characterize the habitat. Line transects were deployed to count and measure individuals
to estimate density, abundance, and size distribution. Population densities were variable,
ranging from 0.6 ± 0.2 (SE) to 70.8 ± 7.6 ind. 100 m−2, representing one of the highest densities re -
ported in the Mediterranean Sea. The total abundance of individuals across the coast was extra -
polated to 163 000, with 87% located in Thau and Salses-Leucate, highlighting these lagoons as
essential for the survival of the species. This study also revealed the diversity of habitats colonized
by P. nobilis. In the context of the pandemic, only the lagoon populations remain unaffected and
provide natural refuges that have disappeared from all open-water coastal areas. However, the
conditions in these lagoons could become unfavorable, leading to the collapse of the last P. nobilis
populations. We therefore propose that Thau and Salses-Leucate lagoons, which harbor the
largest remaining populations of P. nobilis, should be declared as conservation priorities.En prens