Introduction
Mass mortality events affecting the Mediterranean mussel Mytilus galloprovincialis have been reported with increasing frequency across the Mediterranean basin. In the Slovenian Sea, mussel farmers have recorded near-total (≈100%) mortality over two consecutive production seasons, representing a critical threat to local aquaculture. Within the TEAMFORADRIS project (Transnational mussEls preservAtion systeMs FOR the ADRIatic Sea; Interreg IPA ADRION 2021–2027), a monitoring programme was established to characterise mussel health dynamics and identify the drivers of mortality, combining field observations, biometric and biomechanical analyses, histopathological examination, and species identification.
Materials and Methods
Monthly sampling was conducted from June 2025 to May 2026 (12 events) at a mussel farm in the Bay of Piran, collecting 50 farmed and 30 wild M. galloprovincialis per event (n = 960 in total). Visual inspection of mussel lines was carried out, recording the presence of biofouling organisms. Biometric measurements (shell length, width, height, wet and dry tissue and shell weights) were recorded. Byssus thread tensile strength was measured using a force gauge (Babarro and Comeau, 2014) , and the Condition Index (CI = dry viscera weight / shell volume (g L-1)) was calculated as an indicator of physiological state (Lobel et al., 1991). A subset of samples (30 farmed and 30 wild per event) was submitted for histopathological analysis of gills, digestive gland, gonad, mantle, musculature, foot, heart and kidney. The presence of epifauna and flatworm species were recorded.
Results
Biometrics and field observations: CI in farmed mussels showed a progressive seasonal decline from a median of 20.3 g L-1 in July 2025 to a winter minimum of 9.7 g L-1 in February 2026, with a partial spring recovery to 14.8 g L-1 in March 2026. Byssus thread tensile strength followed a similar pattern, with the lowest value in July 2025 (7.8 ± 3.2 N, n = 20) and the highest in November 2025 (12.7 ± 4.5 N, n = 20). Field inspections consistently recorded the presence of biofouling on mussel lines throughout the monitoring period.
Histopathology: The endoparasitic copepod Mytilicola intestinalis was the most prevalent finding, detected in 75% of farmed individuals in June and 40% in July 2025. Rickettsia-like intracellular bacteria were observed in the gill epithelium without a marked inflammatory response (20% farmed, 40% wild in June; 30% farmed, 55% wild in July). Focal granulomas with haemocyte infiltration, areas of tissue necrosis in the mantle and musculature, and ciliates in the digestive gland epithelium were recorded in both sampling events and both groups. Suspected myocarditis, characterised by haemocyte infiltration surrounding cardiomyocytes was identified in 15% of individuals in the July 2025 sampling, in both farmed and wild mussels. The turbellarian Urastoma cyprinae was detected in gill tissue sections from July 2025 onwards (15% farmed, 15% wild).
Associated fauna: Biofouling species were recorded along the hanging ropes in the mussel farm. A predatory polyclad flatworm native to the Pacific Ocean Postenterogonia orbicularis, was present during the whole period of present study.
Discussion
These results indicate a complex, multi-stressor aetiology for the observed mass mortality rather than a single causative agent. The presence of the predatory flatworm Postenterogonia orbicularis, native to the Pacific Ocean was confirmed in all investigated samples. The species were recorded for the first time in a mussel farm in October 2023 recognised as a voracious predator, but the impact on economic loss was not quantified (Ram��ak et al., 2024). The concurrent seasonal decline in CI and byssus thread strength, reaching minimum values in summer and mid-winter respectively, together with a high prevalence of Mytilicola intestinalis, Rickettsia-like bacteria, and haemocytic inflammatory responses observed in histopathological analyses, suggests that cumulative physiological stress reduces host resilience and may amplify the impact of pathogens. Field observations further indicate that biofouling species differ markedly between upper and lower sections of culture ropes, and the potential presence of the tunicate Clavellina sp. and other biofouling species requires further investigation.
A temperature difference of approximately 5 °C recorded between the sea surface (measured at 2 m depth) and the bottom layer at 10 m depth at the study site suggests a practical approach for mitigating the identified stressors. Evidence shows that lower water temperatures reduce trade-offs between byssus function and reproductive investment, maintaining stronger attachment capacity during thermally stressful seasons and decreasing overall mortality risk (Mizuta and Wikfors, 2019). Based on this, the project proposes piloting an underwater cultivation structure positioned at 10 m depth, using vertical cultivation lines to access cooler bottom layers. The infrastructure, developed as an Integrated Multi-trophic Aquaculture (IMTA) approach, would allow direct comparison of mussel health metrics between conventional surface farms and depth-managed systems as an adaptive aquaculture strategy.
Acknowledgment
This work was carried out within the TEAMFORADRIS project, co-financed by the European Union through the Interreg IPA ADRION 2021–2027 programme, P1-0237 Coastal Sea Research, ARIS.
References
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Mizuta, D.D. & Wikfors, G.H. (2019). Depth Selection and In Situ Validation for Offshore Mussel Aquaculture. Journal of Marine Science and Engineering, 7(293).
Ram��ak, A., Bizjak, T., Robi��, U., & Vir��ek, M. K. (2024). The need for innovations to secure the future of artisanal mussel farming in the coastal sea of the Gulf of Trieste (Slovenia). Aquaculture Reports, 36, 102166.