MOESER, G.; LEBA, H.; CARRINGTON*, E.; University of Rhode Island; University of Rhode Island; University of Rhode Island: When mussels don�t go with the flow: seasonal variation in byssal thread production and mechanics

The attachment strength of the blue mussel, Mytilus edulis, follows an annual cycle on New England temperate rocky shores, increasing two-fold in winter in comparison to summer. The period of weak attachment often coincides with tropical storm activity, resulting in substantial dislodgment and mortality each fall. Many previous studies suggest increased attachment strength in winter reflects increased byssal thread production by mussels in response to increased water motion. We present a combination of laboratory and field studies that demonstrate byssal thread production is indeed strongly seasonal, but surprisingly, is suppressed during winter months in Rhode Island. Furthermore, the response of mussels to increasing water flow is nonlinear, with thread production declining markedly at velocities above 20 cm/s. Together, these results suggest that the annual cycle in mussel attachment strength is not driven by environmental influences on byssal thread production. Mechanical tests of byssal threads reveal strong seasonality in both quality (strength, extensibility) and durability (resistance to decay), with the production of mechanically superior threads in winter. This seasonal variation in thread mechanics largely explains the annual cycle in mussel attachment. Future studies will be aimed at elucidating the proximal cause of variable thread quality, such as water temperature, food supply, or reproductive state in order to characterize how environmental conditions influence mussel attachment.