Mussels are a rising star in the world of bivalve mollusks. Not only are they a tasty and nutritious source of protein, but they also have a lower environmental impact than fish aquaculture. However, France, a major mussel producer, is facing a challenge. In recent years, there have been outbreaks of abnormal mussel mortality (AMM), which have caused significant economic losses and threaten the sustainability of the industry.
A study published by scientists from Ifremer ASIM (France) and the University of Montpellier (France) reports the potential of selective breeding to create more resilient mussel populations. The researchers analyzed natural variations in mussel resistance to abnormal mortality outbreaks as a selective breeding tool. By selecting mussels that survive the outbreaks, they hope to develop future generations with greater resilience.
Mass Mussel Mortalities
The European Union (EU) is a major producer of mussels, cultivating both blue mussels (Mytilus edulis) and Mediterranean mussels (Mytilus galloprovincialis). France is a leader within the EU, producing more than 61,000 tons of mussels in 2020, along with hybrids of both.
French mussel production relies on the collection of wild seeds for cultivation. This dependence makes the industry susceptible to variations in natural dispersal.
However, abnormal mussel mortality outbreaks have become a major concern for French mussel producers since 2014. The cause of AMM remains a mystery, though factors such as pathogens, pollution, and climate change are being investigated.
Selective Breeding: A Hope for the Future
Selective breeding has proven successful in improving desirable traits in other farmed species. Studies suggest that selective breeding can improve growth and disease resistance by 10 to 15% per generation.
Early studies suggest promising results for selective breeding against AMM in M. edulis. Heritability estimates ranged from 0.55 to 1.15, indicating potential for genetic improvement, while mass selection for a single generation increased survival by 34% to 48%.
The researchers created 100 mussel families from six wild populations and monitored their survival and growth at two locations: La Floride (LF) and Maison Blanche (MB).
Site Matters: Dramatic Differences in Mortality
The results were surprising. In MB, a significant AMM outbreak occurred in spring 2018. Mortality rates were highest for Mytilus edulis (70%), followed by NH (63%) and Mytilus galloprovincialis (43%). However, in LF, mortality remained low across all groups, ranging from 11% to 30%. This suggests that some site-specific environmental factor at MB might be triggering AMM.
Genetics Play a Role, But Not the Whole Story
The study also explored the heritability of AMM resistance, a measure of the likelihood that a trait will be passed on to offspring. Heritability values were low to moderate across all groups and sites, ranging from 0.12 to 0.37. Interestingly, Mytilus edulis showed the highest heritability, followed by NH and then Mytilus galloprovincialis. This suggests potential for selective breeding to improve AMM resistance, particularly in Mytilus edulis.
The Vibrio splendidus Strain is Not the Culprit
Further investigation was conducted on the possible role of a specific strain of the bacterium Vibrio splendidus isolated during an AMM outbreak in 2014. Mussel families were experimentally infected with this strain. While mortality was observed, heritability estimates were generally lower compared to MB field data. Additionally, the genetic correlation between MB AMM and experimental infection was weak, suggesting that this Vibrio strain was not the primary cause of the AMM outbreak observed in 2018.
Promising Signs of Growth, Room for Improvement in Mortality
The study also found significant genetic variation for growth traits in both mussel species, with high heritability estimates (above 0.44). This means selective breeding programs could potentially improve growth rates. Interestingly, the correlation between growth and mortality was weak, suggesting that faster-growing mussels are not necessarily more susceptible to AMM.
Conclusion
The scientists found a strong species-by-environment interaction, with higher susceptibility to AMM for M. edulis, while it had greater resistance to mortality at La Floride. They recommend that if AMM becomes widespread across sites in the near future, then M. galloprovincialis could offer substantial benefits to French mussel producers.
The mystery of AMM outbreaks in France continues. While the study rules out Vibrio splendidus as the main culprit, the cause remains elusive. In this sense, the research is a crucial step towards sustainable mussel farming in France, as understanding the genetic basis of AMM resistance will be vital for breeding programs.
The study was funded by DPAM of the French Ministries of Ecology and Agriculture through the “MORBLEU” research program and the European Union’s Horizon 2020 program.
Contact
Lionel Dégremont
Ifremer ASIM,
Avenue Mus de Loup, 17390 La Tremblade, France
Email: lionel.degremont@ifremer.fr
Reference
Ajithkumar, M., Lillehammer, M., Travers, M., Maurouard, E., Aslam, M. L., & Dégremont, L. (2024). Genetic parameters for resistance to field mortality outbreaks and resistance to a pathogenic strain of Vibrio splendidus in Mytilus edulis, Mytilus galloprovincialis and natural hybrid. Aquaculture, 590, 741034. https://doi.org/10.1016/j.aquaculture.2024.741034
