New case study: Understanding mussel shell breakage in Scottish aquaculture

Investigating the causes behind shell breakage in Scottish mussels

Blue mussels (Mytilus edulis) are the backbone of Scotland’s shellfish industry. The reasons for shell breakage - a persistent issue during harvesting and processing – are complex and the exact causes remain unclear, however there may be a link to hybridisation with the Pacific blue mussel (M. trossulus).

This project set out to investigate whether genetics, physical traits, or environmental factors contribute to shell weakness. Valued at almost £95k, the project was led by the University of Stirling with partners Fassfern Mussels Ltd and the Scottish Shellfish Marketing Group.  

The study analysed nearly 600 mussels from Loch Sunart on Scotland’s west coast. Researchers examined shell thickness, hardness, and fracture toughness, alongside genetic testing using cost-effective DNA protocols and advanced genotyping tools (KASP). The goal was to identify links between species composition, shell strength, and breakage.

Key findings:

• Shell thickness and hardness were strongly correlated with breakage: thinner, softer shells were more likely to break.
• Fracture toughness showed no significant link to breakage.
• No correlation was found between species genotype and shell strength traits, meaning hybridisation does not directly cause breakage.
• More shells broke after processing than harvesting, but this may reflect handling differences rather than genetic factors.

The project also developed improved methods for identifying mussel species and hybrids, which could support long-term stock management. Due to time constraints, environmental factors such as salinity and water flow were not fully explored, leaving questions about site-specific influences unanswered.

While the anticipated outcomes, e.g., selective breeding based on species, were not achieved, the project clarified that shell strength, not genetics, drives breakage. Future research will focus on environmental factors and explore genomic tools to identify other potential markers for shell robustness. A follow-on PhD project is already underway to continue this work.

The full title of this project is ‘Investigating influences on shell breakage in Scottish mussels’.

Read the case study