Unexpected residual habitats raise hope for the survival of the fan mussel Pinna nobilis along the Occitan coast (Northwest Mediterranean Sea)

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 reported in the Mediterranean Sea. The total abundance of individuals across the coast was extrapolated to 163000, 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.

Automated summary

This study investigates the distribution of P. nobilis along the Occitan coast and identifies the Thau and Salses-Leucate lagoons as essential refuges for the survival of the species. The study also reveals the species' ability to colonize diverse habitats.