How do adductor muscles open and close a bivalve shell?

In bivalve mollusks, the opening and closing of the shell is a mechanical process involving the interaction between the adductor muscles and the hinge ligament.12

• Closing the shell: The adductor muscles, which are attached to the inner surfaces of the shell valves, contract to pull the valves together. These muscles are often differentiated into "quick" (striated) components for rapid closure and "slow" (smooth) components for maintaining tension over long periods with minimal energy expenditure.32
• Opening the shell: The shell opens through the action of the elastic hinge ligament located middorsally. When the adductor muscles relax, the ligament acts as a spring, creating an opening thrust that forces the valves apart.12

This antagonistic relationship allows the bivalve to control its shell position for feeding, locomotion, or protection without requiring constant muscular effort to keep the shell open.2