The effect of spawning of cultured mussels (Mytilus edulis) on mechanical properties, chemical and biochemical composition of byssal threads

• Diameter and breaking force of byssal threads decreased after spawning of mussels.
• Stress-at-break and strain-at-break of byssal threads were similar after spawning.
• Byssal thread lipid contents were less than 0.05%.
• Metal and amino acid composition of byssal threads were similar between spawning.

The blue mussels Mytilus edulis are widely cultured in the coastal regions of Canada. However, their production on sleeves attached to long lines in the water column suffers from fall-offs due to byssus weakening, especially during the spawning period. Thus, we tested in laboratory the hypothesis that the spawning event changes the chemical and biochemical composition of byssal threads, resulting in a significant modification of their mechanical properties. Metal composition of byssus is interesting, as the proteins are cross-linked by complexation with metals influencing the breaking force. Our results show that the filaments' diameter and breaking force significantly decreased after spawning, as compared to unspawned individuals. However, no differences in the stress-at-break and strain-at-break were observed between the two experimental groups. Analysis of the lipid composition of the fibers' distal region indicates a very low concentration of total lipids, i.e. < 0.05% in both groups. Some amino acid and lipid classes showed significant differences in relation to the spawn, but as their specific concentration or differences were very low in the byssal threads, their influence on mechanical properties seems negligible. The metal composition of the distal section of the byssus did not show substantial changes induced by the spawning event. Altogether, our results show that the weakening of the byssal thread force resulting from the spawning event of the mussels M. edulis in laboratory conditions was mainly due to a reduction of the fiber diameter, and cannot be related to changes in lipids, amino acid or metal composition.