Mass Mortality Events in Marine Bivalve Populations

Mass mortality events (MMEs) among marine bivalves, notably the endemic fan mussel Pinna nobilis, have emerged as one of the most pressing conservation concerns for coastal ecosystems. These episodes are characterised by abrupt and widespread population declines, often exceeding 90 % local mortality within months. Driven largely by infectious agents—principally haplosporidian parasites and members of the genus Mycobacterium—these outbreaks are exacerbated by abiotic factors such as sea‐surface temperature anomalies, salinity fluctuations and altered current regimes. The synergistic action of multiple pathogens, together with stressors such as pollution, habitat degradation and climate change, has transformed what were once isolated die‐offs into basin‐wide collapse of keystone bivalve populations. The loss of large bivalves disrupts benthic–pelagic coupling, compromises water filtration services and undermines the structural complexity of seagrass meadows. Improved understanding of the epidemiological pathways, environmental thresholds for disease expression and genetic resilience of remnant populations is essential to inform targeted monitoring, early‐warning systems and proactive restoration strategies at a Mediterranean and global scale.

Research from Nature Portfolio

Recent studies have elucidated the role of mycobacterial infection in silent mass mortality before overt parasite outbreaks, revealing that intracellular bacteria provoke chronic inflammatory lesions in connective tissues of Pinna nobilis, independent of Haplosporidium pinnae presence. This work underscores the need to expand pathogen surveillance beyond traditionally implicated parasites. Collaborative citizen‐science frameworks combined with high‐resolution oceanographic modelling have mapped rapid regional spread of the haplosporidian parasite via surface currents, demonstrating that temperature thresholds above ~13.5 °C and salinity ranges of 36.5–39.7 psu critically influence infection dynamics. These findings illustrate the power of integrating community data with predictive oceanography to forecast MME trajectories and identify thermal–haline hotspots for targeted intervention.

Mass Mortality Events in Marine Bivalve Populations publication trend

The graph below shows the total number of articles in mass mortality events in marine bivalve populations across all publications each year (not limited to Nature Index journals).

Technical terms

Mass Mortality Event (MME): A rapid and large-scale die-off of a species population within a limited geographic area and time frame.

Haplosporidium pinnae: A haplosporidian protozoan parasite implicated in widespread bivalve mortality, particularly in Pinna nobilis.

Mycobacterium: A genus of acid-fast bacteria, some species of which have been linked to inflammatory disease and death in marine bivalves.

Histopathology: The microscopic examination of tissue lesions to diagnose disease processes in organisms.

References

1. Description of Mycobacterium pinniadriaticum sp. nov., isolated from a noble pen shell (Pinna nobilis) population in Croatia. Frontiers in Microbiology (2023).
2. Seasonality in Synergism with Multi-Pathogen Presence Leads to Mass Mortalities of the Highly Endangered Pinna nobilis in Greek Coastlines: A Pathophysiological Approach. Microorganisms (2023).
3. A mycobacterial disease is associated with the silent mass mortality of the pen shell Pinna nobilis along the Tyrrhenian coastline of Italy. Scientific Reports (2019).
4. Tracking a mass mortality outbreak of pen shell Pinna nobilis populations: A collaborative effort of scientists and citizens. Scientific Reports (2019).
5. In the Wake of the Ongoing Mass Mortality Events: Co-occurrence of Mycobacterium, Haplosporidium and Other Pathogens in Pinna nobilis Collected in Italy and Spain (Mediterranean Sea). Frontiers in Marine Science (2020).