Attachment strength of the mussel Mytilus galloprovincialis: Effect of habitat and body size

• Mussel morphology and attachment are functions of local habitat and body size.
• Thickness of the byssus clearly defined mussel strength within habitat comparison.
• Wave exposed site caused mussels to allocate more energy to reduce dislodgment risks.
• Large mussels secrete lower byssus quantity and quality in high flow environments.
• High energy sites limit mussel size with no pace between force and shell area.

Marine organisms adapt to a wide variety of environments, often altering their morphology and behavior in response to local habitat. This study addressed the effects of habitat (wave exposure) and body size on the morphology and byssal attachment of mussels within the same estuary. Tenacity of the mussel Mytilus galloprovincialis was higher at the exposed site, particularly for the smaller size classes. This was largely due to differences in thread thickness; mussels from the exposed site produced thicker and stronger byssal threads. For a given shell length, exposed mussels also produced thicker and smaller shells and had lower gonadal condition. In laboratory flume experiments, both thread production and mechanical performance (strength and extensibility) decreased with increased flow, suggesting that flow alone does not explain tenacity differences between sites. Altogether, these analyses suggest that mussels at exposed sites allocate resources to reduce risk of dislodgment (smaller and thicker shell, stronger byssal threads) instead of growth and reproduction, and these allocation differences between sites are less apparent in larger size classes. The lack of the largest size class (8 cm) at the exposed site may reflect an upper limit to size imposed by wave induced mortality, where attachment strength does not keep pace with hydrodynamic loading.