Pinna nobilis is suffering a widespread mass mortality linked to the pathogen Haplosporidium pinnae. At present, only one population along the Spanish Mediterranean coast located in Fangar Bay (North Ebro Delta) remains clean of pathogens. A few kilometers south, the population in Alfacs Bay (South Ebro Delta) was infected in its more external part in 2018.

This study examined the disease outbreak of the fan mussel Pinna nobilis in the Alfacs Bay during a period of two years in three zones exposed to a summer salinity gradient resulting from agricultural freshwater discharges and distance to the open sea. Monitoring was also conducted in Fangar Bay, featuring lower salinities and no evidence of disease.

The obtained results showed that the salinity gradient of Alfacs Bay (37.4-35.7) was associated to cumulative mortality (100% near the mouth, 43% in the middle regions, and 13% in inner regions), thus hindering the spread of pathogens. In Fangar Bay, lower salinities (30.5-33.5) prevented the disease but individuals were highly vulnerable to storm Gloria which caused 60% mortality in 3 weeks, and near 100% in 6 weeks.

Prado et al. (2020) Pinna nobilis in suboptimal environments are more tolerant to disease but more vulnerable to sever weather phenomena. Marine Environmental Research 163: 105220.