Abstract
The fan mussel, Pinna nobilis, endemic to the Mediterranean Sea, is a critically endangered species facing mass mortality events in almost all of its populations, following the introduction of the parasite Haplosporidium pinnae. Such a unique pandemic in a marine organism, which spreads rapidly and with mortality rates reaching up to 100%, could lead to the potential extinction of the species. Only few regions, involving lagoon habitats, remain healthy throughout the entire Mediterranean Sea. This study describes the genetic structure of P. nobilis across the Gulf of Lion, including confined locations such as lagoons and ports. A total of 960 samples were collected among 16 sites distributed at 8 localities, and then genotyped using 22 microsatellite markers. Genetic diversity was high in all sites with mean allele numbers ranging between 10 and 14.6 and with observed heterozygosities (Ho) between 0.679 and 0.704. No genetic differentiation could be identified (FST ranging from 0.0018 to 0.0159) and the percentages of related individuals were low and similar among locations (from 1.6 to 6.5%). Consequently, all fan mussels, over the entire coastline surveyed, including those in the most geographically isolated areas, belong to a large genetically homogeneous population across the Gulf of Lion. Considering the ongoing mass mortality context, this result demonstrates that almost all of the genetic diversity of P. nobilis populations is still preserved even in isolated lagoons, which might represent a refuge habitat for the future of the species.
Highlights
Recent extinction rates have been shown to be 100 to 1000 times their pre-human levels in taxonomically diverse groups through a wide variety of different habitat types, and they continue to accelerate[1]
The analyses revealed a high level of genetic diversity as the mean allele number per locality (Na) ranged from 10 to 14.6 (Table 2)
The genetic structure of P. nobilis across the Gulf of Lion, based on the analysis of 960 fan mussels sampled at 8 localities, with different environmental conditions, appeared homogeneous throughout the area including samples isolated in enclosed lagoons and harbors
Summary
Recent extinction rates have been shown to be 100 to 1000 times their pre-human levels in taxonomically diverse groups through a wide variety of different habitat types, and they continue to accelerate[1] These extinctions are mainly caused by anthropic activities such as pollution, artificialization, overexploitation, habitat loss and global change, which can lead to the isolation of populations, reduction in their size, and a loss of genetic d iversity[2]. Coastal environments and species life-history traits that can affect dispersal and population connectivity, as found for the pelagic early life stage which represents the main opportunity for movement[4] This dispersal phase allows connectivity between distant areas and is a critical feature of the ecology of marine organisms and evolution, as it drives the genetic structure and composition of populations[5]. There remains a significant lack of knowledge concerning the P. nobilis life cycle, as a recent study has shown that the larval stage could last up to 20 days under controlled conditions[13], a much longer larval duration which would certainly enhance dispersal capabilities
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