In Situ Investigation of Ecological and Molecular Stress Mechanisms Triggered by Marine Heat Waves in Adriatic Populations of the Mediterranean Mussel

Global warming is influencing marine dynamics, with marine heat waves (MHWs) threatening the survival of several species. After observing mussels’ massive mortality for three consecutive years (2022–2024) along the Italian Mid-Adriatic Coast, the present study aimed to evaluate, from an ecological and molecular perspective, the evolution of the health state of the Mediterranean mussel (Mytilus galloprovincialis) population facing the effects of summer heatwave in 2024, in situ. Three MHWs were recorded over the summer, with the second being 41 days long and having temperatures higher than 30 °C. In both sites considered (at the Tronto River mouth and the Frana San Nicola) inside the Piceno coast, the mussel beds experienced a clear decrease in individual density from April (the reference month) to August, with the total mortality recorded in September. The transcriptional levels of the molecular biomarkers analyzed during this time span revealed a state of heat stress with HSP70 (heat shock protein 70) and HSP90 (heat shock protein 90) upregulated in July and August. The apoptotic signal measured through the branchial transcript quantification of p53 and caspase 3 is less clear. The occurrence of MHWs is reshaping the local macrozoobenthonic community structure: the permanent mussel beds that characterized the intertidal and shallow submerged reef along the Mid-Adriatic coast are shifting to a temporary population that renovates yearly.