Discovering new living Pinna nobilis populations in the Sea of Marmara

Fan mussel (Pinna nobilis) is one of the largest bivalve species in the Mediterranean Sea. The situation of the species is critical as it faces widespread mass mortality attributed to pathogens in various parts of the Mediterranean Sea. The Sea of Marmara (SoM) offers a unique environment for fan mussel populations, with some areas hosting alive populations. This study aims to explore and describe new P. nobilis populations in the SoM that are known to be not affected by mortality. An area of 28,200 m² at 47 stations along the 105 km coastline in the southern part of the SoM was explored using underwater visual transects. A total of 544 alive fan mussels were recorded during the underwater surveys, ranging in total shell height from 11.8 to 31.4 cm. The mean density was estimated as 5.3 ind 100 m⁻² although maxima of 18.8 ind 100 m⁻² were recorded in some stations. These density hotspots were distributed from the shoreline to a 10 m depth range and 100 m distance from the shoreline in sandy and seagrass meadow habitats. The presence of juveniles provided evidence of successful recruitment. The distribution pattern and recorded mortalities were attributed to hydrodynamic factors and intense human activities. Potential environmental factors (low salinity and temperature) in the SoM may control or delay the possible spread of the lethal pathogens. Favorable conditions result in mussels’ resilience and survival mechanisms. The SoM offer a promising larval reservoir for the recolonization of affected areas, such as those found in the Aegean Sea, through larval exportation.